Since I last wrote on this subject in January certain facts have come to my knowledge which, I think, favour the theory of five simple colour sensations-red, yellow, green, blue, and violet. Formerly green was held to be compound, because it was supposed to be producible by mixing blue and yellow; but is now held to be simple. We should, therefore, be warned against hastily assuming that yellow or blue are compound, because they can apparently be produced, one by mixing green and violet light, and the other by mixing green and red light. When one looks through a prism at a strip of white on black, which is wide enough to show some white, the white is seen in the middle, dividing the colours into two different sets, one set having red shading rather suddenly into yellow and finishing with a slightly greenish yellow, and the other set having violet shading into blue and finishing with a alightly greenish pale blue. A person of my acquaintance, with a good eye for colour,and who normally sees the colours under the stated circumstances as just described, has recently had his eyes peculiarly affected after taking a strychnine tonic. Without noticing any particular difference in the colours of objects generally, excepting that he seemed to miss seeing some of the finer and richer shades of blue, this person had his attention (one day recently) drawn to a broken cut-glass gaselier pendant, and accidentally looking through a prismatic part of it towards a window was at once struck by a remarkable difference he observed in the colours exhibited as compared with what he was accustomed to. He, of course, at once got a proper colour prism, but still found the same difference. The series of colours beginning with red did not appear altered, excepting that the red seemed, if anything, a little more than usually inclined to crimson; but in the other set of colours the violet seemed richer and more like purple, the blue almost absent, and the light blue inclining to green, changed to the ordinary rich green seen in the spectrum of a narrow slit. On looking through the prism at a shaded lamp, in which was burnt an oil believed to be photogene or petroleum spirit, blue was entirely absent, although seen ordinarily, and a rich violet was seen graduating quite suddenly into a rich green. These peculiarities seem difficult to explain on any assumption, excepting that the sensitiveness to simple blue was much reduced, and that, therefore, the normal eye must be capable of transmitting separate simple sensations for green, blue, and violet, at any rate, and in that case it is obviously most probable that it can also transmit separate simple sensations for both yellow and red. Why should we (mentally) recognise these five distinct colours and no others? It is useless for the "three-primary" theorists to say this or that colour can be made by mixing two of the three primaries, for the" three-primary" theory requires six distinct colours, and six distinct colours do not exist. There is no distinct colour between pure blue (untinged with violet) and pure green, between green and yellow, or between yellow and red; and although violets, purples, pinks, and crimsons are spoken of as between blue and red, blue and red are suggested by every one, and the only colour that can be called distinct between blue and red is one, whatever be its name, in which neither blue nor red preponderates. DOOR HANDLES OF RAILWAY CARRIAGES. [4033.]-PERMIT me through your columns to throw out a small hint relative to railway carriage doors generally. It is simply this: That a handle (similar to those now in common use in most private carriages) be fixed inside the doors of all the three classes-viz., first, second, and third; in connection with the one outside; so as to obviate the necessity, when opening the doors, of putting the windows down when up, as well as being obliged to put the hand and arm outside; thus avoiding at the same time a certain amount of risk. It is an easy matter, and might be done by all railway companies at a very little extra cost when constructing their carriages; and would also be conferring a great convenience to the vast multitudes who travel upon our iron roads. I hope that you will insert my letter, and that the idea may meet with approval. NATHANIEL WATERALL. AMATEUR TURNERS' SOCIETY. [4034.]-WHAT is now wanted is mechanical instruction to amateurs by competent workmen, so that they may acquire an amount of skill and practice not to be attained in private. Many of our readers (this I can answer for from my own personal knowledge) are employed by firms or in offices year by year, and naturally enough take to mechanics as a pastime and recreation. If a society is started in London, as nearly central as possible, open to any respectable man on payment of the usual fees (as moderate as possible, and payable at the option of the member-monthly, quarterly, or yearly), I have no doubt it would succeed. I would propose something like the following: That a suitable place be hired annually, and, as before stated, tools, &c., fixed; that honorary members be admitted at a lower premium; that two or more meetings take place in the year, on which all business should be brought forward; and further, that the committee should place themselves in communication with private firms (for premiums) to allow a few members at a certain time to visit the manufactories, and by that means gain practical knowledge. The advantages to the members would be numerous. They would gain real knowledge and practice in the handling and working of the lathe and its accessories, also an insight into the art of cabinet-making, polishing, &c. Should any member require advice-such as recommending or purchasing any article for the use of himself relating to turning, cabinet-making, &c.-it should be given gratis. Any articles that should be turned out in a finished state by the members should be sold, and the profits arising be carried to the society, unless the same has been finished throughout by one member only. In a few weeks I will advertise an address, whereby I can be communicated with by post only. SAMUEL SMITHER. A TRAVERSING SCREW CHUCK. [4035.]-A TRAVERSING mandril for cutting short screws with accuracy and certainty would be coveted by many amateur turners were it not that its cost and, still more, its injurious action upon the lathe for general purposes more than counterbalance its admitted advantages for that one purpose. I have lately designed and made a tool which seems to possess all the advantages of a traversing mandril, without its defects; and as it can be adapted to any lathe, I hope it may be generally useful. propose to call it "the traversing screw chuck." The following is a description of the apparatus, with reference to the accompanying drawing: A. A steel cylindrical holder, which screws upon the nose of the lathe mandril, having a longitudinal keygroove a a (or it may have a left-handed spiral keygroove of one turn per inch, which would probably work even better than the parallel key-groove; but the latter, as I have made mine, is sufficient). B. A gun-metal cylinder fitting nicely, and without shake, upon A, having a key (b) which enters the groove a a and prevents it from turning upon A, though it is free to traverse longitudinally. The cylinder B ends in a boss, having at its extremity a screw exactly like the screw upon the nose of the lathe mandril, and upon this screw any of the ordinary chacks for holding the work to be operated upon can be screwed so as to run true in the lathe. C. A steel guide-screw (of which there may be several of various pitches) fitting tightly and keyed upon B. the thread of the steel guide-screw C, and partly emD. A fixed screw-guide (of brass or gun metal) fitting, bracing its periphery, as shown in the cross section Fig. 2. The screw-guide is here shown (as I have made mine) held fast in the socket of the T hand-rest, but a better plan would be to have the several screw-guides formed on portions of the circumference of a brass or gun. metal disc, about gin. thick, centred upon an eccentric or cam, on which it could be turned and made to gear with the corresponding screw-guide C, just like the brass screw-guide fixed at the back of the head-stock of Messrs. Holtzappfel's best ornamental lathes, having traversing mandrils. NOTE.-When this apparatus is used the proper screw-guide D is put into gear with the steel screwguide C, and compels the latter, with the chuck and work which it holds, to advance at the rate required for cutting a screw upon the work by means of a single point tool or chaser, held in the slide-rest or by hand. The resulting screw will be an exact copy (as to pitch) of the steel guide-screw C (except that if a spiral keygroove is adopted in A it will have one turn per inch less than the guide-screw). JOHN F. STANISTREET. Abercromby-square, Liverpool, April 16. THE "TURRET" TRICYCLE. [4086.]-As I know several of your readers are interested in the bicycle and tricycle movement, I forward you a sketch of a tricycle I have designed and constructed. I drive direct off the crank without the intervention of connecting rods. I form a slot in the lever which pivots on an arm bolted to the frame. The lever is bent at right angles so that the feet may assist in working; cords lead from the handles to pulleys at tached to the angle of lever, up to the frame, and thenes to guiding axle. By employing rollers on the crank pin the friction may be much reduced, travelling in the slot. I mark the various dimensions on the sketch. The arms on which the levers pivot, as well as the stay, should be strong as well as the levers themselves. have thought of replacing the pair of guiding wheels by a single wheel in a fork under the seat, steering it with the cords, but fear it might be difficult to preserve a balance. I believe you could spin along at a great speed with a single guider. The handles are attached to a disc of hard wood in which the cord runs in a groove. The length of crank arm is 4in. The end of arm on which the lever pivots must be directly under the axis of the wheel, so that the levers may travel to and fro equally. C. TOWNLEY. A NEW AND EXCELLENT SUN-SCREEN FOR ALL TELESCOPES.-CENTREING THE FLAT OR PRISM OF NEWTONIAN REFLECTORS. [4037.]-THE following mode of defending the eye in viewing the sun is the most perfect that can be conceived. A film of silver is deposited either on the field lens of the eyepiece itself or upon a concave lens inserted where the Barlow is usually placed: in either case the silvered side should be turned away from the eye. The interposition of this partly transparent film of pare silver, as brilliant as any mirror, has the effect of turning back all the rays of heat, while it allows light enough to pass to produce the sharpest definition. The film may be made so thick that the eye can bear the whole aperture of any telescope, but it is better to have it only of such a density as to require a rather pale glass screen to be used with it. The action of this simple contrivance is so perfect that the eye-lens is not heated in the slightest degree, nor is there any heat upon the eye. The process of silvering is extremely easy, and may be done by any one in a few minutes. I find the cost of chemicals for three lenses is one farthing. A still better place to insert the silvered concave is between the flat and the big speculum, close to the former. The following will be interesting to all who use Newtonian reflectors. The want of perfect adjustment in the secondary mirror, whether a flat or a prism, is one of the chief causes of their frequent imperfect performance. I have never till now been satisfied with my centreing, nor have any of the published methods been absolutely perfect. The following mode leaves nothing to desire. Together with my dynamometer is sold a very simple and inexpensive microscope which clips on to any eyepiece for reading this power-gange, which it does to the one-thousandth of an inch; around the little bright image of the aperture seen thus in front of the eyelens are certain darker circles which are the images of the inside of the tube, the rim of the flat-holder and the eye tube. If these are not all exactly concentric the fault is in the flat (or prism), which must be readjusted till they are so. Though not so important this little contrivance detects the slightest want of coincidence between the axis of the great tube of a refractor and that of its dye tube. Vicarage, Romsey. 2 If any of the readers of the ENGLISH MECHANIC desire any further information upon either of the above hints I shall be glad to give it. E. L. BERTHON. P. S.-Since writing the above I have received letter from Mr. Browning, to the effect that five years ago he tried silvering the second lens of an eyepiece, and a Barlow lens for viewing the sun. I am ser prised that so admirable a plan has remained so long unused. Mr. Browning says that he found it betten to insert a plane of glass silvered on both sides, hend placed at an angle of 45°; but it must be very dati cult to obtain such a piece of glass with perfectly flat and parallel surfaces, and it was this difficulty which led me to prefer a concave lens. I do not find the heat reflected back into the tube productive of any of the supposed disturbances, or that it impairs definition. Mr. Browning's method requires a hole in the side of the tube for the escape of the heat, and this is not always convenient. Let any amateur observer silver a concave spectacle-glass, and after fixing it in a paper tube, insert it inside his adapter, and he will see sunspots, &c., as he never did before.-E. L. B. SOLAR PHENOMENA. [4038.]-As it is my custom to make a daily sketch of the solar surface, when clouds do not hide the sun from view, I have been able to trace the progress of several phenomena of great interest. Not the least remarkable among them were the shifting movements of a small spot, which I perceived for the first time on April 12th. It lay to the south of a tolerably large opening, situated upon the west limb, in the northern hemisphere. On the days mentioned below, the posi tion angles (the intersection of the wires being at the centre of the larger spot) were as follows:April 12th 5h............................... ......... 242° *13th 233° 14th 33 9) .................................... 152°? 15th 158 On the 16th the pair were so advanced towards the extreme edge of the disc that I was unable to find any trace of them, with a power of 140. It will be noticed that, with the exception of the slight retrogradation on the 15th inst., the motion was from west to east, by the south. This direction was also observed by Mr. T. W. Backhouse, of Sunderland, to be taken by a spot in its revolution round another, which was visible on the northern zone on May 9th, 1870. The line joining them rotated through an angle of 80° or 90° in 53 days. "It is interesting to observe," says a writer in Nature, "that the direction of this rotation (from south to east) is the same as that in which cyclones rotate in the earth's northern hemisphere; in the southern hemisphere they rotate in the opposite direction. This coincidence gives some support to the theory of solar spots being produced by cyclones." (September 1, 1870.) But the retrograde movement of the spot on the 15th inst. (from east to south) has its counterpart in a large opening which I observed in July, 1870. The small companion to it moved from south-east to southwest, through an angle of 35°, in 23h. This also occurred in the sun's northern hemisphere. The solar surface, especially in the equatorial regions, has of late been diversified by vast groups of spots, which, from day to day, have presented a most interesting and astonishing series of changes. Had it struck me earlier, I might have delineated some of the principal groups in detail, to exhibit, in a succession of drawings, their daily alterations in appearance as they passed across the disc. Yet, on several occasions, the complexity of many of these spot-aggregates was extraordinary, and might have severely taxed my artistic skill-whatever that may be! I do not think that "P. S. T." (qy. 11543) would find a revision of double stars, and of other oft-inspected heavenly objects, an occupation of such transient interest as he seems to apprehend; on the contrary, if he once got over a few preliminary difficulties, I feel sure the pleasure of the labour would excite him to continue it. Guided at first by a work such as Darby's "Astronomical Observer," or Smyth's "Celestial Cycle," a telescope would soon cease to be a mere toy in his hands, and would afford him many a pleasant hour of unfailing and instructive employment in one of the most soul-stirring of the natural sciences. But, supposing for a moment that "P. S. T." found his interest in such studies to be on the decrease, he might endeavour, with a good 9fin. or 10 in. reflector, such as Mr. Browning supplies, to aid the Moon Committee in their Selenographical labours. Our little satellite, though so much examined, is still a wide field for research, and can never fail to yield a store of detail to a careful observer, provided with the requisite means. Reflecting telescopes (equatorially mounted) of the sizes mentioned would cost "P. S. T." from £100 to £150, a price about one-fourth of the value of an equatorially-mounted 6in. refractor. As such reflectors perform best in an observatory, constructed on the plan of that described by the Rev. E. L. Berthon in No. 842, Vol. XIV., of the ENGLISH MECHANIC, "P. S. T." would have at his disposal great power and efficiency, combined with small expense, by purchasing one of them. But I have no doubt Mr. Proctor will give a fitting reply to his inquiries, and he could not have applied for an answer to a more competent guide. April 18. W. BROWN. REVOLVING FURNACES-DRILLING MACHINE. [4039.]-SEVERAL years ago I read in a newspaper a report that the Manager of the Dowlais Iron Works had constructed a revolving furnace, which was giving great satisfaction, and specimens of the puddled iron had been exhibited at a meeting of the trade in the Midland Counties. It was likewise stated to be the intention of the Company to erect more furnaces on the same plan. I hope it is not asking too much, if those capable of doing so would let us humble readers know, if such had been the case; or, if not, what was the reason? As it is reported here, Danks's patent has been declared invalid, and the agreement with the English iron masters broken through. A full state ment of this, if true, might be a very proper subject so very desirable, why not do as I said before-viz., for the pages of "ours." A universal angular drilling couple two engines to one train? This would have machine is figured on p. 57 of this volume. How is one decided advantage over the Fairlie, inasmuch as if the ratchet motion used on such machine, as it is not an axle or tire of one of the two engines broke, or any shown in the engraving? A tool for the same purposes part which totally disables an engine, you could at was figured in the Penny Mechanic, August 17, 1868, least use the other engine, but with the Fairlie if one as invented by Messrs. Westroy and Forster, Barrow-end was thus disabled the whole would be useless. in-Furness, Lancashire, which seems to me to have a stouter and better sliding bracket. Those who may have seen both tools would oblige by giving their opinion of both machines for actual hard service. LINUM. COMPRESSIBILITY OF THE ATMOSPHERE. [4040.]-OWING to this property, it follows that at lower elevations, two or more, or many given volumes must be compacted together in a space, which at higher elevations one such volume suffices to occupy. Hence, as the density of air is to that of water, so, though in a far less degree, is the density of the air at any level to that of the air immediately beneath it. It follows, therefore, that the bullet fired horizontally from a gun must be deflected upwards, though in a far less degree in its course through the air, even as it is by striking the surface of the denser fluid, water. I wish to ask your learned correspondents whether they have ever considered that the well-known phenomenon-viz., the rise of the bullet from a gun above the line of aim, is thus accounted for. I beg also to express my conviction that the spinning top is supported, placed or even replaced, in its vertical position from the same cause. If I am right, a top should not be able to spin in a vacuum. I have no means of trying this experiment, but feel convinced that it will not. J. M. TAYLOR. Seer Green Vicarage. THE FAIRLIE LOCOMOTIVE. [4041.]-THE letter Mr. Fairlie has sent (page 41), signed by G. Allan, though giving many particulars, is very far from complete. cylinders of the three engines: and, unless I am mis1st. It makes no mention of the diameters of the taken, the Fairlie had slightly the largest. 2nd. No mention is made of the stroke of either the "Wonder," "Pony," or "Giant." tioned, and though the total weight of the Fairlie is 8rd. The pressure on the driving wheels is not menhalf a ton less, yet if in the "Pony" or "Giant" there Fairlie, the tractive power would, of course, be less. was less pressure on the driving wheels than in the of the engines after they first started. 4th. It does not state whether sand was used by any in fuel, but no mention is made of what average 5th. It is stated that the Fairlie engine is economical amount per mile, and as engines made from the same design and exactly alike, as far as can be ascertained in every respect, are some more economical than others, one cannot be surprised at some difference existing in this case. the different points given on the journey, we shall 6th. As regards steam pressure, if we compare it at find it as follows: Welsh Pony. 150 Little Wonder. 160 165 167 At Weigh-house Not stated Thus, in every instance given, the pressure in the Fairlie was in excess of the others. Here again is another omission-viz., the pressure in the Fairlie at the point where the other engines were pulled up. Whilst writing on the subject of the Fairlie system, might I refer to a statement I saw in print a few months ago, with what truth I cannot say, to the effect that Mr. Fairlie had invented a new kind of buffer to facilitate the passage of trains round sharp curves? It consisted of a single central buffer instead of the ordinary double one. Now, in the "Engineers' and Mechanics' Encyclopedia," published in 1836, at page 571 (Vol. II), is an engraving of an apparently similar construction that was used on the Dublin and Kingstown Railways. I am sorry I was unable to answer Mr. Fairlie's letter sooner, but was unavoidably prevented. Cheshunt, Herts, April 17. A. G. BOYD. [4042.]-GRANT space but for a few words more, because, as I take it, the object of the Fairlie locomotive, or, rather, that which should lead the public to have an interest in it, is similar to that of the ENGLISH MECHANIC-namely, the opening up of new fields of operation and research. And the latter brings the same principles to bear to wit, flexibility of (wheel) base, easily adapting itself to the various curves and inclines, often very great, of the different branches (of knowledge); steadiness; smallness of gange (of type); and length (26 pages solid). Amos Appleyard says (page 96), that the companies could easily build He does far heavier engines than they at present use. not say how this is to be done, seeing that the limit of adhesion is already quite reached, and every inch of width is taken up. So much so, that in some recent goods engines on the Brighton line, the slide valves are put underneath the cylinders to gain room, also to admit of longer bearings on the crank axle. Moreover, it is not found advantageons to couple more than six wheels together. However, our present trains are quite heavy enough, and it is not likely that Fairlie engines would be generally required for the 4ft. 8in. or wide gauge, the Mexican railway being an extreme case, as abounding principle supplies the possibility of building an engine with steep inclines and sharp curves. But the Fairlie for the 3ft. gauge of the same power as our present heaviest goods engines; and it is possible to make a line on the smaller gauge which will pay where the expense of the wider would be simply rainous. So that whole districts in this and other countries hitherto tion may be opened up and become valuable feeders debarred from the advantages of railway communicathe working of the little Festiniog line, on which to the existing trunks. Indeed, so satisfactory has been 35 miles per hour have been run, although its gange, only 1ft. 1lin., is admittedly less than it would have been had it been originally intended for steam, that in favour of a narrower gauge with Fairlie engines. the Indian and Russian Commissions both reported And a bill of the London and North-Western is now before Parliament to extend the Welsh line on the same gauge to Bettws-y-Coed. The difficulty with regard to the flexibility of the steam and exhaust pipes has been also so completely overcome that they are now said to give very little trouble. G. R. 7th. The diameter of the driving wheels is not mentioned, either of the Fairlie or the other engines. 8th. A point is made of the diminution of oscilla-chievous fallacy," and that any plan which permits tion, consequent on the use of bogies; but were not these used on the ordinary engines some years before the first Fairlie was built? and as it is not a thing belonging exclusively to this particular class of double engine, we are not, therefore, obliged to adopt the Fairlie system because we want to diminish oscillation. I must say I agree with what our correspondent "Osa" (let. 3864, p. 41) says in his last paragraph. Now, I suppose, Mr. Fairlie would say he intends his engines for drawing extra heavy loads on the ordinary gradients as well as on very steep banks. Now, as there are engines on one of our main lines, whose load is 40 full or 60 empty trucks, surely Mr. Fairlie would not advocate trains of 80 loaded or 120 empty trucks, making a train of about 650 yards long, causing a fearful strain on the couplings, and rather unmanageable if it was necessary to pull up quickly. Bat if the Fairlie is designed for overcoming steep gradients, why run so much unnecessary power when not required? If it was any advantage to have so much extra power, coupled to one train? what is to hinder two ordinary locomotives being Our correspondent "G. R." (let. 3987, p. 96). though perhaps right as respects the 50 Fairlie, with 5 laid up being about the same as 100 ordinary engines with 10 laid up, yet seems to forget that engine boilers, &c., are not so easily interchangeable as the parts of a military rifle, and that an engine must needs be "laid up" in order to have its boiler or whatever is necessary changed. Then he says that another advantage is the regularity of water-level on inclines; but he forgets that the Fairlie has not an upright boiler, but a horizontal one. Again he says "the contingency of one of the motions breaking down when three cylinders will remain effective," is an advantage overlooked. But, if this is WARMING AND VENTILATING. [4043.]-WILL "E. L. G."-who considers that any one who proposes to ventilate a room in any other way than by withdrawing, or allowing the escape of, its air from its very top, supports a "misleading and misany of the air which has once been breathed to mingle with that which will be breathed, "a murderous arrangement"-kindly explain to us how it is that though all warm-blooded animals do, whenever they breathe, draw back into their lungs a large proportion of the air which has just left their lungs, they are not injured, but benefited thereby? Such being the universal arrangement for respiration, it is, of course, the right arrangement, even though it be in direct opposition to "E. L. G.'s" dictum, and that it is the universal arrangement cannot be disputed by any one who remembers that a large proportion of the air expelled from the lungs on expiration necessarily remains in the windpipe and other air channels, to mingle with and help to warm the air drawn into the langs at the next inspiration. I do not know the proportion of the air inspired which does not enter the lungs; but it is, probably, not much less than half, and this accounts for the proportion of carbonic acid in expired air being much greater in that forced from the lungs by a very deep expiration than that of ordinary breathing, the air so forced being less mixed with air which never entered the lungs at all. It is clear, therefore, that "E. L. G." is quite mistaken in attributing dire effects to the mere mixing with air of a small part which has been once breathed, for that we are doing constantly, yet it is quite essential to health and comfort that air that has been breathed should be quickly removed, and I allow also that it is desirable to let it escape from the top of the room; but deny that it is essential to very frequently not done in rooms wh ventilated, as any one may prove by a fire is burning air enters the ro however high, unless, indeed, it chimney. To make openings t for every cubic yard of inclosed space to let in and out just as much air as is breathed, neither more or less, or anything like it, I will believe to be possible when I see it done. REPLIES TO QUERIES. Has "E. L. G." any evidence to prove that the air In their answers, Correspondents are respectat the very top of a room is materially different ex-fully requested to mention, in each instance, the title cept in temperature from that which has just descended and number of the query asked. from the top from being cooled by contact with the cold glass of the window, when it is not cold enough to condense the moisture by the removal of which the air would be somewhat purified? If not, how will he prove that it is essential to remove the air from the very top of the room? M. R. C. S. DR. CARPENTER AND PERSPECTIVE. [4044.]-How many more times will M. Paris need to be told that perspective is totally unconcerned with what the eye may notice or not notice (p. 119); being matter of pure mathematics, and dependent on the plane of projection chosen? On a vertical plane, all really vertical lines must be projected vertical, and no tower not tapering can have its picture tapering at all. Whether its height be 100ft or 100 miles, a hair's-breadth of convergency is wrong, nor will any levelled camera make its photograph converge one hairs'-breadth. Tilt the camera upward, and, of course, the lines will converge-" painfully" if viewed as a vertical picture, but be equally right, rightly viewed. M. Paris shows that he has the very first notions of perspective yet to begin. DUODECIMAL SYSTEM. E. L. G. [4045.]-THOUGH the last paradox of the anthor of letter 3965 (p. 118) would be far better answered by the "F.R.A.S." than I can pretend to do; yet, as our learned and indefatigable astronomical instructor has, by going out of his way and rôle (let. 8960) incurred, as I contend, the paramount duty of going to the bottom of his geological statements, it becomes mine, however imperfectly, to help in the defence against "the multipliers of £19, &c., by £19." Feet can no more be multiplied by feet or inches, than pounds can be multiplied by pounds or shillings. In finding an area of a rectangle you are told to multiply the units in length by the units in breadth, merely because we have chosen as the unit of area the square whose side is a unit of length (and this not always, for an acre and a rood are only units of area, not related to any particular linear unit). That an oblong of 8 inches by 5 contains 15 square inches is only a short way of saying its area is to that of one an inch long and an inch broad as 15 to 1. There is no natural reason for fixing on the square of a linear unit as the super ficial unit, rather than the equilateral triangle on the same base, or the circle (which is, perhaps, the most natural). If we used circular inches, the length and breadth of an oblong multiplied would not give the units in its area. But the inches in the length and breadth of an ellipse would give its area exactly, by mere multiplication. So, if we used trigonal inches, a trigon whose base was 4 would have its area 16 exactly; but a square or oblong would not have their areas expressed by the product of length and breadth. Thus lineal and areal measures are as distinct in kind as shillings and degrees of heat, and the connection between the units we choose of each is purely conventional. HINTS TO CORRESPONDENTS. 1. Write on one side of the paper only, and put draw. ings for illustration on separate pieces of paper. 2. Put titles to queries, and when answering queries put the numbers as well as the titles of the queries to which the replies refer. 3. No charge is made for inserting letters, queries, or replies. 4. Commercial letters, or queries, or replies, are not inserted. 5. No question asking for educational or scientific information is answered through the post. 6. Letters sent to correspondents, under cover to the Editor, are not forwarded; and the names of correspondents are not given to inquirers. was the co-efficient of restitution less with increased facts. Again, as to the statement that this co-efficient diminishes with the momentum, I have several authorities; my chief, however, is M. Athanase Dupré, who, two years ago, published the results of his experiments, being the most elaborate and carefully executed series ever undertaken with the view of determining the laws of collision of elastic bodies; he experimented with ivory balls of seven different sizes, from 1țin. to 24in. diameter; falling on a marble slab from all intermediate heights, from eight-tenths of an inch to 8in. also, with these same balls, suspended by threads over 10ft. long, and allowed to strike the same marble slab, and each other, horizontally in every possible way, from distances varying from lin. to 40in., and, of course, velocities in the same ratios. The number of carefully recorded experiments is over 200, and in every instance momentum. That this should be the case, I think it would not be difficult to give a good and sufficient reason, if my remarks had not already extended too long; but, nevertheless, I should be most happy to have "Billiardist's" proof to the contrary. I stated in [9440.]-Mathematical Question (U.Q.).- my first communication that there was a very great The diameter of a circle being one, to inscribe in it a variety of matters to be considered in calculating the triangle, the rectangle of whose two sides shall be equal angle of reflection of a billiard ball, most of which tend to the square of half the base of the triangle. Also to to make the angle of reflection greater than that of inshow what are the values of the three angles of the cidence; but there is one cause of a contrary effecttriangle, and of the sector in which the said triangle is viz., the soft nature of the cushions, which by yielding found. Let angle of sector = 2 m; and AC Q be the cause the ball to rebound more towards the perdiameter. Let A CQ unity; C, the centre; A B, pendicular, and, no doubt, is quite sufficient to prothe base, bisected in D by radius E C, equal to ; then duce the results which "Billiardist" describes. My AD into D B remarks and all the experiments with which I am x sin.2 m = altitude of acquainted, have been with hard elastic bodies, such as triangle x diameter of glass, steel, ivory, marble, &c. I should like much to circle (Euclid VI, C.). have "Billiardist's" proof that the harder the blow Draw DF perpendicular with which one billiard ball strikes another, the greater to AC in the point F; is the ratio of recoil to approach, as it is contrary to then AF AD x sin. all experiments with which I am acquainted.-F. Ñ. m; but ADA C x sin. m = sin. m, .. A Fx sin.2 m, therefore, bisect A Fin H, we shall have AH = S 2 if we x sin.2 m, as in the first deduction. Make 2 DIAH; draw KI parallel to A D, cutting Feet, therefore, multiplied by feet, do not make Hind's figures have plain meanings as units of area. The first denomination are square feet; the next, twelfths of square feet; the third, square inches. But the working by multiplication is never used but by schoolboys. The rule of "Practice" is the only rational and practical treatment of such "squaring and cubing." E. L. G. Fastening Loose Window Sashes.-The most convenient way to prevent loose window sashes from rattling unpleasantly when the wind blows is to make four one-sided buttons of wood, and screw them to the stops which are nailed to the face casings of the window, making each button of proper length to press the side of the sash outwards when the end of the button is turned down horizontally. The buttons operate like a cam. By having them of the correct length to crowd the stiles of the sash outwards against the outer stop of the window frame, the sash will not only be held so firmly that it cannot rattle, but the crack which admitted dust and a current of cold air will be closed so tightly that no window strips will be required..-Industrial Monthly. angle of incidence is the acute angle made by the A. P. S. but [10664.]-Angle of Reflection and Incidence. -"Billiardist, on p. 100, thinks I am in error in stating that the co-efficient of elasticity is the force of restitution divided by the force of compression, and that the co-efficient of elasticity diminishes with increase of momentum. If I err it is in good company, as I made the statements, not on my own authority, or from my own observations, but on what, I think, will be admitted to be the very highest authority, although in strictness I ought to have employed the word velocity instead of force. Bat, in the first place, allow me to state that "Billiardist" is quite correct as to the generally accepted meaning of the words angle of incidence and reflection. Sometimes, however, the angles made with the plane struck are employed instead of those made with the perpendicular, and in my reply to the query I unfortunately did so by mistake. Now, as to the meaning of the phrase co-efficient of elasticity, most writers on Mechanica use it in the sense in which I employed it-viz., the ratio of velocity of recoil to velocity of approach. I thought of using the phrase coefficient of restitution as being more correct, but did not like introducing any new term. I have since observed, however, that Sir Wm. Thomson has proposed that very expression as being more in accordance with [10731.]-Fastening Escape Wheel in Lever Watch.-I am surprised to find "West Cornwall" finding fault with my reply on page 100. If the job had been sent to me I should have done it without soft solder. I was influenced by two considerations in my reply-(1) the wording of "S. H. L.'s" query led me to believe that he had little practical knowledge, and not the skill to make the nicety of fit to get the wheel right height for the pallet stones, therefore I gave him a simple method by which he could make a tolerably good job of it; (2) I sent the information in the belief that it would suit his particular case. I spoke of the soldering iron because I had in my mind some men who when they have had a verge contrate wheel or lever escape come loose on the staff, have used the blowpipe and softened both staff and pinion. Not a particle of soft solder in the original composition of the watch. "W. C." surely knows that the fusee and stop are soldered on to the arbor; that all verge colets (both English and French manufacture) are soft soldered on, and I have often met with balance staff colets done the same. For twenty years I have been seeking the best methods of doing all kinds of jobbing work, and I am a learner yet. If soft solder forms the great mainstay of all my operations, I can yet give watch querists a few practical wrinkles.—A YORKSHIRE PIVOT. [11024.]-Stuffing Birds.-"Frank M.," or any other of our readers, would, indeed, want to use their judgment if they depended on A. J. Shaw's information. I will try and help "Frank M." from my practical experience. Let the bird cool so that the blood will coagulate, then stuff a piece of cotton in the mouth to soak up any moisture; dislocate the wings by turning them over the back towards the head; by doing so the bird will be easier to skin. Make an incision from the centre of breastbone to vent, just large enough for the body to pass through; having come to one of the legs, cat the second joint across with a pair of scissors, skin to the bend of the knee, and strip the flesh off the bone; its place, treat the other leg the same; next cat the then take hold of the foot, and draw the bone back in tail within a quarter of an inch of the stamps of the in the rump-bane, and suspend the bird; turn down feathers, and cut off all flesh and fat; then stick a hook the skin carefully and skin to the wing-joints, disjoint, and serve same as legs. When you come to the head skin a little below the eyes, but be very careful not to the body as a guide, lay the skin on a table, and out as cut the eyelids; then cut off the neck at the poll; keep small an aperture as possible at the base of the skull, and take ont the brains. Remove the eyes and roots of tongue, cut off any fat that may be on the skin, and apply any preservative you wish. I use ground alum four parts, saltpetre and pepper one part, and find it a first-class preservative. Previons to skinning take a piece of wire of suitable thickness and measure from the centre of bill to tip of toes, have the wire twice that length and double it in two, and point the double erd with a hammer; do not separate them; point the other ends with a file. Having put in the eyes and twisted some cotton on leg bones, and filled up the aperture in skull with a piece of cork, thrust the double end of the wire through the cork, and let it enter the base of the beak; then twist some cotton or tow round the wire same thickness and length as neck; then separate and form a shoulder on each wire, roll up some tow same size and shape as the bird's body, and twist some thread round it; then thrust the wires through it, one at each side; then carefully turn the skin over your artificial body, in doing so place the wing bones in their right place; then pass the wires through the back of the legs inside the skin, add a little tow if required, sew up the aperture, and fix on stand by the wires; then form a piece of wire same shape as a hairpin, and pass under and through tail into the body to keep tail up; tie the bill with a piece of thread till it sets; then give the bird the natural set, fix the wins in the right position, and pass a thread with a long needle through the body and last joints of wings and tie, not too tight, and tie tips of same at tail. Pay particular attention to the eyes, replace stray feathers with a needle, and brush down with a camel's hair brush.-FREDERICK R. ROHN. [11045.]-The Bug Bible (UQ) was printed by John Day, 1551, with a prologue by Tindall. It derives its name from its rendering of Psalm xei., 5, which reads," So that thou shalt not need be afraid of the bug by night." This edition is very scarce, and rarely finds its way into the book-market.-JOURNEY MAN PAINTER. (11054.]-Calculating Contents of Cylindrical Vessels.-" Excelsior" will see that his formula is incorrect. I gave the more usual and simple one. Had he said, multiply half the circumference by half the diameter to give the area, he would have been right, and that sum multiplied by the height (if vertical) will give the cubical contents.-TUBAL-KAIN. where an eye will have the sun exactly hidden by his globe, but by nothing a hair's-breadth narrower than his globe. Now as he knows that the moon sometimes makes annular eclipses of the sun, and yet sometimes total ones, he knows that her shadow may or may not be long enough to reach the earth, according to its to her own monthly variation of distance from us, which variation of length with time of year, but more according is greater than ours or hers from the sun. If he admits then that the earth is less than 4 times the moon's diameter, he must admit that "our" shadow, though farther, can never extend above 4 times as far. The always extending to the moon's orbit and even much penumbra of even the smallest thing (which is what he falsely calls "shadow") extends, if he will, to infinity. We are in Mercury's penumbra whenever we see him transit the sun, which, in fact, is a small annalar eclipse of the sun. Now, returning to his first subject, what can ho racan by 1130 feet per second being the "one hundred and seventieth part" the velocity of light; namely, some 180,000 miles per second? It is the 170th of the 5280th [11083.1-Treacle Beer (U.Q.).-I see this in the part! But if air and ether are each composed of atoms list of unanswered queries, and as it is a refreshing, (and one or other is, but we have no sure grounds for harmless drink, it should not remain unanswered. It saying both are) we have nothing here to inform as was, and I suppose still is, a popular drink in Forfar- which atoms are nearest. If, however, you take a 6in. shire, and was made in perfection somewhat as fol- cube of air, and let it expand into 8 such cubes, or a lows:-Heat four gallons of water to the boiling point, foot cube, Mr. Barwick will grant, probably, that we may and dissolve in it four pounds of treacle. Let it cool to be quite sure the atoms (if any) are now just twice as 170° Fahr., and then add a quarter of a pint of fresh as far apart. Well, this can obviously be done in gasyeast, or two pints of new ale taken from the ferment-pipes or sewers long enough to try experimentally at ing tan, and stir briskly till thoroughly mixed. Bottle, what rates sound may reach their end. This has been but don't cork, for from twelve to twenty-four hours, done under Paris by M. Regnault, with air varying in during which time the bottles should stand in a density as 5 to 1, the distance of atoms, therefore, as warmish, or rather, not in a cold place. The time to 5: 1, or about 19: 11, and the velocity of sound was cork is when the yeast rises fangas-like half a finger's just the same at a given temperature. At different degrees length from the mouths of the bottles, and the beer of heat, though with the very same number of atoms will be in excellent condition in two days thereafter. in the same pipe, the speed differs, as was well known. The bottles should be pretty strong, as the quantity of Long before this experiment (which was repeated in carbonic acid formed is considerable. I am not sure various pipes, Mem. de l'Acad: tom. 87, pp. 113, 171, that the proportions here given are the best: I write 551), climbers on the Alps, as Stampfer and Myrbach, from memory of thirty years ago. They will, however, 1822, Bravais and Martens, 1844, had measured the make a good beer, but any one who will take the speed of gun reports where the air is only about half trouble to experiment a little with varying proportions as dense as at sea level, and the same equality had may make it better. One thing, however, must be been found. Heat or cold alters the rate, though the kept in mind, the yeast or new ale must on no account atoms be at one unaltered distance; but distance of be added to the solution of treacle till the temperature atoms alters it not at all, at a given temperature. has fallen to the 170°, or at the highest 175-AULD Lastly, let Mr. Barwick observe that long before any of this had been tested by experiment, Sir Isaac Newton had predicted it all mathematically, from the mere mechanical properties of air, as settled in the wellknown "Law of Mariotte." Therefore, what J. Barwick has succeeded in "impressing on" me (whether "anxious to impress on E. L. G." or not) is that however little any of us know of what air and other matter is, we may know it is not a number of things kicking one another, like J. Barwick's bits of needle; and that Newton knew rather more of it, even two centuries ago, than Mr. Barwick in 1872.-E. L. G. REBKIE. [11120.]-A Question of Sight.-Tyndall says that, according to Wertheim, sound travels through iron at the rate of 16,822 feet a second, at a temperature of 20° C., quickening to 17,386 feet at 100° C. Mr. Barwick, p. 128, gives only 11,090 feet per second. I have very little doubt which is right, but Mr. B. will probably state his authority.-SAUL RYMEA. [11120.]-A Question of Sight.-J. Barwick must surely have taught our lively satirist "A. J. V. G." by this time the inutility of anything" wrote sarcastic" with the querists wherewith "ours" has to do. It is aseless to be "agape with speculative wonder," and expect the virtue of an "if," even in italics (as our satirist says he meant to "speak in italics," p. 674) to prevent J. Barwick regarding the sun "as a point," if it happened to land them in novelties they have the distinction of not finding "noticed by any one but myself." By the way, it was not I who asked "A. J. V. G." seriously "why he should regard the sun as a point." He will see (p. 20) I merely questioned his luminous point "illuminating half" his globe, which seems not to have been "wrote sarcastic." Being thus on the subject of Mr. Barwick's last questions, first, I may as well proceed to remind him he is making no distinction between the space whence all rays or the space whence [11120.]-A Question of Sight.-Mr. J. Bar. part of the rays of a luminary are intercepted. In wick states that sound travels through air at a velocity other words, he is regarding the sun "as a point" of 1,130 feet per second, and as many readers of "ours" something whereof you cannot hide any without hiding may not have seen the account of some late experiall. Shadow is not, as he tells us, "the absence ments on that subject made at the Cape of Good Hope, of intercepted rays." It is the absence of all I subjoin the result. Further particulars, and the details rays (or all direct rays) of the luminary that of the experiment, will be found in the Philosophical casts the shadow. The spaces whence a part of its Magazine, Vol. 48, No. 284, pp. 153-154, for Febrarays, but not all, are intercepted, are not called shadow ary, 1872, on "An Experimental Determination of the by any one, scientific or not. There is no call, in Velocity of Sound," by J. E. Stone, F.R.S., Astrocommon matters, to mention such space; and, there- nomer Royal at the Cape, on February 27, 1871." There fore, the unscientific have no name for it; but astrono were 38 observations, and in the reduction of the mers and opticians call it "penumbra." The peculiar equations, the co-efficient of elasticity of the air under distinctness of shadows cast by an electric or lime a constant volume (that is to say, the ratio of the inlight arises from their smallness, and the consequent crement of pressure for an increment 1° F. of tempesmall amount of penumbra round the shadow. These rature to the pressure at 32°F.), was regarded as an unlights may show J. Barwick the shadows of his single known quantity, as well as V, the velocity of sound at hairs, even at some feet distant from him; which the 32° F. The reduction of the equations furnished by sun never can at one foot, or even Sin., because his the observations gavediameter causes all shadows he casts to be bordered by penumbra, the darker portion of which, next the abadow, seems a shading off, destroying its sharpness; but really the edge of shadow is a distinct line that may always be traced with care (at least when received on a white surface), while the limit of penumbra cannot, and the outer portions of penumbra are quite invisible to any eye, even on snow. Hence the moon is not eclipsed by merely touching our penumbra, nor is any thing reckoned as a lunar eclipse but her touching the earth's shadow, the only shade whose limit we can see as a distinct line. On the other hand, we reckon as a solar eclipse every time the moon's penumbra touches our globe, for every point in her penumbra has part of the sun hidden. Her shadow need not touch the earth at all, and does not in eclipses that are nowhere total-and a majority of solar ones are of this kind-and in the minority, where the shadow does reach the earth, it can never exceed two or 300 miles [11166.]-Area of Segment of Circular Ring. wide; so that very few places, in a given century, have "J. K. P." has correctly but very needlessly figured, been in the moon's shadow, those only that have seen on p. 128, the most curved of the forms I remarked a total solar eclipse, while every place has, more than that "Thankful's" segment might have had. I should once, even in a child's life, been in her penumbra. call it as much a "segment" as his own figure, p. 650. Now Mr. Barwick has only (as I suggested to In the broadest sense, anything is a segment that is "A. J. V. G.," p. 20) to hang up a globe or bead in the cut off, whether by one straight cat or two. It is not sunshine, and measure how far its shadow (as distinct a sector unless the two cuts are directed to the centre, from penumbra) extends, which he will find between and there being no hint, either in the query or draw 100 and 112 times its diameter, and about three dia-ing, that this was intended, I could not see my way to meters farther in July than in January, to bave the any such conclusion. I therefore gave "Thankful" a proof he asks, that the earth's shadow does the same rule equally applicable whether he meant "sector" or (and also the moon's). The shadow ends at the point not.-E. L. G. V = 1090 6 feet per second. "L = 0·0019 that our transliteration is meant to convey the true [11208.]-Incubators.-My only wish was, if pos sible, to find an incubator that would hatch to a certainty, and I thought "Shylock's" plan would not, as expensive patented ones, and none of them will do it there is no heat over the eggs. I have got several to a certainty. Brindly's, of Derby, does it the nearest, and the heat there is conveyed in copper pipes over the top of the eggs. I have this night put eggs in an incubator of my own make, and if I succeed I will give you a drawing of it. I have eggs above and below the heat, and a very fine jet of steam conveyed across the top of the eggs, but not touching them. I have from 1,500 to 1,600 eggs per year, many of which I try in my to hear if "Hatcher" got any chickens out of the one incubators, and I must say spoil. I should be glad he made (Cantelo's).-M. O. [11223.]-Stereotyping.-I am under the impression that the mode of stereotyping with the plaster process has been given in back volumes. "H. W.R." must oil his type well, and remove the superfluity with a brush, then put an edging round it, and having the plaster mixed rather thinly pour it on. When dry remove and bake in an oven, standing the cast on edge. He will now want a casting-box, which consists of a tray with a lid capable of being screwed down, and what is known as the floating-plate-a plate of cast-iron with notched edges fitting the bottom of the tray. The plaster-cast having been oiled or plumbagoed is placed face downwards on the floating-plate at the bottom of casting-box; the lid, which has pieces cut off two corners to allow the molten metal to flow in, is then put in position and secured by the screw so as to give the requisite thickness of metal for the plate. The whole is then plunged into a bath of the molten metal, which flows in through the holes at the corners, lifting the floating-plate slightly as well as the cast, but by means of the notches in the edge of the floating-plate the fluid metal gradually spreads over and into the crevices and lines in the plaster cast, and owing to the pressure of the metal in the bath a solid cast is obtained. process is not an easy one for an amateur; the plaster casts require a high temperature to bake them, aud need careful handling afterwards. But such is an outline; if "H. W. R." wishes for details I will help him, or possibly some of “our” friends, who may have succeeded in getting good casts with more simple apparatus, will come to the rescue.—SAUL RYMEA. The [11226.]-Concertinas.-I do not know what is the peculiarity of a "trio" concertina, but a "bass" is merely an instrument supplied with reeds yielding bass tones instead of the usual treble ones; "piano" is one with soft-speaking notes, and a "chromatic" is one with semitones. They are, of course, similar in construction to the German concertina and are as easy to play. The only thoroughly chromatic concertina is the English.-E. M. [11233.] - Marine Engineer. - Presuming that "E. W. B." wishes to determine the proper velocity of the piston I copy the rule given by Templeton: Multiply the log. of the nth part of the stroke at which the steam is cut off by 2-3, and to the product add 7. Maltiply the sum by the distance in feet the piston has travelled when steam is cut off, and 120 times the square root of product will equal the proper velocity of piston in feet per minute. In condensing engines the approximate velocity of piston with stroke of 22in. would be about 150ft. per minute.-E. M. [11237.]-Four-Wheeled Vehicle.-Your carriage would be drawn easier up hill with large wheels than with small ones. The reasons for which you will see fully gone into in the number of the ENGLISH MECHANIC for January 12, 1872 (reply 10054).101, PARK-STREET. [11255.]-Farm Estate Agent's Duties.The duties of an estate agent (read the "General TextBook for Architects, Engineers, Surveyors, Land Agents, Solicitors, and Others," by E. Ryde; and "For Landowners," by J. Donaldson; also the "Book of the Landed Estate," by Robert E. Brown) being thus so multitudinous, it is evident that the person undertaking them must be a man of high and general qualifications, such, indeed, as the most liberal, prac tical, and scientific education can alone confer. In the first place, it must be his business to make himself thoroughly acquainted with the property, and with a things and persons connected with it. He must superintend and conduct the whole of the work that is done on the estate, give all orders himself to the subordinate officers (each foreman in every department), and see that each performs his duties. He ought to have a thorough knowledge of the principles and the best practice of good farming, to enable him to encourage the system of culture best calculated for the soil, the climate, and the peculiar circumstances of the estate. The best practical book on farming I have met with is "The Book of the Farm," by Henry Stephens. Also read Morton's "Cyclopædia of Agriculture," and The Journal of the Royal Agricultural Society of England." For a general knowledge of agricultural chemistry and geology, read the books written by James F. W. Johnston. Of arboriculture, or the raising of forest trees, planting and forming new plantations, the general management of old woods and plantations, and of different descriptions of fences, read the book written by John Grigor, The Nurseries, Forres, N.B.; "The Forrester," by James Brown; "The Transactions of the Highland and Agricultural Society of Scotland;" and "Transactions of the Scottish Arboricultural Society of Scotland." Of surveying for making plans of roads, new plantations, drainage, and computing the contents of pieces of land in exchange between estates, farms, &c., for general improvements; on architecture, for the making of plans for farm buildings, cottages, &c., making specifications and estimates of all buildings and all other works on the estate, see the "Book of Farm Buildings: their Arrangement and Construction," by Henry Stephens and Robert Scott Burns. Also, in the above-named books will be found a lot of useful information on this matter, and the valuation of landed property, of the soil, houses, woods, minerals, manorial rights, royalties, fee farm rents, tenant-rights, &c. He has, moreover, to let the farms, cottages, &c., to the tenants under such conditions as shall induce them to cultivate the land to increase its productivenessthis should be his constant aim. Collecting rents: This is a very simple matter to do when the money is forthcoming-see the above-named books. A little knowledge of law between landlord and tenant is quite necessary. Read "Every Man His Own Lawyer," Dixon's "Law of the Farm," Rouse's "Practical Man," and Archbold's "Landlord and Tenant."THIRTEEN YEARS' AGENT. D" [11294.]-Dividing Metal Disc.-So simple a problem does not require an elaborate answer. Take D= the exterior diameter of disc in inches, and n = the number of equal area smaller dises to be cut out of the same. It would be, a = area of the several discs in circular inches; then for the smallest disc a= d, the diameter in inches, the next size √2 x a=d, then VS x a = d and so on.-TUBALΚΑΙΝ. [11299.]-Tarpaulings for Railway Carriages in the Tropics.-I have seen a material invented by Spill and Co., Bow Commen, which is as pliable as a glove, and always remains so, not being affected by either extreme of temperature. It is the result of twenty years' experiments, and the amicable co-operation of two substances hitherto as opposed as fire and water. It is in use on several English, American, and Indian railways.—AMATEUR. [11818.)-Setting Lathe.-To F. HUME (additional). I don't think the screws between the beds are intended to be moved; but if they are, could you not make a mark to indicate exactly how far you have to set them back after altering? My contrivance for tapering is merely a hardened cone-point screwed into a hole in the collar-plate, or into a separate plate of the same description, if there is not a convenient place in the collar-plate itself, of course taking care to keep the centre at the right height, and set the head up with a hammer. Screws cnt taper with this plan would not be drunk if, as well as pulling the back centre ever towards you, you also alter the mandril-head so as to be truly in line with the back centre itself. But taper screws (true ones) are rarely, if ever, wanted. I think if I had to use your lathe I should take out one setscrew or both and fit in a parallel piece of iron (or brass) on each side to regulate the position of the mandril-head, and the said pieces of metal need only be (say) lin. square or less, and if they had a rivet or stud sticking out fin. on one side, which you could insert into the old screw-hole just when you are putting the head-stock back in its place, they wonld not drop out. Don't pinch your fingers in doing it. My mandril-head is held by a thin wedge on one side.-J. K. P. [11889.]-Astronomical.—It is asked "why does the polar star always occupy the same place when the earth's axis, by reason of its orbit, changes position?" The polar star may occupy the same place as compared with other stars, bat it does not occupy the same place as compared with the Pole, or rather the Pole does not occupy the same place as compared with the star. In the year 1790 the Pole and the star were about 1° 49′ apart; in 1839 the distance between them was about 1° 33', and now it has decreased to about 1° 22' only. Mr. Porter may consult with advantage npon this subject Sir J. Herschel's "Outlines of Astronomy" (a new edition of which, I see by the ENGLISH MECHANIC, has just been brought out), and the astronomical vol. of Orr's “Circle of the Sciences," and, doubtless, other works on astronomy, but I mention these because I happen to know of them. The star does now appear to describe a small circle in the heavens once in twenty-four hours, but this arises from the rotation of the earth on her axis, not on account of her revolution in her orbit.-C. W. M. [11354.]-Fire Balloons.-If your correspondent, William McDonald, refers in his question to fire balloons, the proper material for a fire balloon of 20ft. in circumference will be 12lb. double crown paper, that is paper of double crown size, each sheet measuring 30in. by 20in., and weighing 121b. per ream. To inflate it, a cotton pad soaked in spirits of wine is fastened to the junction of two cross wires arched upwards in the mouth of the balloon, and the balloon itself must be supported by a loop at the top fastened to a head line, so as to hang 5ft. clear of the ground, and fastened so that by pulling a string the attachment can be cast loose. The balloon should be held out above the mouth by two persons, the spirits of wine lighted, and when the balloon is thoroughly inflated the string is pulled and the balloon let go. It is gene: rally the fault that they are let go much too soon, and before they have power to rise rapidly. A balloon of the size mentioned will require some small weight as a car to be attached to it to keep it steady, and the month should be about 12in. or 14 in. in diameter. J. F. E. erect [11887.]-Sundials.—" Philanthropist" must excuse my meationing it, but it appears to me that his reply to this query may mislead some persons. That edge of the gnomon which casts the shadow which indicates the time on a sundial must be parallel to the earth's axis, and therefore, although in an south dial it should form an angle with the plane or face of the dial, equal to the co-latitude of the place, yet in a horizontal dial it should form an angle with the plane of the dial equal to the latitude of the place. For a place having 45° of latitude, the angle would be the same for either of the above-named dials. A dial may be drawn upon a plane in any position, but the above rule for fixing the gnomon must be adhered to.— C. W. M. [11398.]-Metallic Harmonicon.-What does "Zoo Andra" mean (in his reply to query 11893, P. 108) by "similar to an inverted aquarium," and then farther on, saying that this "inverted aquarium is to be partly filled with a solution of alam"? Does he mean an "inverted fern case," or what ?-TUBA. [11398.)-Stinging of Bees, Hornets, and Wasps.-Take a pinch of tobacco, damp the palm of your hand, work and rub the tobacco until you get the juice well out, then rub the spot where stang wel! with it for five or ten minutes. I have found this a perfect cure for bee stings, and used it with success only last Sunday. Let the stingfbe withdrawn as soon as possible.-S. W. [11409.]-Canine.-The dosing with Dr. Rooke's Oriental Pills, recommended on p. 103, should be " for about three weeks," not "eight weeks" as printed. AMATEUR. [11415.]-Scarlet Runners are scarlet runners!i.e., there is only one scarlet runner, a variety of tallgrowing or running bean. There are white runners, Painted Lady, and black-padded or Negro runners; there is also a so-called giant variety of the scarletnot a bit better than the common scarlet as grown by hundreds of cottagers, but more than double the price. Sow as soon as you like, now; dig deep, at least 2ft.; put in rotten dung if you have it; make a tolerably firm trench or drill, lay the seeds 6in. apart at the bottom, and cover them about 2in. When up about 3in. draw the earth round them up to the seed leaves. If the soil is well dug, they will not require water in ordinary weather-i.c., watering at the root-but they do require almost daily syringing. You may, however, give them all the soap-suds you can get either overhead or at the root. Stop them at 5ft. high, or some little distance before they reach the top of your supports. Try a row as dwarfs by stopping them at a foot, and pinching the growing shoots afterwards. If you have no manure ready give liberally of guano or artificial, and they will pay you handsomely. Your blooms fell off through weakness or sourness of soil, shallow digging (or rather scratching), or want of the syringe.-SAUL RYMEA. [11428.)-Surgery.-Having suffered from a thing very similar-viz., a carled shaving passed with the graver, when engraving a steel plate, into the fore part of the middle joint of the forefinger; and in cutting some German steel plate up that was hard, a piece either from the chisel or plate struck into the middle joint of the little finger, and in carelessly using a pump drill it slipped and entered the ball of my thumb and broke about three-sixteenths of an inch off and left in behind, all of which were sore troubles for four or five years. The former was the cause of the other mischief. The severe pain in handling things made me clumsy. It is only dangerous in boring it about trying to get it out. The two former do not trouble me, and although entering on the inside have worked along to the outside just under the first joint. They never festered or gathered, but a hard core, like a wart, kept growing out of the place where they entered for a long time, which, when touched, was exceedingly painful, which led me to cut and bore it about to no purpose. You might as well look for the article in a load of hay. If it appears inclined to gather, take a strip of lint or a small portion of cotton wool, and place upon it wet, and keep it constantly moist. Place over that a piece of oiled silk and a finger stall. This is the best form of poultice, and will greatly assist the opening and the ejection of the enemy. Do not apply every nostrum, drawing salve, or plaster in succession that is recommended, for thereby many lose their limbs and frequently their lives.-JACK OF ALL TRADES. [11486.]-Harmonium-I am "musician and mechanic enough to know" that the channels of a harmonium pan (there are no channels in the reservoir) should be graduated the whole way through; but "Practical Horologist" will excuse me if I tell him he is evidently not a practical harmonium maker. What I wish to know is whether in practice an inequality of (say) 1-64th of an inch in cutting the channel frame for divisions makes any material difference in the tone of the note the unequal-sized channel is intended to give ?-K. T. L. pool," just glances at Dr. Brewer's "Guide to Science," [11442.]-Old Wives' Science.-If "A., Liverquestion, why does the sun shining on a fire make it dull he will at once perceive the following answer to the and often put it ont, is, because the air (being rarefied by the sunshine) flows more slowly to the fire, and secondly, the chemical action of the sun's rays is detrimental to combustion. The sun's rays are composed of three parts, lighting, heating, and actinic or chemical rays. These latter interfere with the process of combustion.-WEE PET. [11442.]-Old Wives' Science.-With respect to reply on page 105, I beg to say, not only for "A.'s" information, but also for that of S. Bottone's, that there is very good foundation for the statement that the sun shining on a fire puts it out, and the following are the only true and proper reasons:-1. Because the air (being rarefied by the sunshine) flows more slowly to the fire. 2. The chemical action of the sun's rays composed of three parts: lighting, heating, and actinic is detrimental to combustion. The sun's rays are or chemical rays. The two latter interfere with the process of combustion. 3. The air flows more slowly to the fire for being rarefied, because the greater the contrast between the air in the room and that which has been heated by the fire, the more rapid will be the current of air towards the fire. Because rarefied air contains less oxygen than the same buik of condensed air (or air that has not been rarefied by the heating influence of the sun's rays, in other words shaded, and therefore condensed). 5. In frosty weather the air being more condensed by the cold than at other times, as indicated by the high readings of the barometer during a frost, supplies more oxygen than a similar volume of warmer air, and consequently falls more rapidly into the place of the hot ascending air, and as we all know, makes a fire burn very brightly.— SCIENCE. ceivers.-The effects would be the same as those of [11444.]-Bursting of Compressed Air Resteam, except the scaldings. Too large for any purpose. A series of egg-ended receivers, 2ft. diameter, 10ft. long, would be far better.-JACK OF ALL TRADES. [11450.]-Adjusting Equatorial.-"Inquirer's" latitude is 53° 50', and as Ursa Minoris is 7° 45° 42'4" from the Pole, it is plain that his circle should read: 61° 35' 43" + ref. 32" 61° 86' 15" north declination at the instant of transit, or above. - If the -THOMAS BUCHANAN. [11451.] Weight for Safety Valves. E. Naylor had the time and means to look up past vols., he would find sufficient information on question:-Obtain a weight 911b., which at 16in. The following is an answer to his specific from fulcrum will give 39.961b.; at 14in. from fulcrum, will give 34 661b.; at 12in. from fulcrum, will give 30 081b.; at 10in. from fulcrum, will give 24.84lb. This is worked out according to Templeton. The effective weight of lever is correctly obtained by multiplying its length in inches by its weight in pounds, then dividing by twice the distance in inches from fulcrum to point of resistance. Thus a 60in. lever, 15lb. weight, at 10in. from fulcram, pulls 4641b.; and at 20in. from fulcrum, pulls 2311b.-MUTUAL IMPROVE MENT. [11457.)-Motive Power for Amateurs.-The sketch shown is of no practical use, except to waste power by friction, and possibly to render the motion regular. Whatever the length of the pendulum (which is the way this would act) from the point of suspension to the centre of gravity of the suspended weight, so would be, in all ordinary cases, the number of vibrations per minute. Better apply your power, whatever it may be, in the simplest and most direct way.TUBAL-KAIN. [11459.]-Printers' Ink.-The recipe given by "H. B. E." (p. 130) is for common red ink, not printers' red ink.-A BARRISTER. [11460.]-The Beehive.-In my reply to Mr. Godden I said that glass hives were dangerous things in a house, but I did not mean that they were likely to go raving mad and bite any one. The particula danger to be guarded against is the breaking of them, by which the bees might be let into the room, perhaps among nervous ladies or children. On that ground only I consider them dangerous, and recommend that they be always kept under cover, for accidents will happen "Philo's" hive in the conservatory is a different affaije certainly; but if his bees ever get loose he will fivesome difficulty in getting them back to the hithe unless he be an expert. The reason the bees go inir the conservatory is not because they see the hive the to or their fellows, but because there is some attrac aroma, probably from the hive itself, which perven |