with loose earth. A 10-horse power engine, weighing The when passing up the steepest incline. The engine REPLIES FROM "AN ADEPT." In reply to "M. P. S.," No. 2082, page 613, I should Whilst thanking your correspondent, "C. R. O., Hants," on page 657, for his courtesy, may I venture to inquire if the idea has never struck him of placing the 12 bourdon pedal pipes in two rows, the five largest at the back and the remaining seven in front of them? This will answer well, and as he states that depth is not so great an object, I presume it will meet his requirements. In reply to "W. D.." No. 2154, on page 21 of the last number, he will find the description and figures of the bellows' valves he mentions in my communication on page 212, No. 242; the other inquiries he makes are not in my department, but no doubt some of the skilful writers who grace your columns with their contributions upon the harmonium will afford the requisite information. I presume your correspondent, "Organine," No. 2190, page 22, is a new subscriber, from the nature of his inquiry. I beg, in reply, to refer him to page 330, No. 223, where he will find I havo given, I trust, a full description of what he asks for. AN ADEPT. MANGANESE BATTERY. strong; but he alone advises "copper" wire; and ON THE METHOD OF LEAST SQUARES. SIR,-The object proposed in the solution of Equations, however formed, is the determination of the value of some unknown quantity involved in the equation, by certain operations with given numbers and accompanied with appropriate symbols which direct these operations. In the case of one unknown quantity, affected by co-efficients absolutely known, the determination of the unknown, in terms of the known quantities, is a problem that offers little difficulty, and provided that the number of equations is equal to the number of the unknown quantities, the discovery of the unknowns is a simple algebraical operation. But when the unknown quantities are affected by co-efficients only approximately known, the resulting value of the term sought will be affected by a certain error. To those who have not advanced into the higher branches of mathematics, it may seem probable that if a certain result absolutely satisfies an equation, then a quantity very nearly equal to that result will very nearly satisfy the equation, nearly being understood in the same manner in both cases. But this does not necessarily follow, and we are led to ask what is the test of approximate equality. Let us take the equation xao, then xa, or = 1. In this equation there is absolute equality. Now, we can give to the word nearly any signification we please. Let us suppose that an error of 0-0001 is a quantity so small that it can be neglected, and that the result of our solution of the equation has given us = 000009 and a 000001, xa then gives us 0.00008, which as it is less than 0-0001 can be neglected. Are we entitled to assume that the equation is approximately solved? 1. Inserting Certainly not. Try the other test, = - a a 0.00009 the values of x and ɑ, we have 0.00001 = 9, which is not by any means a small quantity. Now, the method of least squares assumes that the most probable values of the unknown quantities, In any system of equations, are those which make the sum of the squares of the residual errors a minimum. By the residual error, we mean the value of the equation after the approximate values of the unknown quantities have been substituted. This principle may serve as the basis of the solution of many otherwise indeterminate problems, and its adoption is justified by the calculus of probabilities, but which is of a too mathematical nature to be recapitulated here. We will proceed at once to explain the method employed in solving equations of condition, and to fix our ideas let us suppose that these equations arise from comparisons of observed and tabular places of a planet. If we suppose that the "elements of a planet's orbit are not exact but only approximations to the truth, and if we suppose that the formulæ for deducing a place of the planet from these elements is absolutely exact, every observation will differ more or less from the computed place. This ZINC PLANT, ALIAS LEAD TREE. error, which is called the error of the tables, must neces-(employing dashes to distinguish the particular results) e ax + by + cz, &c. Another observation will give € = a' x + b'y + c'z, &c. 89 19899 54 737 = 41 2211 which, being inverted, give the required weights, ** = a′′ x + b^ y + c′′ z, &c. e3 + e22 + 6a + &c. = (a2 + a3 + a + &c) x2 + 2 x y (až + d'¿' + &c.) + 2 x z (a c + a' c' + &c.) E = M + Nz + P And if we are to make it a minimum, the first differen- And ... Mx + N = O de + &c.) + &c. = 0. SIR,-As regards the letter of "J. T." page 14, that gives a minimum for any one of the unknown The solution by ordinary methods gives, If these values be substituted in the original equations, -0-249,- 0068, + 0·095, — 0:069. To determine the probable error of each unknown israther long and intricate computation: we indicate only the method here, the proof of what is inadmissible. In the first normal equation write-1 for the absolute term, and in the other equations zero, for each of the absolute terms: the value of x then found from these equations will be the reciprocal of the weight of the value of x found by the general elimination; similarly for the other unknowns. Writing then, In a few weeks I decermined to scrape off the var- cannot venture to apply any of them, neither would There can, however, be no doubt, amongst violin TAUTAU. ENGINEERING QUERY-ANSWER TO "RELWOT." 11 its diameter is 22in; the steam port is 15 in. long, and The steam is cut off from the high pressure cylinder at 34 per cent. of the stroke, or 144in., and the diameter of the cylinder being 16in, we have a space of 165 cubic feet to be filled twice every stroke, or 3-30 cubie feet per revolution of the crank shaft; this at 60 revolutions per minute is equal to 196 cubic feet, or 11880 per hour, to which we may add one-fourth for waste in filling ports, piston clearance, leakage and cooling, and we have 14850 cubic feet of steam required per hour; this at 60lb. per square inch above the atmosphere, or 751b. total pressure above a vacuum, requires for its formation 39 cubic feet of water, but the boiler is capable of evaporating 50 cubic feet per hour, se there can be no question of its sumciency. We will now take a look at the proportions of the slide valves, and see how they acquit themselves in the performance of their duty of distributing the steam to the cylinders, but before I do so I shall just state what are the proper proportions and essential conditions of a good compound steam engine as far as its valve gear is concerned. The valve which admits the steam from the boiler to the high pressure cylinder should be capable of cutting off at any point, from, say, 10 to 25 per cent, of the stroke, and of detaining the expanding steam in the cylinder to do duty on the piston to nearly the end of the stroke, and this valve should, if possible, be placed under the direct control of the governor. sure cylinders should be free and unrestricted from the beginning to very near the end of the stroke, in order that no illegitimate or unnecessary back pressure may exist to retard the motion of the high pressure piston, and that the low pressure piston may receive the full benefit of the unrestricted expansion of the steam contained between the two pistons. The communication between the low pressure cylinder and the condenser should also be freely open from the beginning to nearly the end of the stroke, in order that its piston may continue to receive the benefit of the vacuum as long as possible. The communication between the high and low pres valve and those of the ports are equidistant; with Now two serious evils due to this early exhaust at once become apparent, and are plainly illustrated in the annexed sketch, Fig. 1, in which I have placed the cylinders side by side, and attached their piston rods to the same cross-head, so that they make their strokes simultaneously. I do not know whether this is the arrangement of "Relivot's" engine or not, but that is of no importance, as it does not affect this discussion in the least, and moreover it is a common way of constructing compound engines. distinctness, are exaggerated to the inch scale. The The cylinders are drawn to a scale of half-inch to the foot, but the valves and ports, for sake of greater pistons in both cylinders are at the point when exhaustion takes place, viz., 78 per cent. of the stroke, and they are moving in the direction of the arrows, and both valves are at the middle of their excursion, just closing the exhaust on one side, and opening it SIP, So long ago as October last year, in the numon the other side of their respective pistons. The ber for the 22nd of that month, on page 140, Rel- steam from end A of the high pressure cylinder is. ivot" gave some of the dimensions of his compound passing away into the congesting pipe and valve engine and boiler, and solicited the opinions of chest of the low pressure cylinder, thus undergoing considerable expansion and reduction of pressure at practical engineers" upon the same. I replied, asking him to send more particulars, and promising the cylinder, and driving the piston at least 19 per the very time when it should be securely bottled up in to give him the information he required. He gave the required dimensions in the number for November cent, further on its way to the end of its stroke. At 19th, on page 242, and I at once set about writing this letter, but I laid it by for reasons I need not trouble you with, and did not resume it till now. I hope it is not too late to be useful to "Relivot." With your permission I will just recapitulate his questions here. First," Will the boiler supply those engines with steam? Second, "Are the areas of pistons, and ports, and travel of valves, properly proportioned?"; and after stating in his last letter, That those engines are working in connection with water wheel," and that "their load is varying from day to day," he asks "Would an independent cut off valve be advisable ?" The dimensions of his boiler are "Shell 30ft. long, 7ft. diameter, containing two furnace flues, 2' 7" diameter, with grates 7ft. long." This boiler is capable of evaporating about 50 cubic feet of water per hour, with good coal and sharp draft, and the steam from that quantity of water if used in a well-constructed condensing engine, cutting off at about one-third of the stroke, will produce about 100 to 120, or perhaps more, indicator horse power. The stroke of the high pressure cylinder is 42in., or 7ft. for the double stroke, and it makes 60 strokes per minute, or 420ft. per minute piston speed, and the diameter of the cylinder is 16in. The steam port is 11in. long, and 1 in. wide, or 15 square inches, which is very good proportion. The speed of the low pressure piston is the same as the high pressure one, and the same instant that the steam is thus recklessly permitted to escape from the positive side of the piston A, compression be gins on the negative side B for it will be seen from the position of the valve, that exhaustion from that side has just been intercepted; the piston, therefore, during the remainder of its stroke encounters a great resistance in gradually compressing the unexhausted vapour back into the, be sure, a part (and only a part) of this power is report and small space allowed for piston clearance. To stored to the piston on its return stroke by the re-expansion of this compressed vapour. But why compress it at all? The evil resulting from doing so can be avoided by using a properly-constructed valve motion. At the same time that the above bad work is going on in the high pressure cylinder, similar bad mauagement and prodigal waste are going on in the low pressure one, for the steam, which up to this instant has been driving the piston on its way, is suffered to escape. to the condenser from end C, instead of being kept on the piston at least 19 per cent. longer, and compression also takes place on the other side D, as in the high pressure cylinder, for the communication of this end of the cylinder with the condenser is prematurely cut off, and the vacuum applied to the wrong side of the piston. But I have yet to point out the greatest evil resulting from the bad proportions of these valves, which is o be found in the restriction of communication between the two cylinders. I have said at the beginning of this letter that one of the essential conditions of a good compound engine is" that the communication between the two cylinders should be free and unrestricted from the beginning to the end of the stroke," but how do we find it in this case? The valve of the low pressure cylinder cuts off at 36 per cent. of the stroke, that is to say, the communication between the two cylinders is improperly intercepted at that point, see Fig. 2, which shows the position of the several parts at that instant. The low pressure valve F has just closed, and no more steam can pass from the exhausting end B of the high pressure cylinder to what should be the receiving end C of the low pressure one. The high pressure piston has to spend its energies in cramming the unexhausted steam into the passage of communication and valve chest of the low pressure cylinder. The 36 per cent. of steam which has gained admission to the low pressure cylinder makes a faint effort to drive the piston by its expansion. It is but a faint effort compared with what it should be, for it is unsupported by the great body of steam which should have free ingress to its assistance, but from which it is cut off-isolated-by the premature closing of the valve. If that engine were mine I should make the following alterations in it : 7 16 would make the ports in both valve chests exactly alike in their transverse dimensions, that is to say, would cut in. off each of the admission edges of the ports in the high pressure one, and fin. off each of the exhaust edges of the ports in the low pressure one. I would then cut 2-in. off each of the admission edges of the low pressure valve, which would leave it in. of lap, and I would make its exhaust edges 7 in. apart, or the same as the exhaust edges of the ports, and I would then reduce the travel of the valves from 7in., as at present, to 5in., by putting on a new eccentric, which should drive both valves, and this eccentric should be set to give the valves in. of lead. I would do away with the present high pressure valve, and introduce a compound valve, as shown at Fig. 3, which consists of three pieces, viz., slide valves G and H, and an interpose plate I. The valve H should be worked direct from the eccentric by rod J. This valve is made with an exhaust cavity K, whose edges are equidistant with those of the ports, and two admission passages L L' straight through its solid ends; the admission edges of this valve have of lap, the same as the valve of the low pressure cylinder, and of course the same lead. The interposed plate I has two ports cut through it exactly opposite the steam ports in the cylinder face, and of the same dimensions; this plate dies loosely upon the back of valve H, and valve G lies loosely upon the back of I, and all three are pressed up together by the steam. I is prevented moving from its proper position by studs or stoppers fixed to the sides of the valve chest. Valve G is a plain solid plate, having in. of lap at each end beyond the ports in plate I; this valve is worked by a "link" M, one end of which derives its motion from a rod N, pinned to the main eccentric rod at O, and the other end is worked by a rod P, from a small eccentric Q, fixed on the shaft beside the main eccentric. Q has 24in. of "throw," and it is keyed on the shaft with its centre dine, making an angle of 158° with the crank (I am now speaking of a horizontal engine). The effects of this construction are-that when link M is lowered so as to bring eccentric rod N into line with valve rod R, the movements of valves H and G will be very nearly simultaneous, and steam will not be cut off till the piston has performed 95 per cent. of its stroke, and exhaustion will take place at 97 per cent. But wben the link is moved from this position, simultaneous action of the valves ceases, and the movements of valve G become in anticipation of those of valve H, and this anticipation increases gradually till eccentric rod P is placed in line with valve rod R, when steam will be cut off at 10 per cent. of the stroke, or 24in., and any amount of admission between these extremes is to be found between these two extreme positions of the link. It will be seen, therefore, that this simple apparatus possesses the inestimable advantages of a variable cut off between very wide limits, united to an invariable exhaust taking place at a late period of the stroke. The effect of these alterations on "Relivot's" engine would be that the communication between the two cylinders would be kept open during 97 per cent. of the stroke, instead of being prematurely cut off as it is now at 36 per cent., and the steam by its uninterrupted expansion between the two pistons would continue to do good duty upon the large piston for 61 per cent. longer, at the same time relieving the small piston from the enormous back pressure which at present paralyses its action and reduces its useful working effect. Also the communication between the low pressure cylinder and the condenser would be kept open during 97 per cent. of the stroke, instead of 78 per cent. only as at present, and its piston would, therefore, continue to receive the full benefit of the vacuum for 19 per cent. longer. If "Relivot" should wish to hear from me further, I shall be always at his service, with your permission, through the medium of the ENGLISH MECHANIC, and I shall not again keep him waiting four months for an answer. JAS. BASKERVILLE, 5, Westland-street, Limerick. WRAY'S OBJECT GLASSES. SIR,-In reading over "F.R.A.S.'s" letter, it gave me great satisfaction to hear him speak so well of Mr. Wray's object glasses, as I have a 4in which I got from him. On Jan. 24th, 1869, I had the pleasure of observing three occultations of the stars 115, 119, and 120, in Tauri. These stars were quite bright and clear until they disappeared behind the moon. In July I had some good views of Saturn, although he was low at the time. Venus has shown very sharp, so much so that some of my friends thought it was the moon that they were looking at. On Jan. 25th, 1870, and on March 5th, I had the pleasure of seeing six stars in the trapezium of Orion, and two small stars a little below in the nebulæ. instrument. Since I have read "F. R. A.S.'s letter," I must acknowledge that I am proud to be the owner of such an I think that I was the first who made mention of Mr. Wray's object glasses in the ENGLISH MECHANIC, some time ago; and as an act of gratitude to Mr. Wray I cannot refrain from writing, as his name is now before the readers of the ENGLISH MECHANIC. If not trespassing too much, I wish to say a few words of encouragement to my fellow working men. When I first made a start, 25. was all MECHANIC to help me on-no "Chats on the Stars." 1 that I could spare for an object glass, and no ENGLISH had not the privilege that the working men have now of getting every question answered through the columns of the ENGLISH MECHANIC; but perseverance has done more for me than money could ever have done; end now that I have overcome the difficulties that I have had to contend with, and am in possessionof a good telescope, I do not wish to put it under a bushel, but I am ever ready to bring it out to any one that can make it convenient to call on me. Working men, think of this; and let your motto be Nil desperandum, D. ALSTON, Clyde-street, Merefield, Rochdale. THE HORIZONTAL WINDMILL. SIR,-I take the liberty of sending yon for publication a plan for this purpose, on a scale of 1-12th, capable of doing real work, and suitable for driving two pair of usual 4ft. stones-or, say, three, as one may be frequently up for repair. The sails are enclosed in a windcap, because the open horizontal windmill cannot produce profitable work, on account of the wind only catching one sail at a time, and that for an instant only, besides the insuperable difficulty of getting the sail frames back against the wind; whereas in the common vertical windmill all the sails act at the same time, and moving edge on, atmospheric friction is very slight. In the horizontal enclosed form it will be seen that five sails are always acted upon by wind concentrated by the funnel-like openings. The latter are 24 x 15, wind there will always be a partial vacuum on the lee side, which acts as an exhaust, so that nearly the whole of the concentrated blast is available for work. The mill-house must be, of course, a substantial brick building, either square or round, according to fancy. The windcap above may be of thin board, but the joisting must be strong, in order to stand the strain of the upper gudgeon. The regulation is a very simple matter, because the twelve sails are on rollers, which can be reefed er let out by hand gear round the main shaft, or better, perhaps, by means of a self-acting governor. The pressure of a good working wind of 20 or 30 miles an hour is found to be from 3 to 41lbs per square foot, or at least at double that upon the sails. HENRY W. REVELEY, Reading. science. PHRENOLOGY AND BIG HEADS. SIR,-Having for many years taken great interest in the study of phrenology, I beg you will allow me to say a few words in reply to some of your correspondents, who have, through great ignorance, endeavoured to throw discredit and ridicule on the It is a great mistake to say that phrenologists assert that big heads have always the most power. All true phrenologists maintain simply that size is a measure of power, with this reserve-viz., all other things being equal, and they will invariably maintain that temperament or the quality of the brain is most essential to the effective working of the organs; therefore an averaged-sized head with well-proportioned organs and an active mental temperament will produce far greater and better results than a large head with ill-balanced organs and a slow lymphatic temperament. I make this statement from personal knowledge and many years' experience. I know a man who has a head 23in. in circumference-which, by the way, is a very large size-and although he has had many advantages, has produced little either good or great; and on the other hand, I know a man with a head only 213in., but the preponderance of brain being in the upper region, and with a most remarkably active mental temperament, who has taken a most prominent part in the political and literary world. I may observe, for the information of those little acquainted with the subject, that temperament is, in regard to the human brain, much the same as blood or breeding is to the horse. Self-made men have, I believe, invariably big heads or remarkably active temperament. S. H. A. THE IMPROVEMENT OF OLD COTTAGE AND ing a greater intensity of heat given by the ordinary SFR, Although many valuable improvements, especially in the action, &c., of old upright cabinet or cottage pianos may be carried out, to the great improvement of both touch and tone, it is obvious that no great augmentation of the power of the latter can be got without resorting to the use of either more or thicker strings, which usually necessitates employing some means to do what, in many instances, would greatly benefit their owners also, viz., to "strengthen their backs." In a former series of papers on this subject, written by me, one of which was printed in No. 239, Vol. IX., I mentioned a very effective method of doing this in new instruments, employed by Messrs. Rolfe about A.D. 1854, for which Mr Moly Beaux afterwards solieited a patent, which patent was not completed. In this plan, each wooden bracing is grooved to admit the central web of a T iron bar; of course this may be done to an old piany, but it is done much more easily before the bracings are fixed in their places; so, instead of doing it to an old piano, I would suggest the following method of forming grooves or recesses for the Torangle iron, which. instead of weakening the original bracing aids considerably to the total strength of the instrument. To form these recesses conveniently, I would insert additional wooden braces between the original ones, leaving from in. to fin. space between them, according to the thickness of the angle iron which is to be inserted. These additional wooden braces ought to be well fitted against the back of the wrest plank, so as to form a sound joint when glued, also under the wrest plank if shouldered on. Should the belly bars be in the way of the additional bracings, they may either be grooved to admit the belly bars, or, what I think preferable, two pieces of wood, of the required section, inay be glued and nailed on the front surface of each brace, leaving a groove or recess between them to admit the belly bar. As it would be difficult to fit each brace so that it shall bear firmly under the wrest plank and on the bottom, I would recommend to cut them about in. too short, also about in. shorter at back than front, and to form a perfect joist between the brace and the bottom, by driving in 2 hard wood wedge, having first secured a cleat on the bottom (in front of the brace), so that the latter cannot be forced forward against the belly by the act of driving in the wedge. The new bracings will of course be secured to the wrest plank at top. Lot only by glue, but also by a strong screw, or what is preferable, a screw bolt and nut, bearing against a large washer; the hole for this bolt should be bored 1-16th inch large, so that it may not act as a dowel, and prevent the wedge from forcing the shoulder of the brace up against the under edge of the wrest plank; this bolt also forms an excellent clam to press the glued surfaces together, which is an important consideration when the work has to be done by an amateur, or by a workman who has not plenty of iron clamps at hand. It is obvious the recesses thus formed between the bracings are capable of receiving either the central webb of a bar of T iron, or one of the members of a bar of ordinary L. or angle iron. The latter is now kept in stock by Messrs. Moser and Sons-and by no other London firm I know of-of very suitable sections, say 2 x 1 x 1, or 3 x 11 x 5-16th. From three without also washing out his individual life; both he Not long since I heard an harmonium, by Ramsden, whose strident tones might have satisfied our MECHANIC's princess "Alexandra" herself. This instrument, which drowned every other in the room, had but one and a half row of reeds, whose tones remarkably resembled some of those German concertinas which so greatly surpass their English namesakes in power and-if I may be allowed to coin an adjective-also in horniness. Possibly great pressure of wind had something to do with the matter, for I find the steel reeds of my Day and Munck's colophon assume this quality when forced. I suspect the physharmonica with a swell (not, of course, the organist himself, although some of them are both great swells and heavy swells to boot), with its free reeds blown by 10 or 12in. wind, has something of this strident character, and sounds very unlike the tender reeds of an Evans's or Hermann Smith's chamber harmonium, which are delicately blown, and perhaps also-as all good tender children should be-well wrapped up to keep them from squalling. two or more ranks of keys, that perhaps "Eleve That any free reed instrument will ever render that REFLECTORS and REFRACTORS. SIR, The telescope was next directed to the two nebula before-mentioned. We shall now only give our impressions of the latter (M. 13 Herculis); and what a wonderful sight we beheld when it was brought into the field of view! With a low power a small, bright cloud, all sparkling with diamond points of light, but with a higher power how magnificent the change! Stars piled upon stars, not one trace of the cloud remaining, but star-dust, even to the very centre, where they seem so infinitely close, that one wonders they each can still be seen separate from the other. Here is a test for the perfection of figure of a speculum, than which none could be more severe. To completely resolve this superb cluster, even to the very centre, with such an aperture, is work any man might feel proud of. On the second evening I examined this object most carefully, and for some time, and could most distinctly trace the two dark rifts first seen in This wrapping up is a very effective method of sub- the Earl of Rosse's great telescope. They were best duing unpleasant sounds. Every paterfamillas knows viewed with the highest power I used (350). -to his horror-that his infant's eries are strident, But enough has been said to show the extreme indeed, if heard in their native nakedness, but become excellence of the speculum I was employing, and the quite bearable when its dear little head is under the sufficient adaptability of the mounting described in a bedclothes. It won't do to let the infant breathe too former letter. Let every amateur take courage, and freely, or he becomes "the son of the sleepless"; and if he only possess a little ingenuity and patience, he even if this mode of quieting sometimes be fatal to it, may do even better than I have done. The above is we have at least the consolation of knowing that its but a short account of one or two nights' work-very departure to the world of spirits diminishes that favourable nights, I will admit, and such as do not pressure of population on the means of subsistence come very often, but when they do amply repay the which political economists so earnestly deprecate. earnest student any small additional outlay he may No doubt the reader will say there is nothing new in have made in procuring a really good speculum or this, it having long been familiar to "ye family man,' "object glass. Whatever he does, let the beginner but if not new itself, we have not long since become beware of cheap object glasses or specula; they always acquainted with a new application of this very old break down when such testing times" come, and fact, which new application well deserves letters prove ultimately a loss rather than a gain. patent not only to be granted gratuitously, but also that its inventors should be fully paid for their ingenuity. I need hardly say that I allude to the late Shoreditch infirmary practice, in which a noisy pauper -what right can paupers have to make a noise ?-was duly and effectually quieted by having a cloth forcibly held over his wicked mouth, while the nuss prepared a sufficient dose of morphia to quiet him effectually until the day of resurrection, on Count Fosco's principle of assisting tired Nature's tendency to permanent repose. Verily we do improve in the treatment of insane paupers, for not only do we liberally provide wet cloths for their crying necessities, but also expensive medicines, dispensed with a liberal hand by nusses, to assist their progress out of all mundane miseries. To return to our dead sheep (muttons, as the French say). I very strongly suspect the reeds whose draw stops are duly labelled clarionet, hautbois, and most of the other varieties of timbre we hear from the harmonium (or rather ought to hear). Too often the real tones are very far indeed from that instrument, are much more due to the different areas of the spaces between the reeds and the pallets (within which spaces the air, under the influence of the reeds, becomes resonant), and the quantity of what I take the liberty of terming bedclothes (interposed between the reeds and the hearer), than to any other causes-of course I mean the extent to which the reeds are covered up. I am told the popular voix celeste-which can only be termed celestial on the hypothesis that the angels practice singing the music of the spheres (whatever that music may be), with remarkably thin reedy voices-is produced by another dodge, but instead of enlarging and perhaps yet further exposing my own ignorance on this not very celestial subject, I would recommend the reader to peruse Mr. Hermann Smith's paper on it, for an expert can, if he will, almost invariably give us more information than any mere amateur, however clever he may make the very common mistake of thinking himself. But few modern harmoniums which I have seen are constructed with the means of blowing their reeds with wind of different pressures, without using the expression stop. Myers and Storer, 1839, and Hewett, 1838, patented methods of doing this, with only one bellows, and, I believe, others have since invented other ways of effecting it. The thing seems to me so very desirable, especially for instruments with I would rather possess a really good 24in., or even 2in. object glass, than a cheap 3in.; and this reasoning holds good, a fortiori, with regard to specula of respectively larger aperture, as with them the evil is increased in a threefold degree. There is one fact, however, which I have learned from constant observation; it is that when the air is unsteady there is no use in trying to use an aperture of 8in. perhaps this may seem too strong an assertion to some, but certainly the position in which my telescope is placed-a position very similar to that described by Mr. Purkiss (near a large city-has taught me lits truth. Many nights come when I can use smaller apertures, and see planetary and lunar details sufficiently well to learn something of phenomena taking place in those bodies, which a larger aperture would not even suggest. Nights often come when it is vain to attempt to divide even such a star as a Herculis with the whole aperture, or see the transits of Jupiter's satellites, yet a lesser aperture of 5 or 6in. will accomplish both tolerably well; unfortunately too, these are the kind of nights which predominate in our climate. This I say with great deference to Mr. C. Key's opinion, which is deserving of the highest respect, but it has forced itself again and again on my notice, and were I asked to advise one of your correspondents who was anxious to possess a telescope which he could use on most bright nights, and was not willing to keep a large one in reserve to use on good occasions only, I should strengly recommend a refractor of from 3 to 4in. There is a calmness of definition in a refractor, arising from the air between the object glass and eye-piece being shut in, and nearly free from disturbance, which has greatly tended to induce a preference for that instrument over the reflector. This freedom from air currents is frequently mistaken for a greater sharpness of definition, whereas it may be stated with all confidence that a reflector, if it be a really good one, is, on favourable occasions, equal to the very best refractor of the same aperture, only those favourable occasions do not come quite so often with the former as with the latter. The position of the eye-piece in the Newtonian telescope, as it is generally placed, has something to do with these tube currents, for the heat from the observer is almost necessarily brought in contact with the air in front of the tube, and with the surface of the tube itself; it would be very difficult to estimate accu rately the exact ratio of disturbance originating from this source, as it varies so much under different circumstances, nor can we hope to eliminate it entirely until the good old form of the Cassegrainian or Gregorian telescopes come again into general use. An open tube could then be used with great advantage, which seems rather to increase the disturbance frem this cause in the Newtonian telescope. There is, however, no comparison whatever in the relative greatness of this source of indistinctness, between the Newtonian and front view construction. Of all the forms ever devised, the last-mentioned telescope is so completely at the mercy of thes troublesome cyclones that, leaving out of consideration the spherical aberration, sharpness of definition is almost, if not quite, impossible, in such a telescope --certainly, if the correspondent, who sent you a sketch of his telescope lately is the happy possessor of one of those large heads, so highly spoken of in a recent number, very little light, and that of a very tumultuous character, can ever reach his 12 n. mirror. was While speaking of this form of telescope, sorry to read the abrupt manner in which your talented correspondent "F.R. A.S." took up a remark of "Arcturus' with regard to spherical mirrors. Your worthy correspondent has always been so courteous in his replies, and so ready to assist us at all times, that we should expect, for one who has taken so much trouble to enlighten our minds on the mechanical minutiæ of speculum grinding and polishing, and to teach us lessons which are rarely, if ever, learned from professional astronomers or opticians, better treatment at his hands. I can hardly see how F.R.A.S." would, in any degree, trench upon the just estimate he has formed of the absolute necessity for the parabolic curve, if true definition is to be attained, were he to confess with Sir John Herschel, "that for this construction it is needless to insist on the parabolic curve." My own experience is, that the spherical aberration of a mirror, parabolised in the usual way, is quite equal to, if not greater, than that of a good spherical one, when tilted at an angle of $ and 4 degrees, and that experience was learned from a most perfect 4in. speculum by With. For preserving the silver films of these glass mirrors, if the observer does not wish to remove them from the telescope each time he has finished using it, I have found no plan so efficacious as to turn the tube itself into an air-tight celi; this is easily done by having it made perfectly stanch in all its seams, and having a lid for the mouth which fits most closely and neatly; to this lid is attached, by a string, a long roll of some light fabric, which has been steeped in chloride of calcium, and thoroughly dried in an oven or at the fire, before inserting it into the telescope. It will quickly absorb all the moisture in the tube, and as no more can creep in so long as it is closed, will keep the silver surface perfectly dry and bright. I have often thought, however, that some of your clever chemical correspondents would be conferring an incalculable benefit on the possessors of these telescopes if they could discover some method for precipitating pure aluminium on glass; it is of an exquisite white colour, of very close grain, and receives a higher polish than even silver itself, and might tend to render them quite equal to refracters in light-grasping power, as they are at present in defining power. I see in a letter from one of your excellent practical correspondents, that he paid £2 for a "focussing metion" to the eye-piece of his telescope; now for about one-sixth of that sum he could have procured a plate lens, secondhand, with rack motion. Having taken out the lenses, he could easily have fitted the spring-tube of his eye-piece to this, and thus have obtained a most excellent and simple focussing motion. This plan is not mine, as I feel sure many of your correspondents have seen telescopes so mounted. The most pleasant "finder" possible, can be formed by placing a rectangular prism before the eye-piece of the finder, and so turning the rays out parallel to those from the large mirror after they pass the "plane." By this plan you have only to turn your eye from one chair, and need not strain either your neck or your eye-piece to the other, while still rechining in your eyes to find what you may wish to observe. I cannot close this letter without two words of thanks, first to yourself for the successful manner in which you have conducted your most excellent journal of late, and the abundant store of really useful information you have managed to obtain for its pages from those whose "fame is in all lands," an secondly, to your generous and patieut correspondent, Mr. Purkiss, who has made a real discovery in the science of speculum testing, and one of the very greatest value to all those interested in the subject. I have tested it myself, and believed it possible, with due attention to particulars, to attain, by the means he suggests, a mirror of any size, perfectly free from spherical aberration. P.S.-I am vesy glad to see Mr. Wray's name brought before your readers in a recent number by the Rev. Mr. Webb. I have two object-glasses by him, one a 2 8-10ths inch, already mentioned, the most perfect of its kind I have ever seen, the other a 4in., recently obtained, which quite surprised me by its light grasping power. On two occasions I have distinctly seen the 6th star in the "Trapezium" of Orion, a test I used to consider far beyond such an aperture. I do not mention these matters by way of advertisement, but to assist those in search of really good object glasses, and to whom the price is a very important consideration. If the amateur can save £5 in the purchase of an object-glass, and obtain a really good one, he has the money in hand to purchases battery of eye-pieces, no mean consideration after all. HYPERION. EXTRACTS FROM CORRESPONDENCE. NOMMES DE PLUME-" Neptune," in a letter full of congratulations, says :-" One of the interesting features of the ENGLISH MECHANIC is the great variety of assumed names under which correspondents write. Where they all come from I cannot imagine, |