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the link is derived from the excéntrica through the rod», and from them alone ; and no matter what part of the link, be it the end or be it the middle, is in gear with the valve-spindle, the nature and amount of its vibration is ever the same ; and its position with regard to the valve-spindle has about as much to do with modifying that motion as the handle of the stoker's shovel has.
Fig. 2 shows the forward gear end of the link in gear with the valve-spindle, and the reversing lever (not shown) in the end notch of the rack. In this position all the movement», and attitudes, and angles made by the link daring a revolution are of course identical ■with the middle gear position; it is impossible they could differ in the slightest degree ; and all that Mr. Harrison says about the centre of motion being 2in. higher up or lower down, and about the disengaged rod receiving all the angk-я, and about " that end of the link having almost double the vibration," ia pure unmitigated nonsense, without a shadow of foundation iu reality or fact, and his three diagrams are on a par with the text they are intended to illustrate.
I have already said that in Figs. 1 and S the centres of the excéntrica are placed diametrically opposite each other, and without lineal advance, and the object of placing them so has been explained—viz., to illustrate the impossibility of constructing the ordinary link motion with a motionless centre; but excéntrica set in this way are not suitable for working a slide-valve, becanse ail slide-valves have more or less of lap and lead, and, therefore, require the excéntrica to be set with more or less of lineal advance. I have also to remark tiiat the excentric rods shown in Figs. 1 and 2 are unusually abort as compared with the length of the link and the throw of the excentrics ; and I have purposely made them so in order to exhibit in a conspicuous manner and on a large scale the influence which the crossing of the rods has upon the movements of the link, for short rods are more influential in that way than long ones, other dimensions remaining unaltered. James Baskerville.
1242] Sir,—Mr. Harrison commences his letter very well, by apologizing to Mr. Whuatley for his unpolite reply to that gentleman's letter. He also confesses that he knows very little about the link motion. Such a confession on his part I think quite unnecessary. But he spoils all by the "would-be satirical way" in which he alludes to my letter. Allow me to correct his assertion that my diagram (Fig. 2, page 425) is a copy of Mr. Baskerville's; the dimensions are taken from the workiug drawings of an engine now running on one of the chief railv.ays in Ireland.
Now, sir, if Mr. Harrison will not agree with either Mr. Baskerville or myself that the link does not swing on a centre, I hope ho will accept the authority of D. K. Clarke's "Railway Machinery." In that work he will find a series of diagrams showing the curve described by the working point of the link, in full, backward, middle, or full forward gear, whether hung from the centre or bottom end. Now, one of the defects of the curved link is, that if the motion is set to work with ¿in. lead in mid-gear, it will become ii-16ths or mere when working in full back or forward gear; this difference in lead increases as the rods arc shortened, so that the general practice is, in designing motions for tank engines or locomotives in which the motion must bo short, to substitute for the lifting link the stationary reversed link with the reversing gear attached to the pulling rod, which works up and down in the link: a constant lead is claimed for this motion. Now, as discussions in your valuable journal should be conducted in all good nature, allow me to say that if Mr. H. would wish for a set of diagrams I would be
most happy to send him a set showing the position of the link in all parts of the stroke. If he will not accept my authority, all I can do is to refer him to the work above. In the mean time, I think if Mr. H. will examine the diagram (Fig. 1, page 425), he will come to the conclusion that the link moves to a small extent along the motion line, on account of the rods drawing the link, top and bottom, towards the excentrics where they cross. This occurs even when the excentrics are placed diametrically opposite. But engineers require the link to open the ports even when in middle gear, and for that purpose they give the excentrics an advance of l-16in. which with lin. lap on valve, 5Jin. travel of excentric, and, of course, all olher dimensions suitable, would open the ports ¿in. on each side. As for the letter of "R. H. M.," in which he states that with the excentrics in the position shown on page 425 the rods ought to be crossed, I really do not know what to say about it. Either I know nothing at all about the link motion, or else "R. H. M." has ideas about it totally at variance with all known theories. James F. Ryan.
A HANDY TENT FOR PHOTOGRAPHIC
 Sir,—I send you a sketch of a handy tent for the benefit of any of your photographical readers who may be contemplating the practice of their art during the holidays. It is taken from the Philadelphia Photographer. Drive a stake in the ground so that it will be about 3ft. high; now take a common umbrella, tie the handle firmly to the stake, then place a double sheet of black and yellow calico over it, leaving a small hole in
pellets, Ac. A pellet in a ring at the heaâ and tail of the horse. The device is partly surrounded by a beaded circle. In very fine preservation. Gold; size, Jin. by Sin.; weight, 95 grains. On the- fiat rim of the obverse are marke, looking rather like a legend. The type of this coin ia similar to the first class of E vans (" Coins of the Ancient Brilon«," plate B, No. 8), and is very like those ir Kuding ("Annals of the Coinage, Ac.," plate I., No%. 1, S, and 4.)
This is the first ancient British coin that is fcnoir» to have been found at Brighton; a copper one of a different type is mid to have been found there (see Evans, p. 206, plate Г\\, No. 12), but Mr. Evans was not satisfied of the fact. I purchased my coin of the man who found it, and myself know the exact find spot. Henry W. Henprey, M.N.S., 4c.
EMIGRATION: WHERE TO GO—WHO ЙНОПЛ> GO—HOW TO GO—WHEN TO GO.
 Sir,—If Emigration is still entitled to some space for discussion in your columns, I should like to say a few practical words on the subject, and I would divide my observations into four parts.
1st. Who should go.—I would advise none to go -who have not had some experience as practical farmers, or who cannot form a society in which the cultivation of the land shall be the principal object, and to which each individnal shall bring an experience in some craft useful on a farm; this would in no way exelnde such mechanics as smiths, carpenters, wheelwrights, &c, as I am of opinion that a practical farmer, a smith, and a carpenter emigrating together (with their families, and if they have grown-up sons and daughters so much the better) would be sure to succeed if they go to the right place.
2nd. Where to go.—To a British colony, or to the United States, for practically I think the choice is the* limited, as emigration to foreign countries—that ia to say, to non-English speaking countries—for the ordinary British emigrant is the height of folly. The British colonies may be divided into the Australian, South African, and British North American. The first and second division are difficult of access on acconnt of the distance and consequent expense of passage, and so for the present I will pass them over, and take Canada as the colony to be compared with the States. It has great advantages. It ia prosperous, fertile, rich , well-settled ; and, moreover. English, Irish, and above all, Scotch, will find themselves "at home" there. Land is to be had cheap, and the intending purchaser or tenant may rely upon being juitly dealt with, both by the Dominion Government and the Canada Company, in whatever engagements they may mutually enter into. They may also be certain that when cleared and cropped, the laxd will ultimately amply repay their labour and outlay; but, oh t the weary work of clearing a Canadian ** concession" (I write under the supposition that the emigrant has not the capital to purchase or reut a cleared or partially cleared farm, but that he simply brings his labour and bnt little money with him). The felling of the forest, and the grubbing up and splitting the stomps is dreadfully disheartening work to those who look upon a farm Ы only so much land to be tilled ; whereas, all Canadian uncleared land is a forest to be reclaimed; add to this, the farmer 1ms to make and maintain his own road, as far as his concession extends; and, lastly, all the unallotted(Goverument) lands are far in the backwoods— far from towns, villages, or railroads. In this description I exaggerate nothing, nor do I suppress anything that could brighten the picture. The climate is extreme^—in summer dreadfully hot, and in winter severely cold—but, in justice to the winter climate, it must be admitted it is not disagreeable or unhealthy. It is dry, bracing, and cheering, so that to sum up I do not advise emigration to Canada for the class of persons I am advising to leave England; but I do most strongly advise such to go to the Western and Northwestern States of America, and there settle on the prairie lands, offered now on most favourable terms by most of the great railroad companies. Of these States, and of their advantages and capabilities, enough has beeu published for every one to have an idea of what they are; but I know how suspicious all English—i.e., British emigrants, are of all American land prospectuses, no matter by whom issued. Now I am not going to select any of the many railroad companies who offer land to settlers, but I know that with hardly an exception they are all genuine, and it would be hard to advise intending settlers in their selection.
3rd. How to prepare for going, and how to go.—I would advise (always supposing a society formed) that the farmer should go first and select the land; this on most of the railways he can do by going to the agency of the company at New York, and there seeing on the map the disposable lands, and then before laying out one penny, or rendering himself liable in any way for anything, he can see the lauds himself and make his bargain; then, if he is satisfied, let him send passage tickets from America to his friends at home, by one of the great eteani lines from Liverpool, London, or the Clyde. He will find this the best way; as if he has Euglish gold or bank notes, he will enjoy the advantage of the premium on gold which exists now in America, and is likely to continue. I do not specify any particular Hue of steamers, but I most emphatically warn every one against any sailing vct**U whatever.
4th. When to go.—Preferably the early spring (say) April, and I should say on no account go after the end of May, as later the heat is terrific, and make« a stoerage passage, even on tho best appointed steamer, a perfect misery. At any time tho
TUBNTNG BOTTOMS OF CYLINDER COVERS.
 Sib,—According to promise I send a sketch showing how the bottoms of cylinder covers are turned off without having any hole through them. The cylinder cover is fixed in a dog-chuck, that is a chuck with four movable jaws in it, worked in and out by four screws во as to suit any sue article that would want to be turned. I have shown tho cover fixed in this kind of chuck with the cover reversed from one side to the other. Fig. 1 shows the cover fixed for turning the outside, fig. 2 for turning the inside. Fig. 2 is the best way to commence turning the cover first, for this reason: By turning off the rim so as to fit the bore of cylinder first, when reversed it can be fixed as shown in fig. 1 by the rim, and then the outside, likewise the face of cover, can be turned off and finished, but if fixed as in fig. 1 first, when reversed as shown in fig. 2 in tightening the jaws, it would be apt to make a mark where the jaws come, so that fig. 2 would be the best position to hold it first, fig. 1 next. J. С Molton.
A FOUR-WHEELED VELOCIPEDE.
 Sib,—I enclose photograph and description of a light four-wheeled velocipede which I have had in nee for twelve months, and from which, during that time, I .have derived much pleasure and benefit. I may state that I had it constructed by a friend in the business, therefore if any "edit is due Ido not at all claim it. The material used in its construction is chiefly good ash; with the exception of the crank axle, very little iron is used. The wheels are all ЗОщ. in diameter, with bent ash rims, and hooped with leather instead of iron. I give preference to the leather on account of its being lighter and almost noiseless. The hind wheels are the drivers, the front ones being used to steer by. The crank axle is made of tin. round iron. The cranks are Hin. throw, and placed at quarter centre. There is a ratchet on the nave of each driver, но that the working parts remain at rest in going down hill. The ratchets are made in the form of hoops with the teeth inside, aud put on the waves like ordinary rims. The catches work inside those ratchets and fall into the teeth by their own weight—no spring being required, and consequently no racking noise. The material of frame is 1 jin. by lin. Bent ash elliptical springs are used, Ilia. by jin. The ellipse is 19in. by lOin.; one side is secured to the frame, and the other side has the ends left about lOiu. apart: across this space is secured a strong leather strap, upon which is laid the seat. These springs are by far tho best and lightest which I have ever used. The propelling is done by ordinary hand and foot levers. Tho whole weight a little under 571b. With respect to speed, though the drivers are small, I can easily run a milo in six minutes, though for a long journey I find that six mile an hour in ample, if health and comfort are to be attended to. I look upon the velocipede as giving the most pleasant and valuable means of exercise, and I think it will be acknowledged that any exercise taken in due moderation, and continued for a length of time, is far more beneficial to the human system than such exercise taken severely and of short duration. Such а consideration as this compels me to ndvocato light velocipedes, small drivers, and moderate speeds.
 Sib,—Now the dark evenings ore approach ing, will you, through your valuable paper, allow mo to impress upon my brother bicycle riders the necessity
remarks apply especially to those gentlemen who possess bicycles with rubber-tired wheels. I have, for the last two months, been using the "Phantom Wheel" with rubber tire. It is very comfortable to ride. Its merits are faithfully represented in the pamphlet published by the manufacturers.
I cannot agree with a remark made by one of your correspondents, "That it is better to sit well back." To ride a velocipede properly you must sit upright; if the saddle is put far back on the spring it compels the rider to lean forward, which looks very ugly, and if persevered in will prove injurious. I know a youth who walks quite in the present Grecian Bend fashion, and his deformity (I call it) was produced by constant bicycle riding with the saddle too far back on the spring; but I trust to see him, and many others, shift their saddles and avoid this bad habit. R. L.
 Sir,—Except to the few readers of the English Mechanic who are shorthand writers, the discussion which you have printed on Pitman's and Lewis's systems of shorthand must be unintelligible. Only those who have a knowledge of the alphabets of the two systems can understand the rules for writing which Mr. Grierson has quoted from them. I think it is possible to give, in few words, an impartial description of the alphabets of the two systems. I will try. If I succeed to your satisfaction, Mr. Editor, I shall be obliged if you will allow your readers also to see my attempt.
Writing is, professedly, a representation of the sound» of the words we speak. Thus, wo begin the word paper with the letterp; the first letter in sound is i, etc., because the letters p and $ are appropriated м the representatives of the sounds we make at the commencement of these words. In tho use of the consonants we are tolerably consistent. We do not write I when we speak f, nor p when we speak/, though we do, in some words, use Ch for /. We want six more consonants to complete the English alphabet, namely, types for the italicized letters in chnmp (corresponding
to the heavy or vocal pronunciation of this letter in jump), she, vision (or zh, the sound concerning which Mr. Grierson asks, "how often is it required ? ") thin (which is as simple a sound as the first in tin), then (as simple, or single, as d in den), and sing (a single sound of the nasal cuss, made at the throat).
But when we come to examine the vowel sounds, and the letters we employ to represent them, all is confusion, arising from the inefficiency of the alphabet of vowels—«. e, i, о, u. Everyone will give the same sound to the letters pa pin the word ряр, but let the syllable er be added, and the sound of a is altered from
pap er, to pa (pay) per. Every single vowel type changes its sound many times in different words;' as mate, mat,/all, want, father; me, met. I need not carry the reader through all the changes which may be rung upon the five vowels. The same thing occurs with the double vowels. The diphthong ou has different sounds in sound and journey; eo in people and George,
Now, the difference in the alphabets of the two systems is this: Pitman's, or "Phonography," gives a shorthand character for each of the useful consonants in the English alphabet, that is, for b, d, /, g (as in go, never used for the first letter in gender), h, j, k, I, m, n, p, r, «, t, r, Mi, y, x, (18); for the six consonants enumerated above; a distinct sign for each long and short vowel (12), as in rather, lather; mate, met, 00., and for the four dipthongs i, u, ou, oi, as in bind, (in which t represents a sound different from that in bin), tune, (different from fun), bound, boil.
We have thug in Phonography an alphabet of forty letters, which, when written, represent, without defect or mistake, the very sounds we utter in speaking English. If anyone should complain of the number— that it is a large unwieldy alphabet—his complaint lies against our language for being so rich in sounds. It will be observed that the letters e, q, x are not included in the phonographic alphabet. They are omitted because they are not wanted. С is represented by e in cat, by « in city, and by sh in $ocial. Q, with its following», is represented by few; and x is either Ju, as in extant, or kz, as in exact.
In Lewis's system there is no such alphabet of sounds, and I must deny the correctness of the term "phonetic," as applied to it. Ho gives a shorthand character for each of the twenty-six letters of the common alphabet, with additional signs for th in them, sh in the, and ch in cheap; and leaves unprovided with signs the sounds in pleasure, breath, wrong. His alphabet thus contains but twenty-six useful letters to represent forty sounds. These consist of the twentythree useful letters of the common alphabet (c, q, and x being unnecessary) and the three consonants just enumerated.
As for the numerous sounds which the five vowels, a, e, i, o, u, represent, he leaves the writer to employ them as they are used in the common spelling, or omit them altogether.
Hence, the reading of Phonography is SO easy that it is impossible to make a mistake when it is correctly written; and the reading of Lewis's, and all the other common systems of shorthand, is so difficult that the deciphering of them has passed into a proverb to express uncertainty and confusion. Isaac Pitman.
IGNITION BY ELECTRICITY.
[2491 Sib,—I Saw an enquiry a short time since in our paper about ignitiug ga* by electricity. Perhaps the enclosed cutting from the American Technologist will afford some of tho information "W. H. B." asked for:—The idea of using electricity to ignite inflammable substances or explosive compounds is not new. Lightning, which is nothing but an electric discharge on a large scale, produced in nature's laboratory, has effected this from time immemorial. The ignition of alcohol, ether, gunpowder, Ac., by the electric spark, is one of the oldest experiments in this branch of physics, and during the last one hundred years has formed one of the standard experiments in the philosophical lecture-room.
Electricity produced by friction was proposed and practically applied to the lighting of gas; the small amount developed when the feet are rubbed on the carpet in a very dry room is often sufficient to charge the person, so that by touching the gas jet a small spark will appear, which will ignite the gas readily if the latter be turned on to a proper degree. Many devices have been contrived for the development of this frictional electricity. Thirty years ago machines were made in Groningen, Holland, with a glass disc, the whole being enclosed in, a glass case, nearly hermetically closed, and provided with chloride of calcium or other hydroscopic substance, which would absorb all moisture, and thus keep it dry, во as to ensure the obtaining of a spark in all states of the weather. Bat<-beider, of New York, patented a small friction machine, which was hidden in the ornamental globe attached to the lower central part of a chandelier. When the gas was turned on, and a knob which rotated in an inside disc or vulcanite three inches in diameter was turned, a spark was conducted to the burners and the gas was ignited. Vance, of New York, invented a gas lighter in which a similar machine was placed above the handle, and turned by means of a small handle. The insulated metallic rod attached to the lightor was thus charged, and when brought against the gas burner it emitted a spark, thus lighting the gas. Cornelius, of Pliiladelphia, patented a small cup, lined inside with silk, and closed with a stopper of vulcanite, the whole being attached to the gas bracket. When the gas was turned on and the vulcanite stopper lifted out of the cup, friction enough was produced to develop an electric spark, which was conducted to the tip of the burner by means of a -until insulated metallic chain. In 1800 Van der Weyde, of New York, presented to the Smithsonian Institution, at Washington, a simple syringe of vulcanite, which, by sliding the piston along the interior of its cylinder, produces electricity enough to do the same thing. It was, in fact, a small and very convenient instrument for developing small amounts of electricity for experimental purposes.
In 1803 Wesolewski patented a very neat and very small frictional machine of vulcanite, in which a lamp containing bisulphide of carbon was lit by a spark, the latter substance having been found to be moro easily ignited in this way than any other liquid. The blue, non-luminous flame of the bisulphide served to light a candle or lamp. But the odour of this compound is во offensive to most persons that the apparatus had a very limited sale, although it was pushed most vigorously. The inherent uncertainty of all methods of obtaining electricity by friction caused this agent to be abandoned by the knowing ones, and the voltaic battery, induction sparks, or maguöto-electricity was substituted foi it. Lately, however, there has been produced a small frictional machine, made entirely of vulcanised indiarubber, and hermetically enclosed in a casing, which also contains a condenser of the same material. By giving the handle half a dozen turns, a strong, snapping spark appears between two conducting wires which project from the apparatus. Tho machine may bo hail of different sizes, the smallest size not being larger than a common paper-collar box, and of about the same shape. The large size has the same general farm, a slice of a cyl nder, and is one foot in diameter, and threw or four inches thick. It is now extensively used for igniting gunpowder, nitro-glycerine, 4o., in blasting and mining operations. W. It.
RECTIFYING THE CIRCLE APPROXIMATELY.
 Sin,—There must be many ways, equally simple and much more exact than any lately described, for obtaining with ruler and compasses alone the approximate length of a given entire circumference, or aliquot parts thereof. .Thus, Hutton erects at the ends of a diameter two perpendiculars, one = 3 radii, the other = tan. 30% and the lino joining their tops is within a hundred-thousandth part nearly = the semicircumference. But I conceive that the real problem, to be usefully solved, must apply to every arc, whether commensurable with the circumference or not; and comprises three cases. 1. To rectify particularly any given arc. 2. To lay off on a given*circle an aro practically of a given length; and 3, to construct an arc snbtonding a given angle, and practically of a given length. For the first of these I venture to doubt if tho following has been, or will soon be surpassed. It is probably now, being derived, but considerably improved, from two invented and published as new by Professor Raukine in 1867.
longer and quicker, becoming at least, — Hen«—I
choose the fraction — of D E, to reduce «тог on the
Ü whole throughout all possible arcs.
Mr. Drach's very curious discoveries were not proposed, I conceive, as geometrical approximations, but as arithmetical abréviations of the process of multiplying or dividing by 3-14150265, &c., for which they arc surely most remarkable, and up to these first nine or twelve figures, very useful. In fact, he represents the above nine by a combination of factors that contain but five significant figures;thus (3— -3008007) * 1*05. Pushing the principle further, he shows how the effect of these twelve, 3'Ш59265359, may bo got from the same ii, and two more, namely, adding (3- -008007) x ■00000000012. This is perhaps the limit of utility; but if exactness to 16 decimals (that is, to a thousandth of a hairbreadth in the circumference of the earth) should ever be needed» he obtains it by one figure more, subtracting from the last factor ЧЮКМЮОиОиЧКЮ. The extension» ho has contrived beyond this, to 18 and to 31 decimals, ore wonderfully curious, and that is all; but we must remember tho arithmetical processes are abridged in exact proportion to this reduction of the number of significant figures, or rather of figures above unit v. *'• ^- *■*•
P. S.—As far as the above number is qnoted at all, it should be correctly 3-Шо1*265358!)7932384. Though the 17th and 16th places might be 01 without greatly affecting society, the sixth must not be а 1, as twice printed in Mr. Proctor's letter.
THE SATELLITES OF URA2ÎTJS.
 Sir,—I have endeavonred to comprehend the movements of these bodies as described in astronomical books, but I cannot succeed in making the data given bv Sir W. Herscbel agree with certain statements of other excellent observers of these distant bodies.
Sir W. HerHchel in 1708— the epoch of the ascending node in long. 105 — found the plane of their orbits to lie 81° south preceding, and 70 north following. These, in our modern method, would be Pos. = 189° and 11°; this would make the inclination to the ecliptic about HO- on the W., and 100 on the E. side respectively. He had previously found tho motione to be retrograde. Now when half a revolution of Uranus had been performed—say in 1840—the ecliptic would be similarly inclined to the circle of declination on the other side, so that near the descending node the plane of the orbits would be inclined about 10° to the meridian, in Pos. = 170 and 350 . In figs. 1 and 2 the apparent orbits are
Given the arc A B, whose centre is at C. To lay off on the tangent A G Un approximate length. Bisect the arc, and bisect again tho half next A, by the secant С D, meeting the tangent at D. Draw the chord, В A, and produce it to E, so that A E = half of В A. Join E D, and make D F — two elevenths of D E. (This division is only necessary in the case of very large arcs, approaching a semicircle, or at least a quadrant. For any under an octant there will be no sensible difference if D F be as little as an eighth, or аз great as a quarter of D E). From F, with radius F В strike an arc В G, which will cut from A G the required length.
The error is positive, or A G longer than the arc, when this is a quadrant or anything less; but at some arc I have not determined, between 00: and 180°. the error vanishes; and for arcs approaching a semicircle tubs is reversed, or the line A G too short, the defect, however, barely reaching a ten-thousandth in the extreme caso of rectifying a whole semicircle by one operation. By distinct calculations I find
The error in 191 = + about 3,800,000 of the arc. 1
The beauty of the method is, that whereas in other rectifications, as far as I know, however "small tho error for arcs of small curvature, it constantly increases with their angle, and at length so rapidly as to make the rule applicable only by dividing them and taking their half or quarter; it here ceases to increase, reaches a maximum for some arc between 45; and 90Q,
probably never reaching 30,000; and then so decreases with further increase of curve that the quadrant is rectified more truly than an octant. In still moro carved arcs it decreases to nothing, passes to the opposite side, and thus barely reaches a sensible amount for any arc, even np to a semicircle. For arcs below 60° the errors would bo eveu less if we modo D F =
-DE, and tho positivo maximum be less; but as it
would oocur at a lower arc the change of sign would
be at a point below 90е1, and tho negativo error increase
ing to the careful examination of some "bitter beer.' I found no trac*3 of acetic acid by optical analytds. I saw some isolated cells of ToruXa and, horrible to relate, a goodly number of bacteria and vibriones, which appeared to be in the liveliest condition. Under mv ¿th, &c, the spectacle was most edifying and appetising. W. P.
C£ELAUD GALVANIC BATTERY.
[2fi8] $sb,—There would seem to be afloat amongst, your readers an error as to the name of the inventar of the so-called "gravitation " galvanic battery.
"What is in a name?" will not hold in this matter; for the mistake has already caused a misconception, iu the mind of at least one correspondent. The nomo should be Callaud not Calland. In the year 1M62 Callaud, of Mantos, called the attention of the French Academy to his new battery, in which the poron* cell was abbeut. and the liquids ware kept from mixing by "superposition." Veritc, of Beauvais, the maker of the wonderful .astronomical aback of Besançon Cathedral, hit upon tii« «une idea some time biter (ltk'3).
Both these iu ven tore requiring electric currents for clock-work motion, bad not to trouble themselves about the great objection to this battery—that th* liquids «tust not be âiiahen; otherwise, it is stronger, more steady, and uMsleoMnlphate of copper than Daniell'e.
E. Kern ля.
 Sia,—Your correspondents " Bernardin " and "Oceola " are, I think, in error in regard to their statements about the non-existence of a mammoth skeleton in Great Britain. There is a perfect skeleton of the mammoth in the Natural History Department of the british Museum, and several parts of another skeleton, together with a specimen of the hair, which was founi embedded in ice. I would advise *' Bernardin" to viel the British Museum (if he hasnotalrcady done so), ела I have no doubt he will be as much pleaded with oar English specimen as with any to be seen on the Continent.
An Associate Of Kino's ColleOe, London.
THE FIELD BOILER,
 Sir,—Permit me to correct a statement in your last issue regarding the " Fiold " boiler, which ie likely to mislead. After having stated that upward« of 60,000 "Field" tubes are now at work in this country, you say :—" They are not likely, however, to be generally adopted for factory and other uses, where more liability and neglect exists." In answer to this statement, will you allow me to state that upwards of 370 "Field" boilers have been made np to date in this country, and with few exceptions ore all employed, for factory purposes? The number of tubes and boilers mentioned above do not include those made by Messrs. Merryweatker & Sons /or their well-known steam fire engines, as they have the sole right of employing these boilers for that particular parpóseIn addition to this I may mention that a very great number of "Fiold " boilers are made on theContincnt» amounting to about ten times the number made here iu England, one firm alone having turned out as пишу as 150 boilers a year during the last few years.
I trust that this statement may satisfy yon and your numorons readers that the "Field" boiler is as safe and reliable for all stationary and factory purposes as any other boiler. Lewis Olricü.
shown as projected on the meridian, the strong lines denoting the plane of the orbits, tlw thin lines the meridian and circle of declination, and the dotted lûtes the ecliptic. The arrows indicate the direction of the motion of the satellites (as I suppose) in the half of their orbits nearest the earth at the nodes. A telescopic view is, of course, obtained by an npsuk-down view of the figures.
Now, in 1851-2—when the apparent inclination was on the same sido of the meridian, i\s iu tig. 2—Mr. Lasse 11 observed the satellites at their greatest elongations to be in Pos. — 82J:J south^prece-diug, and north following, say 187.15 and "¿°- ^-uw ^ись tu^s a8ree with their discoverer's conclusions V
Mr. Hind, in "Solar System," p. 121, says that Miidler found the axis of the planet inclined at an angle of 15-' 26' to the circle of declination, ltH3, Sept. 28; but whether the N. pole was N. of this circle, or between it and the ecliptic is not stated.
I should feel greatly indebted to Mr. Proctor, "F.R.A.S.," "Omicron,'' or auy other competent astronomer, who would kindly help me to understand these matters. Etudiant.
MICROSCOPIC EXAMINATION OF ALE.
 Sie,—It may interest "John Barleycorn'' to hear that I devoted a few spare minutes the other even
THE DiDUCTORIUM.—IRON IN BATTERIES.
 Sir,—Your readers owe thanks to "Indnctorium"for his description, p. 472,| even th>ugh tho mode of construction described bo well known. I think. he would find it less trouble to pass the wire through tho melted paraffin while winding. It is very easily done by fitting a cover to the vessel, in which proper shaped holes or rollers are provided at extreme opposite sides; and also a frame carrying a roller, which goes to the bottom of the vessel, which is then placed as far away as is convenient over a regulated gas burner. He would find it on improvement, also, to increase the diameter of his discs gradually from the ends, the shape of tho coil being brought to that of the curves of the magnetic force generated by the primary.
In reply to his question, I have not particularly measured the vacnnru my Tate's pump gives; I only know that it has answered the purposes I have us«d. it for bettor than other forms.
In reply to Mr. Preston, p. 497, I think he is correct, as I have no doubt that the inner turns of the secondary are by far the most effective; as tbe outer ones increase in length, their resistance is increased, and the force is used up in overcoming it, and as the force itself diminishes, also, the useful effect is much lowered ; tho real objection togreat length is, of course, the increased primary and resistance to the battery, and, therefore, greater battery power is required.
Will Mr. Jarman, p. -147, say wherever ho got iron from containing 0"> per cent, carbon, as it would be a great curiosity, as it is about ten timos the Uëuoi proportion?
The figures of codt given by " Omega," p. 404, as comparing the Bunsen and Slater's iron coll, differ considerably from my results, but I am unable to compare them, because the conditions are unknown in his caso, and so much depends on tho actual strength of the acid, and accurate results can only bo obtained by working from Urn equivalents of the materials. I rather suspect "Omega's" nitric arid was weak, as this alone can explain the difference in the amount oi work if the measure capacity of tho cells was the samo. Of course, independent of local action, line will consume faster than iron, as its equivalent is 83*6 again* 38.
Another matter is that the comparison is not a fair one for another reason—viz.r that equal numbers of the cells are used; now 24 cells are very groatly in excès* of the number which can ho advantageously used to decompose water, or, more correctly, dilute acid; and as the electro-motive force of the Bansen is the highest, tlii? tells most against that battery.
With "Omega's "other remarks I entirely agree; and I may add that the electro-magnetic engine for use , with which the battery was mainly devised is a very excellent and powerful form. Siuxa.
EXTEAOTS ГЕ0М CORRESPONDENCE.
AN "ENGLISH MECHANIC" SOCIETY FOR LONDON,—"Tometer" says:—" I am much surprised and grieved to Uud that there are so little hopes of a Mechanics' Mutual Improvement Society bcmij established here in London. I should have thought there mast be subscribers enough to form a hundred societies .iuch as that proposed by a correspondent a few months Mx<j. I believe if a meeting were held there would be fbnty of attendants, but very few care to write, as Ilícito e em all to think 'there will be plenty of others write without me/ and, therefore, none write; so the thing drops. I really do hope something will bo done in référence to this important subject, and, I should say, the sooner tho better."
À8TTIONOMÏCAL NOTES.—I trust the "Fellow of the Royal Astronomical Society" will not be moved by Mr. Frith, or any one else, to falter in giving us his (kind and valued "Notes." Let him be assured that there are lots of quiet-going fellows like myself, who, though thtu map not express it as I do now, enjoy those *' Notes" particularly, and scan them always with keen appreciation.— Gimel.
MECHANICAL MOVEMENTS.—This part of our paper is very interesting, and I have no doubt when it is completed, it will go into the form of a book. True, the Enolisu Mechanic itself is a convenient "book," ы/.е (not too big), but still tho "Mechanical Movements" would form a nice neat volume; and some of tho more attractive items could be expanded by a system of notes, copious and explanatory, and a full iudex would (as ever) make the book complete,—
MIRAGE.—Mr. W. L. Carpenter, writing to Nature, mention* a mirage which occurred on tho coast ol Portugal, on the same day as the remarkable one seen in tho Firth of Forth, July 22 :—" A reference to my journal shows me that on the same day wo wero dredging on the Portuguese coast, withiu sight of the Ferilhoe and Berlinga Islands, about forty miles north of Lisbon. The bearings of these islands and their exact distance, calculated by the aid of the known height of tho lighthouse, gave us, of course, an exact position, which our 'dead reckoning' also confirmed. Several solar observations, both for latitude and longitude, were taken by two of the offlcors during the day, both of whom always arrived at the same result, but this was so widely different from our position as previously determined by two other methods, that we were forced to the conclusion that there was a very false horizon. It was the only instance of the kind during the inuuth I was at sea."
AN IMPROVEMENT IN GALVANIC BATTERIES.— Mr. W. Poole Lovison, of Cambridge, Mass., in a letter to the Journal of the Franklin Institute, says :—" In the upring uf 1869, while making use of a small bichromate of potash battery, I discovered that tho addition of nitric acid to tho mixture of potassic bichromate and sulphuric acid, contained in its porous пцн, oonforred upon it the virtue of steadiness, without involving tho evolution of annoy iug fumes. For over two months, during last summer, I had in almost constant action a combination oí twenty-three large Bunsen cells charged with dilute .sulphuric acid and the triple mixture mentioned, and 'setup* openly upon the floor of my room. Not only did I work about it with perfect comfort, hut left choice brass instruments in its immediate neighbourhood with impunity. Its cuergy never fluctuated, ^and after remaining for ваши time etondy, declined, precisely as if the electro-negative plate« were bathod in nitric acid only. To a cooled mixture of pótasele bichromate solution and sulphuric acid (perhaps preferably in atomic proportions) add nitric acid. The proportion of nitric acid may be greatly varied, as its office is merely to transfer oxygen.
FIXING PHOTOGRAPHIC PRINTS.—A correspondent of the British Journal oj' Photography gives the following method for fixing prints:—"Allow the prints to get saturated In a solution of hyposulphite of «oda, three ounces to the pint, and then well squeeze them. This two or three times repeated is quite sufficient. The prints, after the first pressing, on being separated, immediately absorba fresn supply of hypo, solution, as a sponge would do, which in its turn is eliminated by pressure, carrying with it all the salts of silver; and, as the pictures are such a short time (five or six minutos) in the fixing solution, the tone is not lowered iu the least. The prints at this stage have rather a curious wa^hcd-out appearance, caused by tho paper being semi-transparent; but they resume their brilliancy on drying, and are in the best possible condition for effectual washing. The advantages of this method are— n great saving of time, ditto of material, and an approximation to, if not perfect fixation."
COAL FORMATION OF THE LOFODEN ISLANDS
(NORWAY).—It appears that in the Island of Ando, one of the Lofoden group, a seam of ooal has been die-covered, which, in physical and other properties, bears • strong resemblance to the well-known boghead, but the thickness of the layer is irregular, and does not, as i.w as has been ascertained, exceed SOin. The diameter of this ooal basin ia about 2,300 metres.
EEPLIES TO QÏÏEEIES.
*0* I» their answers, Correspondents are respectfully requested to mentiou in each instance the title and number of tho query asked.
[3983.]—ENGINE INDICATING.—As nobody else appears to answer this query I will try-, thouyh only a learner myself. Belore you begin to adjust the indicator, see that it is perfectly clean, free fioin grit, and well oiled with tho best oil. Next fix your indicator to tho taps in the ends of the cylinder if pons i Ue, as the sides aro objectionable ; next gear your baud to some couvepiont part of the engine to get the required stroke for the Indicator. If a beam engine, the radios bar ; if a horizontal, yon must gear a lever from the- crosshead to the roof of tho engine-house; then fix your band towards the tap, so as to got about a Sin. stroke, more or less, to suit your Indicator; then carry your band towards the cylindcron small pulleys, over the indicator; then couplo your band, but be confident your band is long enough at tlrst, as you can shorten It with a running loop at pleasure. You must be careful and seo that your stroke is not too long, so as to cause the indicator barrel to stop at each end, in which case it would produce a defective figure. This done, wrap your paper round tho barrel, sharpen your pencil to a very fine point and let it touch the paper as lightly as possible, but before you take the figure see that your indicator is in working order by letting it work a few strokes.
Should the diagram, when taken, resemble tho above, it will be a very good one, there not being much room for improvement, as it approaches very near tho truo expansion curve. Then to proceed to measure it up, you must divide the length of the figure into ten equal parts, then measure through the centre of each division from the vacuum line towards tho atmospheric line, then from the atmospheric line to the steam line. Should it be a Richards indicator, it will be 161b. to the inch, the same as the above. Then add each side up, and divide by ten, the number of divisions in tho figure; then add the quotients together, and the product is the mean pressure per square inch throughout the stroke ; in the above case it is nearly 15jlb. To find tho horse-power, multiply the square inches on the piston by tho mean pressure per square, then by the number of feet the piston travels per minute ; then divide tho product by 83,000, the equivalent of one-horse power, which will give you the number of horse-power the engine is turning. "Hopkinson on the Indicator" is a very good book for explaining tho action of the slide-valve. "The Discussion as to whether High or Low Pressure Steam is preferable in point of Economy," with an appendix, by Edward Ingam, of Bancroft Mills, Oldham, and Bourne's "Catechism of the Steam Engine," likewise his handbook, are very good books.—W. W. H.
[4011.]—PRICKING ORGAN BARRELS.—" С W. A." gives what sooius a funny process for doing this. Now without pretending to know how it Is dono In practice, I remember seeing a description of the process In the Magasine о/ Science, oí which I think only three or four volumes ever appeared, nearly thirty years ago. It is thore stated that the barrel is mounted in its own frame, with a division plate attached, and is moved forward for each successive note, and a tap with a small hammer administered to the corresponding lever (or whatever it is called), which, having a sharpish spike underneath, leaves a mark on the barrel. If the note is a long one, then a mark is made at the beginning and another at the end of the note, and tho two dots joined with a pencil mark. The barrel is afterward« taken ont and a peg or a staple, long or short, as necessary, driven into every mark. I do not know how musical-box barrels are done, and believo that they are never, or at most, only rarely done in this country.—J. K. P.
[4181.j— SUN-DIAL.—Whatever serves as a gnomon (let the dial face how it may) must have an edge or edges parallel with the earth's axis, that is, In this hemisphere leaning from south to north, and in London 3$J from the vertical; moreover to be available throughout some hours, it must have its chief planes or sides facing due east and west. What "W. П. O." means, therefore, by " supposing it to be placed exactly opposite the point in the horizon where the sun first appears on June 21st," namely, something like 40 north of east, I cannot tell. Any object will cast no shadow on the longest day at noon, if its north side reclines (in Loudon) '¿H from the vertical, and its east and west do not overhang. The difference between " mean noon" (or twelve o'clock) and apparent noon (or twelve o'sun) being on that day only seventy seconds, will not affect the question, because there are always two full minutes during which neither the east nor west face casts a shallow, this time being takon in their turning through the half degree of the sun's diametor. It is not generally known that dials (and those of elegant forms) may easily bo made to show clock time, exact to a quarter minute, throughout the year; and even to a second (except in a few days of midwinter and fewer at midsummer) by having two gnomons, used alternately for nearly half a year each. I propose this problem to the ingenuity of souio of your readers.—E. L. G.
[4174.]—GEOLOGY.—Fossil plants from the coal measures grew at some period botween half a million and three hundred million years ago, but which of these dates may be the nearer the truth no means exist of guessing. A* the Cumberland strata dip in leaving that county, and are overlaid by others, these again by others, and those by others in succession, till, by the time we
reach London (not to say Ipswich), the aggregate thicknesses have summed up to about Un miles, and as the most rapidly shoaling parts of present sea-bottoms, the Gulf of Venice, or the mouths of great tropical rivers, are uot receiving a foot of sedúuent ¡ter annum, "Veritas" may reckon a minimum of time for the deposition of tuerten miles, bat only a minimum. The fished-up cable showed that the Atlantic is not depositing ha\f an inch oí its chalky mud per annum. Certain sandstones, thai there is much reason to believe are the quickest deposited strata, show laminations about forty in an inch, that were mo>t probably each the effect ¿I a twelve hour*' tide. Those are about all the data that geo-chronology has to theorize on.—E. L. G.
[4383.] — WARPED CABINET LID. — I /ear that "J.B." (the second ) will find that even if he succeeds in straightening his warped cabinet lid by "Cabinet Maker's" method it will warp again. Because As the part which he sicells bv damp in the manner proposed gets dry it will naturally shrink as beiore, and so twi«t as before. In order to prevent this, therefore, I would advise him if and when ho gets it straight by the mode prescribed, which would be helped by a few weights, to have grooves mado across the grain of the back of the lid to the depth of half its thickness, and in width and number in proportion to the size of the lid, and if into these he fits with glue and screws, slips of perfectly dry wood, I think he will find hie work likely to itand, which otherwise I doubt.—J. B.
[4361.]—METHYLATED SPIRIT.—This composition is not allowed to be used In the preparation of any medicine to be taken internally—i. e. to be drunk; and several parties havo been hoavilv fined for selling it flavoured with various essential oils no as to render it potable. It is prepared by mixing nine parts grain spirits, generally from 60 to 65 o.p., with ono part wood naphtha. Both its preparation and sale are hampered with stringent fiscal regulations. "T. L. H." has misinformed the querist in this ease. I have had less or more to do with methylated spirit ever since the act came in force—now thirteen years since.—A Rkvhnuu Ofpicbk.
[4380.] — COLZA OrL,—" Bernardin" is mlstakeu when ho says that col/.a oil is the same as rape oil. Colza Is extracted from tho seeds of Brassica olerácea or B. campestris oleífera, which yield from 36 to 40 per cent, of oil. Rape Is obtained from the seeds of several kinds of Brassica, but principally of B. пария, which yields about 33 per cent. (sp. gr. -9128), and B. rap<r, which yields a smaller quantity, but of a specific gravity of -9167. The specific gravity of the true colaa is -9136; but as might be expected, it is extensively adulterated. The mistake seems to. havo arisen from the Customs' returns classing all seed oils under the one head uf "rape oil."—Kküaf.
[4381.]—COURT PLASTER.—Tho following Is the method described by Dr. Paris :—Black silk is to be strained and brushed over ten or twelve times with the following preparation :—Dissolve $oz. of balsam of benzoin in 6oz. of rectified spirits of wine, and in a separate vessel dissolve Ioe. of isingglass Iu as little water as possible Strain each solution, pour together, and let the mixturo rest, so that any undissolved parts may subside. When tho liquid is cold, it forms a jelly, which is warmed and applied to the silk. When the silk coated with this mixture is quite dry, it should receive a coat of a solution of 6oz. of tincture of benzoin and 4oz. of chian turpentine, to prevent its i cracking.—H. U.
]4382.] COPPER COINS.—The first coin named by "Hastings," (quory 4882, page 454), Is one of Faustina junr., empress of Rome ; if the hair Is put up something after the way of the present chignon; if not so, one of Faustima sen. The second coin is one of Hadrian or of Vespasian, emperor of Rome ; if the head on the coin Is very large It Is ono of the latter emperor, If not so, one of the former. Thev are called first or large brass,— W. J. Eooleston, Dewsbnry.
[438S-] THREE COINS.—The first of the three coins described by S. S. Wright is a shilling of Eliiabeth ; the othor two are 17th century tokens, struck In Antrim and London, by R. Young and J. Evin, during scarcity of copper coins In the reign of Charles the First ; none of them are worth much above sixpence over their Intrinsic value.—W. J. Eqglebton, Dewebury.
[4384.] COPPER COIN.—The copper coin of " W. B." engraved In query 4884, is probably a very early British or Gaulish coin, too early to be assigned to any ruler.— W. J. Egöleston, Dewsbnry.
[4396.]—ELECTROS.—I am sorry "Faber" does not find my composition suit. I think he is sparing of the rotten stone, but remember I had failures in casting off metal of large diameter, as tho stuff stuck in deep places. I do not remember having over much trouble in Ыяскleadlng. I suppose there is blacklcad and black lead at the shops. The recipe was given me by a relative M.D., F.R.S., who got it from a professional, but so long ago (1842), that I do not remember quite all about Us succès» or otherwise. I do rocollect that I soon gave up electrotyping, and that some of my early casts were very good. I think tho stuff is all tho worse for remeltiug repeatedly, at least my impression is that I was more successful at first than afterwards.^!. K. P.
[4397.]—SCREW TOOLS.—I am sorry not quite to understand the last words of " Faber's" query (p. -455). The words "and in top,'' appear a misprint. I suppose the only way to mako an Inside scrow tool from a screw plate is to screw In a piece of steel, as In making u tap, and then filing it thin by removing half the material from the top side and one-quarter more from undeside, and ail tho thread, and more than that, say half the breadth, from tho back, to form an ordinary screw tool. I have made an inside screw tool—vU., for a nut lain, diameter and ¿In. pitch, by flattening the end of a steol bar gin. square, and drawing it out on one side just like the "bit" of a key, and then cent о ring it at both ends, and treating it as if it was a bar, with an outside screw to be out on it in tho ordinary way, and it acted perfectly ; really could not have been wished to be better. I may as well mention here, as many a workman (socalled) Is not awaro of it, that inside screw tools should bo cut on a left-handed /too, and I have seen many from first-rate shop* that were not done so. Also, I cannot see what "Faber" can want with screw tools во small as 4in., as the Internal screws are so much more easily tapped,and I think any pitch whatever, tip to about 30 or 35 to an inch, is kept In stork at Back's, besides some finer onee, so that be can hardly be obliged to make his own. I do always make mine, having formerly been to the trouble (and very great, too) of making a set of hobs, and think nothing of making more if requisite, as I want them, but in those days aliquot pitches were not to be had at the shops.—J. K. P.
[4399.]—INTERNAL GEARING.—I do not much approve of this plan in a general way, but to anyone intending to use it I would remark that the pinions should be as large as сап be got in. Also, that internally geared wheels are commonly cut with a circular cutter in a frame specially made for the purpose, and are attached afterwards to the plate or rim of wheel that they belong to; of course it is difficult to make small ones this way. "G. W. A." has very nearly hit upon the best plan of making an internal wheel, when he says, " drill a series of sixty holes, 4c.," but the chipping " away neatly" is the difficult part. The way to get over this is to have a driller similar to mine figured November 12, last year, (and which you must have in order to »lot drill any part of the teeth any way), mounted on a plate attached to the slide-rest, so as to be itself on the radius of the work, and slot drill the teeth by traversing to and fro, not on the line of the radius of work, as "G. W. A." proposes, but parallel to axis of lathe mandrel and lengthwise of the teeth. The tool referred to will work comfortably inside a wheel of »[in. diameter, and another I have intended for a different purpose would go inside а -l *. ítj. wheel. Of course a drill or "grinder" made to the right shape is required. All this is not easy to describe without a drawing, and I cannot stop to make one now, but will give more particulars if called on to do so.—J. K. P.
[440ft.J — TELESCOPES.—Most telescopes have the screw parts cut so as to act as springs upon and retain the sliding tubes in any position. If " G. F.'s " telescope is so made a few slight blows with a hammer on the beak of an anvil, properly applied, will set these springs in and make them act more strongly; if, however, there are no such springs, and there are no means of cutting or making them, a ring of thin kid, fastened with shellac, and oiled, will give the required etiffness of draw.—J. B.
[4409.]—FIXING IRON CRAMPS. — " J. W." will readily fix his cramps or other ironwork in stone by the use of Roman cement and bits of scrap-iron driven tightly with punches into the hole to jam tho cramp. When the cement is set it is quite as strong as lead.— J. B.
[4410.]— TURNING SPHERES.—Q. Yorke advocates boxwood for chucks. I do not. Maple is much better wood for the purpose. Nearly all our chucks are made of the bottom ends «f maple poles, and last for years. If "Young Amateur" finishes one side of his backgammon men before he cuts it off the cylinder he will have to chuck once instead of twice. J. D. L.'' had better get a practical workman to turn his billiard balls, as I can assure him it is no easy job to turn a perfect sphere. He will only spoil his ivory.—Wood Spoon.
[44I2.J—WRITING INK.—Let " J. G. M." put two or three drops of carbolic acid into his inkstand, or about thirty to the pint of ink. The ink cannot then go mouldy, nor will this addition spoil the ink.—Spero.
[4412,1— WRITING INK.—"J. G. M.";will probably find that a small piece of camphor placed in his ink bottle will prevent the growth of the " mould " fuugus.— H. P.
[4412.]—WRITING INK.—A few clove? (say two or three) put into an ordinary-sized inkstand will prevent it from going mouldy and also preserve its liquidity.— Wahsrof.
[4414.] —HEIGHT OF OBJECT.—If " Enquiror " will take two tubes and connect them in such a fashion as ahall enable him to case them to form £_ or any angle, he will provide himself with an instrument which will enable him to get approximately the height of any object, or width of any river. He may use it thus:— Suppose he stand« on the brink of a river, the width of which he wishes to know, let him measure on his side a given distance in paces right and left of some tree or building on the opposite bank. From each end of his measured base-lino let him view the tree, keeping one tube parallel with his base line. This will necessitate his adjusting the other tube to a certain angle with re
§ard to the other one. He must register those—easily one by a piece of string fastened midway between the ende of each tube ; then turning his face landwards, lot him describe a precisely similar triangle, which will be pretty nearly an equilateral one—if the process be accurately gone through, it will be quite—and measure the distance from its apex to base. This will give the width of the river. Or he may measure the angle in degrees, and reproduce the triangle on paper to scale, and work the problem out at home. Some years since, I interested some school-boys much by putting them up to the dodge. I hope it may be useful to " Enquirer."—
[4415.]—CURIOUS PROBLEM.—" Saul Byrne a " asks us, in page 478, What would be the result if an irresistible force came in contact with an immovable body? In the first place, I should like to know how this gentleman proposes to define the expression, an " irresistible force ;" at what point does anything obtain the property of being "irresistible?" Secondly, What is an immovable body? All bodies with which we arc at present acquainted are both movable and moving.—S. A. R.
[4415.]—CURIOUS PROBLEM.—Very! An " irresistible "force comes into contact with an "immovable" body—well. Why, sir, it would be a case of the Kilkenny cats—only there would be no tale to tell.—H. P.
[4415.]—CURIOC8 PROBLEM.—If, as I presume, "Saul Rymea " supposes his " immovable body" to be also "perfectly hard," the force with which tho H irresistible," by which I understand "perfectly hard" body would impinge npon it would be infinite, and therefore impossible in nature and inconceivable to the human mind ; under such circumstances, therefore, his question is unanswerable. But if either of the two bodies bo capable in tho slightest degree of alteration of form by impact tho " energy" is expended in producing such alteration.— J.B.
[4416.]—PHOTOGRAPHICAL.— Tost your bath with litmus paper,if acid, add a few drops of amimnia until
it is neutral; then test the strength and make it up to 60 or 60 grains per ounce. Take care that you do not float the paper too long; one minute is quite long enough for the present warm weather. I do not float longer than 30 seconds in summer with the sample of paper I use. I should recommend " John" to change the paper, as it is a very bad sample if it discolours the bath to the extent he says. "John" must also take care that the paper is perfectly dry before printing it. By attention to the above points he ought to succeed; if not write again and give particulars of make of paper, strength of bath, description of toning bath, Ac. Regarding brown negatives he must give more particulars before I can recommend a cure, but I fancy he has been messing his bath until it is useless.—Operator.
[4416.]—PHOTOGRAPHICAL.—"John" may flrM advantage in adding a little, or perhaps a good deal of fresh silver to his solution; 60 grains to the ounce of water is the thing, and with this strength three minutes will sensitize the paper. Also make a new toning bath as follows :—2 grains bicarbonate of soda (common bread soda), 2 grains gold, 802. water. This will give a good and rapid tone. When the excitingbath becomes brown, the remedy is kaolin. Blend a very small quantity, just what will muddy the bath, and filter. The quantity of albumeuized paper has a good deal to do with brown baths. Try some other house for papor. Are you buying cheap choiniculs or dealing in a second-rate house? Don't do either.—Paddy.
[4418.]—SILVER COIN.—In answer to " A Young Beginner," I beg to inform him that his coin is a Roman denarius of the family of Antonia. It is one of a series struck bv Mark Antony in honour of his lésions. They bear: Obverse, a galley and rowers. ANT. AVG. III. VIR. R. P. С (for Antonius Augur Triumvir Rei Publicae Constituendae; Antony Augur Triumvir, for the reestablishing of the Republic). Reverse, three military standards, the centre one surmounted by the Roman eagle. LEG. (for Ligio, Legion), and the number. Tho first legion has LEG. PRL (very rare); the 2nd, LEG. II.; the 3rd, III. ; the 4th, IUI. or IV. ; the 5th. V. ; tho 6th, VI. ; the 7th, VII. ; the 8th, VIII. ; the 9th, Villi, or IX.; the 10th, X. ; the 11th, XI. ; the 12th, LEG. XII., or LEG. XII. ANTIQVAE ; the 13th, XIII.; tho 14th XIIII., or XIV.; the 15th, XV.; tho 16th, XVL ; the 17th, LEG. XVII. or LEG. XVII. CLA8SICAE; the 18th, XVIII. orXIIX. or LEG. XVIII. LIBVCAE; thel9th, XVIIIt. or XtX. ; the 20th, XX.; the 21st, XXI.; th e 22nd, XXII. ; the 23rd,
diaphragm of copper plate pierced with numerous small holes, and the apparatus, for the sake of convenience, is usually arranged in two columns :—1, the analyser 2, the reetifier. It is worked by steam at a pressjtf of 51b. or 61b. to the square inch and produced In au ordinary boiler. When the analyzer Is under full steam preseure the wash is introduced at the tap through а pipe, to be afterwards referred to, and flows In a continuous stream through the holes in the diaphragms, meeting in its passage with the steam, which gradually raises it to the boiling point and causes it to part with the spirit which it contains. The steam, carrying with it the impure spirit, is next conveyed by means of a close pipe to the bottom of the rectifier. This vessel is traversed by another pipe whose convolution* are arranged horizontally throughont its entire length, one end being connected with the wash reservoir and the other with the top of the analyzer. The wash, in the first instance, runs through this pipe to the analyzer, and, being cold, causes the watery particles of the mixed steam and spirit to condense upom the outside of the pipe, whilst the vapour, richer in alcohol, rises from one compartment to tho other, gradually parting with the wator until it reaches the "spirit plate ; " here it is collected, and contains about 95 per cent, absolute alcohol.—A Revenue Officer.
[4427.1 —CASE HARDENING.—If "W. H. B." will make his cylinder rod hot and then plunge it in, or cover it with powdered prussiate of potash, ho will find it hard enough to resist filing.—Semper Par At Us,
[4427.1-CASE HARDENING.—I do not think if you case harden with prussiate of potash, in plenty, that your work would scale in the fire. But it must be croinut smooth afterwards, and what will a little scale signify then? If it is of iron a little hardening won't hurt it perhaps, but it should be of steel, and east sttel, not shear. I once made one of the latter, and will never do another. If of cast steel, as it should be, and well fitted . it need not be hardened at all. It is not like a mandrel that is everlasting by running with heavy preseure os it. It never does move with any stress on it except whea drilling, and very little then, and at all other times is. held fast by its set-screw. I would rather fit three double-bearing mandrels than one hardened back centre cylinder.—J. К. P.
[4432.1—ISOMETRICAL.—This is returning to first principles. Fig. 1 represents a cube of lin. in iaonie
ХХШ. ¡theiith, XXIV. ; the 25th. XXV.; and the last, the 26th, XXVI. One has CHORTIVM PRAETORIARVM, and another has CHORTIS SPECVLATORVM. Legions If. to XXIII., are common, and worth 2s. or 3s. each. —henry W. Hexfrey, M. N. S., Ac, &c.
"я- " also answers this.
[4423.]—PHOTOGRAPHY,—"John" and "J. B." both complain of bad tone to their photos. "J.B." should not try to get his tones too black, or they become cold and raw. I like a nice purple tint and always get it with my formula, which thoy will find iu some of the back numbers of the English Mechanic, and to which I dare say many of our readers can testify. The reasons of the brown, leathery appearance may be many. There may not be enough silver iu the paper, or gold iu the toning bath, or too much sodium, or a badly salted paper, or under-exposed print. Old nitrate of silver bath* are better than new if treated as follows:—Pour the bath Into a large-mouthed bottle, thou take about lgr. of pure permanganate of potash in los. of water, aud pour in (a drop at a time) evervthreeor four minutes. The reason for not pouring it alliu at once is, because then it would turn nearly all the silver ¡n your bath into permanganate of silver. Now shake this well and lot settle. When you see the bath has a slight pinkish tone you may be sure that all organic matter is expelled the oath; place it in strong sunlight for about a week or in diffused light for a fortnight. There will be a precipitate ; decant or filter (always keeping the bottle steady). This done, add fresh bath to make up for loss, at the rate of OOgrs. per ounce for positive, and 50grs. per ounce for negative. Now test your bath for sensitiveness; if not sufficiently sensitive add more silver, always weighing. In fact, weigh everything. "John " need not bo at all surprised at the bath discoburmg after a dozen sheets, when I tell him that the earn1? thing has occurred to me after two sheets. All that has to be done is to filtor the bath through kaolin and add silver to make up for losi. What does "John " want with Mack negatives? Nothing can bo finer than a iznodbroim negative, always providing, of course, the shadows are clear. And now let me once more advise them to look at my descriptions of formula in back numbers, and success to them and all other fellow-workers in the "black art." If they have uot enough, let them "ask for more," like Oliver Twist.— Mus.
[4124.] — COFFEY'S APPARATUS FOR DISTILLATION.—This apparatus differs in construction from all ordinary forms of still, for by its uso a pure marketable spirit can be produced from the fermented wash by one distillation. It consists of a number of rectangular frames of wood about a foot each in depth, and of an area proportionate to the quantity of wash to be distilled in a given time. These are suparimposel horizontally upon each other, each being furnished with а
"A" Q P
tri cal perspective with a circle on each face. As the lines OD, OB, OA, are equal and equally inclined to the paper und to each other they are contained by a circle (dotted). Fig. 2 is the side elevation of the cube. The way to get the angle of inclination of A N to the horizontal is to construct a triangle A N M, whose sides are in tho^ropertion of 1, ^¿T v'eTrespectively, as that is the ratio of the lines, A D, A N, and D N; aud the way to do this is as follows :— take M N — lin- *nd make M Q = M N; then join Q N and make MA = QN, and join A Nt
then, О N = VM Ni + МО» = */S¡7
and, A N = */A M* + M N3 = ■/£"
and, therefore, л A M N is similar to л О A N, and the
angle NAM = angle О N A. Now to find the pro
Sortions of the projocted circles. As every circle may e supposed surrounded by a square, the side of square obviously equals the diameter of the circle, or JF = О В, or, that is to say, J F is the measured diameter of the circle to be projected. Measure off A P = J F, and draw P R perpendicular, then G К = P R and SL = A R, and this holds good for any sized circle, as they are all exactly in the same proportion. Of course an isometrical drawing comes out larger than a perspective one, as the vanishing lines are not foreshortened, but exhibited of their full length, and the amount of this iu
JS 1-782 creaseis j^ = r^j = 1225, or 22i percent, larger in
lineal dimension. I do not give the proof of the last two sentences, as any one may easily make it out for himself, with a slight knowledge of geometry and of the principles of projection.—J. K. P.
[44ЗД— MARKING COTTON CLOTH.—If "B. D. F." will get somo ordinary, but clean, gas tar, and pat it into au iron pan, and slowly drive off by heat the gas remaining, he may then apply it to marking goods of any sort by a stamp or otherwise, aud it will resist all attempts at discharge by acid or alkali, bat it will not show white. (This is used by all bleachers and dyers in Lancashire.)—Wajisrof.
Г4131.]— TAPS AND DIES.—TO W. REED.—A lefthanded male screw may be made from a right-handed tap, with a specially made screw-stock or equivalent contrivance iu which the blank tobe screwed is steadied) on one side by a blank die, and has the teeth of a righthanded tap forced against it on the other. It is not worth doing, as it cannot be done well, aud there are other and cheaper ways of getting a screw made if It requires doing decently. Workmen are, or used to be, supposed clever, if tbey knew this trick. I accidentally/ struck a triple thread with ordinary dies a short timo since, but then I was using them only for the purpose of