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judge by is the movement observable among the stars, aud, as the stars are themselves in motion, it is not easy to separar-e the effects of their own motions from those due to theenn's.

Still, this problem has been attacked and fairly vanquished by astronomy. II is true we tanaot here get certainty, because we no longer have absolutely certain relations to deal with. But, as it is certain that, supposing the sun to be moving in a definite direction, there would result a balance ot stellar motions in the contrary direction; we can obviously deduce the direction of the sun's motion, by noticing the direction towards which the stars show the most noteworthy balance of motion. This has been done, and in many ways. The result is that astronomers point confidently to the constellation Hercules and its neighbourhood, as inclnding the point towards which the sun is speeding. His rate of motion his been estimated at about 100,000,000 miles per мшит, or, hetweou four aud five miles per second.

I am only concerned here with the sun's proper motion, in so far as it relates to the earth's motions. The conception that the snu may be speeding through space with this enormous velocity seems, at first eight, so contrary to the fundamental hypotheses of modern astronomy, that many persons have been led to believe there isa reil contradiction. They sec that the earth's real path iu space can no longer be looked upon as a closed curve, yet Ko;.ler speaks of it as an ellipse; that, again, the earth, according to this new idea, is not trave'ling in a conic section, yet Newton says, all bodies under the influence of a central attracting force, such ns gravity, must move in some conic section or oilier.

In reality, however, there is no difficulty. Modern astronomy only requires the earth and pianola to movo in ellipses »t-tiA reaped to the miu, nor in space. Suppo-ing the sun at rest, and the planets moving in their present ellipses around him, and that then under the action of far distant funs, our sun began to travel through space, the plsn'ts would not (ла Mr. Eeddie tear-), be left behin I. They would be under the influence of those attractions as well as the sun. They would obey them then, precisely as ho doee, obeying all his influences as before. Thus their motions, with respect to him, would not be changed even by a hair's breadth.

Conclusion.

And here I draw these papers to an end. They have occupied mm h moro space than I contemplated when I began, and yet each part of my subject has been dealt with far less completely than I could have desired. The fact is, a hook, and no small one, is wanted to present at due length all the argumenta which enforce upon astronomers the assurance that tho Newtonian system is correct.

Wonld snch a book be a desirable addition to modern astronomical literature? I think not.

I beHeve that all sensible persons, even if, not capable of fully mastering the evidence in favour of modern astronomy, feel yet no doubts as to its truth. For them, then, snch a book would be of little service. For the paradoxists—dividing them into two wcil-marked parties, the knaies and the fools—what can be done? One cannot make the blind see, let one talk ever so fluently ab out light; and those who, having sight, refuse to бее, arc blinder than the blind, according to tho true old proverb. Towards the paradoxists, then, of either »rder, one should act as Sir John Herschel is said to have done, when a rampant one told him he hid squared the circle. "Indeed," said the greatest living astronomer, raising his hat, "then, sir, I must wish yon good morning."

But while a sot treitise, in defence of modern astronomy, would be a waste of labour, I believe, aud hope, the matter I have just completed may have been of nio to many readers of this journal. The greater proportion of them, those in fnct for whom the English Mech Amc is specially iut-nded, luve not tine for a thorough investigation of the subject, yet possess fie power of appreciating to its fullest extent the argument by which modern astronomy has been established. Knowing for whom I have written, 1 have not feared to be dry whero my subject needed close and consecutive attention.

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has had a prize medal awarded to it, and is known as Admiral Fitzroy's prize medal barometer. But although perfect in itself as an indicator, and although clearly comprehended by those who are accustomed to the reading of such instruments, experience shows that to make it universally useful, its readings must be rendeied in clear and definite language. Its price is within reach of all, and it only wants to be made intelligible to the most ordinary observer. To meet this requirement, Messrs. Davis & Co. have improved the apparut s in the manner shown in our engraving. Their Fitzroy barometer forms the base of the new instrument; this consist« of the mercury tube with the cistern in one pieco, which is enclosed in an oak case. On the lower part of the case is a thermometer, und opposite to this are diagrams of Admiral Ht'.roy's storm warning signale. These a*e surmou ited by tables ex plaining their m-aning. T la novelty is in the upper portion of the tarom-ter, which will be I tee о to terminate in a circ ihr dial about 10

inches in diameter, la tho centre of this dial is the ordinary standard barometer scale, with its two verniers for taking the readings. These verniers are set to the rise or fall of the mercury by means of a rack and pinion arrangement, worked by the two keys Been just below the dial. So far, we have the ordinary arrangement for taking the readings; but as this is not always clearly understood, Messrs. Davis have supplemented it by placing on the right side of the dial two columns of remarks which refer to changes in the weather, one column ahowing the winter, and the other column, the summer changes. On the left hand of the dial these two columns are repeated, and are intended to record the weather of tho previous day. The indications areeffected bva simple and ingenious arrangement in connection with the verniers. On looking at the engraving, it will be seen that there are two hands or pointers radiating from the centre of the dial. These pointers are centred loosely on a pin fixed behind the tube, and each passes through a slot cut in the verniers, which thus hold them up. Now when the verniers are raised or depressed, the hande must follow the same direction; and according as the verniers are placed at the height of the mercury, ao the pointers atop opposite to a statement of the weather that height indicates. The right-hand pointer is set to show the weather expected to-day, whilst the left-hand is set to record the etat« of the weather yesterday. The mechanism of the instrument is exceedinglysimple, and is not liable to get out of order.

PROCEEDINGS OF THE ROYAL

ASTRONOMICAL SOCIETY.—Mab.. 11,1870.

W. Lassell, Esq., F.R.S. In The Chaib.

(SPECIAL B.EPORT.)

AFTER the minutes of the preceding meeting had been read and coufirmed, the first paper communicated was from W. Marth, Esq., containing observations of position angles, aud apparent distances of the Satellites of Uraans from the primary, which was ordered to be printed forthwith. Lieutenant Herschel communicated, through his brother, a paper remarking on a curious appearance, as seen on the Sun's di«c. The author remarked that in October last, while preparing to observe the prominences of the Sun will a spectroscope, an unusual [phenomenon presented itself, consisting, apparently, of a train of dark shadows and light streaks on the solar disc, which were at first thought to be sparks in the tube of the telescope, but tho permanence of the phenomena did not permit of this explanation. The possibility of this extraordinary appearance being occasioned by the transit of a system of meteors, next occurred to tho observer, and preparations were made to observe the circumstances of the transit with all the care possible. The image of the disc of the Sun was thrown on я screen, and the equatoreal telescope set in motion. The apparent size of the shadows was shown on tho screen, and they were observed all that day. On looking at the Sun the nexi day,the same phenomenon was observed; the objects were still seen passing in a continuous stream. The direction was 150° E. by N., or perhaps two streams not very far distant. The dark objects were out of focus when tho Sun was in focus, and were still seen when the Sun was partly obscured by cloud, and notwithstanding the Sun's light they were seen after they left thedisc ; and the smaller they were, the less was their velocity. The direction of motion of one only wa- retrograde-, and one entered slowly, and then changed its direction. Every care was taken to notice overy pec nliarity presented, till the observer convinced himse If that he was watching the motion of a number of locusts, passing in continuous succession between the instrument and the Sun, and however curious tho phenomenon might be in natural history, it was clearly not of an astronomical character. The author endeavoured to account for some of the peculiarities seen in America at the t'me of the Eclipse last year, by referring them to tho same cause.

Mr. Stone remarked on the necessity of » change of focus, in attempting to explain peculiarity of this niturc, which w.s the means of effectually settling the question.

Captain NOBLE remarked that in a prcvl number of the "Notices," would be found tho! marks of Mr. Reed, on a similar question, whose' feSJTWcoQ'* lei Mr. Dawets to focus for the wcllkaown "thistle seed."

CtpZMia Noble communicated three short

papers to the Society. One on occultations of

^am by the Mooa; an observation of Venus, near

her inferior conjunction ; and an observation of

the Zodiacal Light. In the seoond paper Capt.

Noble said that the state of the sky prevented

him seeing th« planet till abont two hours after

her conjunction, when he saw a very narrow

thread of tight, not quite Bemi-circular, and saw

■the body of the planet projected on the sky beyond,

sensibly darker than the surrounding sky. On

the 3rd March, 7h. 40m., the author saw the most

brilliant exhibition of the Zodiacal Light that

he ever witnessed in this country. Besides its

brilliancy, two peculiarities were noticed, namely.

that it was very nearly perpendicular to the

horizon, and the axis inclined to the ecliptic at an

angle of 20°.

Mr. Browsing exhibited some drawings of Jupiter, and remarked upon the many changes of form and colour of the belti of Jupiter, the principal of which seenu to hare been a sensible -widening of the Eqiiatoreal belt to nearly doable iU former dimensions. Mr. Browning invited the attention of observers to a consideration of the Planet, which the peculiarities it presented rendered interesting at this moment.

The President seemed to suspect some Blight exaggeration of the colour and outline of the Equatoreal belt, but Mr. Browning defended his drawings, which appear to have been carefully made.

The last paper was from Mr. Proctor, having reference to the Corona seen in total eclipses of the Sun. Mr. Proctor said that at the next eclipse of the Sun, which occurs next December, in Spain, the attention of astronomers wonld be <lrawn to the Corona with a view of explaining the difficulties that it now presents. The auihir of the paper contended that the phenomenon witnessed conld not be owing to terrestrial causes, for in cases of total eclipse the Moon is seen projected on the corona, which could not be if the light wu occasioned by the Earth's atmosphere. He added that in an annular eclipse, the centre of the Moon has been seen darker than the edge; Ukii that in partial eclipses the edge of the disc has been seen projected (dark) on the background of the sky, as often ss the observer hss looked for this particular, which would seem to disprove the existence of an atmos-1

Sheric glare. Mr. Proctor argued from Captain oble's remark concerning the visibility of the dark part of Venus when in the neighbourhood of the Sun, the existence of a continual brilliancy ■which might be owing to the richer part of the Zodiacal Light, which increases in brilliancy as it approaches the Sun, and in the olose neighbourhood. oE the Sun itself doubtless merges into the Corona. In concluding his remark*, the author conjectured that the Zodiacal Light itself was the collection of the richer portion of the meteorio streams, perhaps incandescent, where near to the Sun. He remarked that numerous meteor streams must exist, since the Earth meets with 56 of these systems, and that if the Great Comet of ISC3 was followed by a train of meteors similar to that that accompanies Tempers, the near approach of these to the San must produce an effect which ctn easily be supposed to resemble that witnessed where the Corona is brightest. Mr. Proctor hopes that the Society will form a committee to censider the best ireais of setting at rest the vexed question of the Corona, and made some suggestions which be thinks likely to produce that end.

It was asked at the meeting if the Government were proposing to render any assistance to astronomers that intended to go to Spain to view the Eclipse of the Snn next December?

The PRBaiDEJTT replied that the Government certainly had not propoted any such measure, and were not likely to, but that the Council were now discussing the mode of application to Government, and the best way to derive results that were likely to follow from the observation.

Mr. Stone observed that he hoped that three observers might make observations on the polarised light, as two wonld probably differ in their results, and the observations of the third would be needed to settle the question. If anvthing had appeared to be settled by the Indian" Eclipse, it was the question of the polarised light, which the observers agreed to hive been polarised in a piano passing through the centre of the Sun, but the •American observations had again opened the

question. Mr. Stone hoped that the observers would not confine their observations to any particular locality in the Corona, but examine all positions of it.

MODERN CHEMICAL NOTATION.

By Da. Fssd. Ht'lTss.

(Concluded.)

Whin the atomicity »r basicity of ths clement should be expressed by the symbol, then the letter ii provided with another figure at the top, but this figure mint be a Latin one. ,The sign On means accordingly that one atom of oxygen possesses two equivalents, or better, that one atom of oxygen can combine with two atoms of a monogenona element. Tliis4atter way of expression is the more general one. When a number of atoms of the same element form one group, then the Latin number at the top is the expression for the capacity of saturation of that group, or, as it is also termed, fot the atomicity of the group, or still otherwise, for the qnantivalence of that group of atoms. In /this way we write Osii or Mn,vi, and understand by it, that 6 atoms of oxygen can combsse with only 2 atoms of a monogenona element, that 2 atoms of mangaaium can combine with « atoms of a monogenous element.

The same rnles apply to groups of atoms of different elements-for example, COn indicates that one atom of carbon is combined with one atom of oxygen, and that this group is able to combine with two atoms of a monogenous element.

When, however, a formula should be quite a rational one and express the function of each equivalent of the various elements forming the compound, then the letters and figures are not always sufficient, and we have to enrich the chemical alphabet with a few moro signs. Instead of using a Raman figure at the head of the letter to indicate how many atoms of a monogenona element the element represented may combine with, horixontal or radiating strokes are nsed, as many in number as the Latin figure indicates. Thus one often finds <>~ instead of On, or

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However, the letters, and, if necessary, the horizontal strokes, are quite sufficient to make the rational formula of a substance perfectly clear. It is, as a chemist of high standing said, perfectly useless to attempt to give any other representative to the atom than its letter. Any drawing cannot but lead to too mechanical a view of the constitution, and wc see iu the present periodicals plenty of proof of the nonsense which such mechanieal views, when used to too great an extent, will bring forth.

As the whole system of notation will become clearer by citing a few examples and discussing them, we append here an assortment of modern formulas and some explanatory notes as the conclusion of this paper.

Elder formula.

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The shove formulas are sufficient to show us the use of the chemical alphabet.

The first of these formula; as well as the second and third need no comment. Potassic sulphate is represented by a symbol which is intended to convey an idea of the manner in which the single atoms are connected The sulphur being tetravalent is connected with two atoms of oxygen, which are (if the compound must be saturated) again combined among themselves. Thus the groap SO, has still two equivalents of sulphur left free to attract other atoms, which we find to bo oxygen atoms, each eombined with an atom of potassium. The same formula would apply to hydro-potassic sulphate in one atom of potassium is replaced by one atom of sodium. The group SO, we find in formula (7) representing alum to occur 4 times. Alum is a sulphate like hydropotassic sulphate. Each SO, is connected by 2 atoms of oxygen on the one side to the aluminium, on the other to either hydrogen or potassium. The metal aluminium has. like all metals of the same group, the property of occuiring as a group ortwo atoms, which by their combination saturates S of the B equivalents or which they naturally consist. The two aluminium atoms are directly united with one atom of 0**Ken; saturating two more equivalents or aluminium, so that at last we have the greup Al,0 being tetravalent. This group replaces four atoms of hydrogen in four molecules of sulphuric acid, forming the compound 9

4 SO, j"' *n which finally two atonn of potassium become substituted 'or two of the hydrogen atoms.

■ The sign I is a remainder or the theory of types; it now is used to indicate that tho two parts which it connects art held together by the elements it points to. H J O mean that the two atoms of hydrogen are held together by one atom of oxygen. In organic chemistry this modern notation is invaluable. Inorganic chemistry, however, can be taught just as well by means of the old notation. Still the funner has its peculiar advantagos, and will no doubt help to further the knowledge in that department as it has done in organic chemistry, where the constitution of bodies is so well studied that we can deduct almost every property from the formula. We can, with a certain degree of accuracy predict the properties of a substance from its formula in inorganic chemistry, but not to the same extent as it is possible in organic chemistry. Here we derive from the formula tho specific gruvity of a liquor at boiling point, the boiling point itself. A well written formula tells us how many hydrogen atoms can be exchaaged for a metallic atom, or for acid ndicals. It tells u> at once whether the substance has more the properties of an alcahol, or of an- acid, or of a base. It tells u», as we havo discussed in our last paper, the specific gravity of the gaseous state and the quantity of heat necessary.to raise the substance's temperature one degree.

It is evident that the chemical formula will not be perfect until it represents all the properties of matter as it does now a few. And when chemistry will make as rapid progress in the next twenty years as it has done in the last twenty we shall soon arrive at that perfection of our knowledge which Berthollet thought to have arrived at some 7" years ago when he published his " Kssai de Statique Chimique." • 1 Hoping that I may have helped sonic of the readers of tho E.-vqlisu Mechanic to an understanding of the leading principles of the modern theory, 1 conclude my paper, asking the reader's indulgence for my want of style. I leel that I have not the English language sutliciently at command to expose my ideas always so clear as 1 should Uke to have done.

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EMIGRATION.—Professor Goldwin Smith writes to a friend In England a letter on emigration as a, means of escaping the economical difficulties and perils which he fears are gathering round England. Tho professor commends the Southern Stales of America, especially V irgiuia, as better north tho attention of emigrating Englishmen than eitbor Canada or the Northern States. Ho Buys, the Southern States are more English than any othor portion of America, and that Englishmen in those Slates would find both a gonerous welcome and an ample Hold for their euter

Sri8e,nelther of which are to be so readily found in the ortheru Slates, where Englishmen do not and cannot lratcrnlse with tho Irish, German, or native American population. "In the Northern States" nays Mr. Smith, "the English emigrant now finds himself, I nin sorry to say, not only in a foreign but n hostllo country. And as a practioal and likely means of securing a solid home in Virginia, he mentions that there is a Virginian Land Company iu Now York, presided over by i.eneral imboden, ono of tho Confederate Commanders—that he ban had communication with the company nud seen its papers, and that we are likely to hear of ifs movements in England. It should be added that he intimates that English prcdelections for Canada are a mistake, both on tho grounds of climate and other natural and agricultural difficulties in comparison with Virginia and other Southern States.

ROYAL NATIONAL LIFEBOAT INSTITUTION. —The annual meeting was held on March 16. Tho report stated that there wore 220 lifeboats on tho coasts, and that duriug the pa-t year 871 lives had been saved through the Institution's agoucy. I no pum« voted as rewards hail amounted to £2705, and .s« silver medals had been granted. Tbo income, during the twelve months, barf exceeded £J0,ouO. and there was a balauco in the society's favour of upwards or £6000.

10

ENGLISH MECHANIC AND MIRROR OF SCIENCE.

[march 25, 1870.

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LETTERS TO THE EDITOR.

[We do net hold ourselves responsible for the opinions of our correspondents. The Editor respectfully request« that nil communications should be drawn up as briefly as possible.]

•»• All communications should be addressed to the Editor of the Iímimsii Mechanic, 31, Tavistockstrcet, Govern Garden, W.C.

AH theories and Post Office Orders to be made payable to J. Passmore Kdïaîdb.

I would have every one write wbat he knows, and as much as he knows, but no more; and that not In this only, but in a]4 other subjects: For such a person may have some particular knowledge and experience of the nature of such a person or such a fountain, that, as to other things, knows no more than what everybody does, and yet to keep a clutter with thie little pittance of his, will undertake to write the whole body of physicks: a Tice from whence great inconveniences derive their original. —Montaigne'» Etseys.

CONES FLOUR.—BARIUM CHLORATE—TEST FOR ARSENIC—SODA CRYSTALS, Ao.

8iR,—On page 633 of your valuable journal a correspondent, stguing himself " One of the Old School," has expressed an opinlou that my statement relative to the adulteration of cones flour with rice is incorrect. Now, we are aware that revet wheat is the S roper wheat to use for tho manufacturo of coues our; but 1 вяу that rice flour is wholly and partially substituted for that article, according as the price is intended to be high or low. Some bakers at Eton pay 36s. to 41s. per sack for good flour, and for cones from 20s. to 25s. I do not mean to say that 20s. is tho regular price, because of course that varies; this flour, cones for dusting, аз supplied at the present time here from 23s. to 25s., consists of nothing else but rice flour; and on applying to various bakers iu the neighbourhood for a sample of cones, I iuvariably obtained pure rice flour. Ou one occasion I inquired lainutely into the sala of this cones, and I found (a rice flour variety) that it was labelled "cones," iuvoiced "cones," and inserted in the account as cones. The bakers about here, as I have said before, pay 30s. to 41s. ; and if they pay (now) 23s. to 25s. for coues, it is hardly likely that it can be pure wheat flour. Once, and once only, I obtained information to tho effect that nearly the same price was paid for cones as for flour; and when it was found that there oxlsted a cheaper variety, 2ÜS., it was obtained directly.

After having expressed my opinion on the matter from direct observation, I will take "One of tho Old School " to a work called "Adultération Detected in Food and Medicine," by Dr. Hassall, paie 277.

Dr. Hassall says: "Dr. Paley, of Peterborough, brought a sample of flour seized ou susptcion, and which the baker called 'conejjlonr.' On subjectlug this to microscopical examination, it was found that it consisted entirely of rice flour."

Pago27fl: "It appears therefore that cones flour is rarely to be obtained r/enitine, but is subject to an enormons amount of adulteration: further, that some of the samples do not contain a particle of wheat jlour, of which aloue they should cousist."

Twenty-two samples of "coues flour" were procured chiefly In the metropolis: the following is the result (this is also taken from Has-all's work). At the same time, I think it is a mlstnko to say that bean flour Is found in cones. Tho samples below, In which that article was found, were iu all probability Inferior, adulterated varieties of wheat flour :—

1. Contain« rye and rice flours. 2. Entirely rice flour. 3. Contains rice flour. 4. Contains nee Hour. 5. Consists In great part of rice flour, (i. Genuine. 7. Almost entirely rice Jlour. 8. The same. !>. Chiefly rice and bean flours. 10. Contains much rice flour. 11. Not cones at all, but consists of rice, Indian corn, and bean flours. 12. Bean and rice flours. 13. Contains much rice flour. 14. Contains barley flour and alum. 15. Genuine. 16. Barley and rice flour. 17. Consisting in great part rice flour. 18,19, and 20 were genuine. 21. Not cones at all, consisting entirely of rice and Indian corn flours. 22. Consisting chiefly of rice, with some Indian corn flour and much salt.

Now, the preface to the above work is dated 1857, and "One of the Old School" will perceive he is decidedly behind timo when ho linde that Dr. Hassall found In cones flour that which I ststed I found, In my letter on starch. I am glad to llud ho can "conscientiously affirm that no rice, boons, or auy other Intermixture ever entered a sack of his." He say« that some of the "aew school" add dry lican or ichite pea flour to wheat flour, to (/¡те it strength, but that " cones should cousist principally of rice, and yet sold and accepted as сопел flour is a novelty." We (all of us, I may say) read the English Mechanic for two pur

Íloses—tiret, diversion, tho principal motive, and nstruction tho second. "One of tho Old School" then must own that he has not subscribed for naught; he has gained a wrinkle, aud that is, that rice flour is sold as cone«, labelled aud sent by rail as cones, paid for, aye, and the purchaser satisfied by payinn- rice flour pilco for Cones. Hassall says that "there arc several kinds of colics in the market, the beet being nearly twice tho price of the worst, the adulteration being usually In proportion to the price." I ran tell. directly 1 see a loaf, which kind of cones the baker uses, real cones or rice flour.

Barium Chlorate.—Л s there seems some difficulty in" Pyro"genli.g an answer to his quory relative to the manu fin- uro of barium chlorate, I will furnish him with the vari >us minutlas :—Tnko SU8 parts oi barium oxide iburvta), finely levigated, and suspend ir. iu water, keeping it ¡u a state of suspension—i.e.

not rotting it sink to tho bottom of the vessel; place the vessel containing the above in a sand bath, and submit it to a temperature of about »0° C. (ISM0 F.). Then take a retort furalshed with a leading tube, which dips into the vessel In the sand-bath; place in the retort 54o parts of manganese dioxide (Mn.O-j), cover with hydrochloric acid, and apply heat Chlorine will be liberated, and pass over into the solution containing the baryta, whl.'h latter will be decomposed. Barium chlorate and barium chloride will be formed, and will co-exist In the solution together. The chlorine must be passed In until the mftklness has disappeared. More manganese dioxide may be required than is stated If the sample be very Impure; but if 600 parts are taken, that will generally be found sufficient. The reaction which takes place may be thus represented :—

(BaH,0,)» + Cl„ = (BaOVs + Ba (ClOj), + (H,0), The action of tho chlorine being continued for a short time after the milkluess has disappeared, the olear liquid (If necessary) may be filtered, and tho filtrate evaporated to dryness. 2000 parts of water are then added to the residue, the whole raised to tho temperatare of 10 ° C, and set aside to cool; the barium chloride will remain iu solution, aud tho ctilorato crystallising out. 316 parts of this latter salt would be formed by taking the above quantities.

Htdroflüosilicic Acid.—" Taffy," although professing to correct me in the barium chlorate preparation, has fallen into an error himself with regard to the above acid. "Pyro " can prepara It thus :—Take one part each of fiuely-powdored fluor spar and powdered glass, or flue sand, and mix with 0 part» of concentrated sulphuric acid (H, SO,). On applying heat to the mixturo, gaseous, silicou tetra-fluoride is formed: (H,S04), + (CaF,i, + SIO, = (CaSOj), + SIF4 +(H,0), Л precipitating glass is then Liken aud partly filled with distilled water. Some mercury is then placed in the bottom, and the tube delivering the silicon tetrafluorido Is made to dip into the mercury; the gas as it passes out of the mercury is decomposed by tho water, forming gelatinous silica and hydrofluosillc acid. This latter Is not "a gelatinous mass," as " Taffy" states; the silica may be separated by filtration through a cloth, while the clearliquld is hydrofluosilicic acid (H,8iFc). This acid, when concentrated, fumes strongly in the air¡ when added to potassium chlorate, an insoluble potasslllcon tctrafluorlde is formed, liberating chloric acid. This chloric acid solution may be poured ofT the insoluble precipitate and neutralised with barium carbonate, and followed out as "Taffy " states. The following reaction takes place on passing SIF4 into water :—

(SIF<), + (H,0), = SIO. + (H,'S! F6),

Purification Of Chloride Of Silver.—" W. H." will And that heat is the proper method for separating paper from his silver chloride. If he dissolves the chloride by means of ammonia, he must recover it by spontaneous evaporation. If he attempts to drive off tho water by the application of heat, he will And that fulminating silver will he deposited. I merely mention this because I find "Ernest " and "Beta" had both forgotten to mention it. and " W. H." instead of recovering his chloride, might lose his head. The excess of ammonia might bo neutralised by the addition of an acid before evaporation.

Test For Arsenic—Let "Claudius" proceed thus :—Procure some puro zinc, free from arsenic, and plaoe It in Marsh's apparatus with the substance to he tested. Add sulphuric acid, and when the gas is being given off freely light it, and hold a crucible lid in the flame; if arsenic or antimony are present, there will be a metallic deposit upon the lid. To discriminate between antimony and arseulc, the metallic deposit must be submitted to the action of a drop or two of yellow sulphide-of ammonium; antimony dissolves,and arsenic remains unaltered. To test the cleanliness of the apparatus, use the zinc and sulphuric acid alone. If clean (free from arsenic) aud the materials pure, there ought to bo no deposit upon the lid. Araenle may also bo detected when In solution by giving a green precipitate with ciiprammonium sulphate. The precipitate consists of arsenitc of copper.

Powdering Phosphorus.—" M. P. S." can reduce phosphorus to an Impalpable powder by dissolving it in bisulphide of carbon, aud letting the latter spontaneously evaporate. By-the-bye, 1 may as well add that tho latter operation Is best performed in the open air, if the operator is blessed with the seuse of smelling.

Soda Crystals.—In the manufacture of soda crystals, the first stop is to form a sulphate of soda. This is done by placing a quantity of common salt In я decomposing pan. and running In an equal quantity of oil of vitriol. When hoat is applied, hydrochloric acid is given off, aud may be condensed iu towers or scrubbers. The mass is then raked out upon the hearth or roaster adjoining tho decomposing pane, aud submitted to a somewhat higher temperature. The «a'f-cal'e thus produced is mixed with an equal quantity of limestone and two-thirds its weight of coal. A charge of this mixture isinlrodnced into a reverberating furnace, heat applied, and when completely fused it is raked out into moulds. This product is called black ash. This product is digested with warm water, the clear solution evaporated to dryness and heated with sawdust, to convert any caustic soda into the carbonate. Tho residue is then dissolved in hot water, decanted ofT from auy insoluble matter, and run while hot into crystallizing pans. After the first crop of crystals have separated, the mother liquor is drained ofT, aud evaporated for a second crop. Does "Soda Crystals" iufer that his decomposing pans hold 120 gallons, or that that Is tho capacity of his crystallizing pans? If the latter, they would hold iOVivt. of soda crystals, deposited from solution. Ну a little calculation he will be able to arrange quantitities iu the proportions given above that will suit his plant.

"Practical Chemist" (Query 2П54] is easily answered. There exists no acid termed "hypochloric" acid, ilypochlorous acid is kuowu, having the followiug molecular formula, UCIO.

Сеопсе E. Davis.

STRAINS ON GIRDERS. Sir,—Will you kindly allow mo to make a few remarks In connexion with J. W. Bedford's notes on "Strains on Girders" which appeared in your laust impression? Tho action of a load placed at the centre of a girder is as follows .—Let А В represent a girder

VT 20ft. span, 2ft. deep and W = 10 tons. — is supporte.»

2 at A, and the reaction at A «aused by the bent lever А С D Is increased 5 times in compressing D. Tlicro

W 10 fore, strain at centre = 5— = 5 — = 25 toes. 2 2

The general formula tor finding the strain opees wrought iron or other girders at centre la as follows s

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W= load, equally distributed, i, length In term« oí depth, s, strata of compression at top flange, or W x Í

tension at bottom flange. 8= . Now, 10 ton»

8 placed upon the centre of the girder in question wilx be equivalent to 20 tons equally distributed. Hence, 20 x 10

the formula becomes = 25 tons strain at centre.

8

The breaking weight of this girder may be foamo,

by the following method :—Let a = sectional are» ui

bottom flange in inches, d depth la inches, and s

span in Inches. 744 tons a constant for wrought iro»

il s il » 74 4

girder or tube. = breaking weight in.

tons at centre. Let tho girder in question have a.

6 x 24 x 74-4» bottom flange area of Gin., then =

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QUESTION FOR MILLERS.

Sir,—In answer to "C J.'s" personal question, I beg to say I have only in one instance had the trouble he complains of with the neck of the stone spindle, which happened some time since in a mill I was) carrying on off-hand, when I bad to take the stones in hand myself, and flayed two high sides of the bedstone, and when I had dressed them two or three times myself the neck went all right, and has been no further trouble the last two or three years. I will also add that I rectified the running balance at the same time.

To the second question, I never yet saw a thick tooheavy stone running satisfactorily on a short spindle, and 1 have always kept the greater part of the wch/fe* of the stone lelow the bearing in my own mills, «p shall continue to do so. Some years since one vmt were fitted with universal iron, like the snip* compasa, with the bearing near the face of the stone*, but thev had to be removed aud replaced by croaa bar and driver. R. R. Smith

REFLECTORS AND REFRACTORS.

Sir,—It is nearly eight months since I last wrote to you on tho above subject, and I then promised your readers, should they so desire it, to supply them at some future time, with a further description of the 8Jln. mirror and Its performance. From tho kind allusions which some of your correspondents hare made to this promiie of mine, and to the matter contained In my former letter, aud from the increasing Interest with which, T am glad to say, tho subject of practical astronomy is every week being iuvosied in your columns, I feel I cannot do better, during a period of enforced rest from professional labour, than state what I have fouud that performance to be, and drop some further hints to amateurs, who, like myself, are anxious to reuder their home-made telescopes as perfect and useful as possible. I had thought of giviue a brief resume (with the Editor's kind permission) of the work doue during the post half-year, but found id would occupy far too much space and trespass toomuch on your indulgence, to think of such a thing.to I shall simply stato the work done on one or rae favourable nights last summer, and leave the reader to form his own opinion of what tho telescope has since accomplished on similar nights.

A short time after I had written the former letter, describing the mounting of tho 841n. mirror, which I had obtained from Mr. Browning, of the Minories, in tho end of June or beginning of July, we, in this neighbourhood, wero favoured with four nights fa succession of most exquisite definition, a boon indeed to those who possessed really good instruments, which they wished to test. On the first of thoso occasions, iu the early oveulng, just as the sun was sotting, I brougbt out the large telescope, and having inserted a large field eye-piece, giving a power of 35, and a Held of ¿4 degrees, directed the telescope to the position iu the sky where the planet Saturn would pi-obnbly be found. After some little eearchnig, I suecesslully piokert him up, shining with a pale silvery light, in a sky which appeared, with that power, of a deep violet hue. Viewed under such circumstances with a lure» telescope, and with the evening light still bright, ihe planets present a most beautiful appearance, onewhich often tempts the observer to »eulfct for a timo the use of higher powers, but of course, if the physics) details of celestial objects are to be investigated, they must be used.

The ilerluitlon I knew to be most excellent, for, with a little expérience, one look at tho circle of light which Is formed iu the cye-plece, wheu a bright object

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is aughrjF oat of (ocas, will tell this, if the Illuminated •uxk is steady and calm, without any bubbling or wavur motion, after the telescopo has been for some tins ta tbe air. the observer may prepare for good work- Thi<* steadiness, too, is not in tbe least affected try it* beautiful pulsations of colr.ured light which czar frmo the larger stars, but refers equally to them. It was peculiarly so on this occasion, for having fssericcL first a power of 100, and then, as the evening darkened, 1S7.1 was enehanted and completely chained to tbe spot with the exquisite appearauce of the planet sJaAuTO. without one trace of haziness or of indistinotns»s ot outline; it seemed chiselled out iu colours of vrareit silver and pale sky Hue, and golden yellow. The dark division in the ring was sharply marked all round, without a break in its continuity, spreading oat into a dark band at the " ansx," and uarrowiug at the nunor chord of the ellipse into an Infinitely tine black Una The transparent ring too was so evidently and beautifully seen, of a dusky grey colour, and clearly less bright on the side next to the Tings, than on that next the planet. This I took pswtlculnr note of, and examined it most carefully ou the two followtag cresting*, when it was voritte.1 beyond all doubt. It is not noted ta the engraving of this planet in the interfiling tittle work by Mr. Lockycr. copied from Sir De L* Hue* sketch, but is very well marked iu tbe lithograph on the frootisBaeco of Mr. Browning's pamphlet. On the occasion above mentioned, it was plainly seen, not by myself alone, but by two other <*?ntlemen. who were eKaminiag the planet with me. "VTenlso directod oor-ettention to the outer ring, and searched with the greaiaat care for some trace of a ■division, but could sot detect any line which might be rightly eo called; there was a shading sometimes 'visible, more, gueatesThtwever than clearly seea, which might possibly suggesttbe idea, but did nut satisfy rue. IiurUie the cine i was observing the shading on the transparent rtojf. and searching for a division ou the outer one, vrc of the satellite* bad come into view. Ou the iccoDd evening I saw the sixth, and bad glimpses of the seventh (If it was not-a very minute star, tor not having' anv ephemerines Of the satellites, I cnuM not absolutely ideutify itJ, with the six satellites however, so clearly seen, and tbe body and rings of tbe planets so beautifully {leaned, yoar reader will believe me when I say. I never saw a more wonderful <tt moreiSACinatiiig object. There was ooe thing aboat this strange planet which

the compouents ol this star are only 0''55 apart, and
it was with fear and trembling I approached it on the
first night, but what was my joy und delight, when I
had applied a power of 300, to see the discs clearly
separated by a black division, yet looking square!
vl hen, however, I had applied a power of 330, the
division was perfect. Again and again I looked at
this beautiful object, two 8 magnitude stars, like most
minute circles oi light, lying so exquisitely close to
one another yet perfectly distinct. I thought I
detected a slight difference of colour iu tbe com-
ponents, aud hope, when the calm summer weather
conies ogaiu, to make many more observations ou this
afar, for a most wonderful and interesting object it
certainly #i.

White' speaking of this star I mny mention, that ft
seems somewhat more easy to divide than 7'J Andro-
meda;, which 1 have nicely asperated ou some favour-
able occasions since; perhaps this cany arise from the
greater tnngnitude of the latter, i will finish what I
have to say on the subject next week. H.

shoddy manufacture. T'« at used for opening raw cotton is called the vy'tlUno. which name lie makes use of, but that used In tho shoddy manufwlnrt. Is called tin Iviltey, Now, the tc'UUfwiiig or scutch>ng maclilue, is composed of a feeding apron and two sots of rollers and beaters. The first set of rollers, turniug in opposite directions, drag tho raw cotton through, and transfer it to tho first pair or beaters, which, revolving with great rapidity, thrash out nearly ail the sand aud other impurities which the wool contains; the second pair of rollers and beaters complete tho process.

It will thus be seen that " Scrutator" has jumbled together the two machines, owing to the slmillarity of tho names ; " rushed headlong into print," to use his owu expression, without taking taking tho trouble to investigate and thoroughly " master" the subject; has drawn hasty conclusions, and tried to criticise what he apparently knows nothing about.

tflTCHE MANITO.

METEOROLOGICAL.

Sir,—Would you, or some one of your readers, give

Information how to observe aha dally changes going

on In the horizontal aud vertical motion of the needle,

as well as the intensity of the magnetic force?

Is there not some moro simple way of observing and recording these variations and earth-atorras, thau tho complex apparatus in use at some observatories 1

J. W. A.

KECKN'T QU4KRIBS AND REPLIES FROM

MILLERS.

8m,—Mr. J. Sharpe, in euswerto several correspondents, denying that 1 am the inventor of the universal joint, described on page02tl. No. 256, says that he has h .d the same working In his mill since 1*86, while the description he gives of it seems to represent quite another thing Perhaps Sir. Sharpe will be kind enough to send a drawing to your excellent engraver

of what ho menns by the following scoleuco:-"Tho , nwrwiTEn PORTION <»' THE MOON

oval ring of mine has the Journal, at the foci of the THE ILLUMINATED FOKlllLN W Iuk. aiwrv trausvurse din.., which admits of the inner riug. or \ Bin,—When I eailod yeurattention to au apparently driver, being shorter, and therefore stronger, and i anomalous appearance in the illuminated part of the leaves more room between them "—without which tbe moon, It was with the view of affording or eliciting following sentence aootns quite Incomprehensible— , information for the more observant of your renders. viz., that tbe centres of all bis journals arc in tho ou n subject which probably Ihm puzzled many of same circle, which could not bo so in uilue. 1 main- tliorei, but I had no Idea of Its being so followed up or tatn that the centres of uiy Invention arc in a circle ; ( jor,„ iong B time. When tho letter of "Sigma " nptbereforv, if bis Joint is anything ollleroiit to tills, as | p„nrwi, r thought his satlifactory eiplanation would lie says, all his centres cannot be wilhiu one and the [mvi, i^,, generally accepted, and though I think his same circle. My object iu forming the oval ring was | diagmros might have been Improved, I forbore to

to bring all the centred within ii circle, so as to cause the stone to oscillate alike on all the lour bearing' Tho evatres are also in the same plane or level. It is therefore n perfect universal jolut in evury respect, and

■delighted me, even more than all I hav.j yet described. [ *■ |{ stands is purely aud simply my owu Invention

''1 also repeat my former assertion that it is superior

(until convinced to tile contrary! to anything at pre

I refer-to thecol.turs on the ball and on the rings—it is only on such nights as these 1 am speaking oi\ that this phenomenon can be aeon to any advantage, and to see it well a large aperture is absolutely necessary. I do not know how most asitrenoiner.* acC'Hini for these colours, whether they think them intrinsically Inherent In the planet or its atmosphere, or to some extent can<ed by the vapours in our own atmosphere, through ■which the light of the planet has to penetrate, certain it is that they are much better seeu and brighter. •when, the planet is near the hor.zou, provided only high power* can. be used, than when it is more overhead. 1 believe, toe, the larger the aperture the more

trespass inure on yonr space, or to tox your engraver furthor. But us Mr. Bcanlslcy cannot accupt the explanation of •• Sigma " and has sent you a diagram, I cannot help writing a few wonts on the subject, .specially as hladlugrani is pretty nearly what 1 would have sont myself, and helps to prove "Sigraa's views instead of his own. If any of your readers

Mechanic the./trsi n«ir thing, as lie deplores so much IbeaiiiiouuV/ ol all hitherto sunt respecting milling? Tuos. .EVANS, otookport.

GALVANIC HANDS.

Sir.—Without any wish to find fault with other suggestions. 1 send you a description of a band of simple and cheap construction, and of the efficiency -clearly are they marked, so that the possessors of large ef tne action of which 1 have had several practical telescopes have a very great advantage iu this respect, proofs. A strip of copper aud oue of zinc, of any over those who are not so fortunate. On the night in | desirable length, are to be j»ineil(by rivet or auy other question, I fonnd that a power of 350 showed them I preferable model, at the ends, the joint whipped most admirably in my reflector of sjn. I could well together by thread, and vurnished with shellac In ■iatMsinc. however, the much greater brightness and j alcohol; the extreme ends having a hole punched in twenty they would have in o 12m. or liiu. speculum. . each, for fastening with a silx ribbon, or ether nonTne contrast between tbe delicate sky blue of the conductor. To apply and brlug tho same into action, polos of the planet, tbo golden yellsw, alternating wind a piece of list spirally round the united stripe of

with silver and grey on tbe belts, and the blending of ill these hoes on tbe rings, was Indeed a lovely sight, rbut should be looked for and seen to bo at all -appresoiated. Tbo beautiful appearance which Saturn presented on that occasion, lingers In mv mind still, and altbongli I have seen the globe of Japiter under equally favourable circumstances, with for more ■marked and darker hues on his belts and brighter tints altogether, the soft dolicate beauty of the planet Saturn surpasses iu perfection aud loveliness oil else lha\e ever seen in the infinite amplitude of space. I haTe dwelt thus minutely on these observations of fcaturu, because they have made a lasting impression on my lnhid, and showed me what a perfect speculum Will accouiplUIi under favourable circumstances. But to continue my description from my note of these ssreuings.

The planet Saturn is setting, and at last drops behind the rool of a neighbouring house, so 1 nm reluctantly compelled to turn the telescope to other sObjects and surely here are a great variety of most interesting- doable stars.some of tbem excellent tests, ana all very favourably situated for observation. Here In a- Aqulljc. * Ophiuohi, r Ophiachi, r\ Cor Bor. c Cygrji. /r> Bockis, all Just as well placed as they could be. Here ore also two remarkable nebula;, M 67. the annular nebula in Lyra c, M 13, themaguitlcent globular cluster In Hercules. Let us take them In tbe •hove order and give you our Impression of them as we saw them on those admirable nights.

TAquil*. I have divided this star more beautifully wen a 23iH. achromatic, by Wray, to ef course do not mention it as a test for an t^ln. speculum, but certainly ij>e eroallness of tbe discs and the large space between taem. does surprise one who has been accustomed to uescopes of much smaller aperture; a power of 100 •eporatfcd this star well.

«m ^'"chl, r Ophiuohi. The former a beautiful eiteTy \ ''"''l' aividod; components with exqui

,.'***- *• much more difficult object for a reflec*"T '"*" fT ot the previous stars, as it is of the third rnsagriituu-; nowevet,on three of the evenings referred to. It wa« l«r,q,ifully and completely divided, and rrequently since that time, whenever the air was fine *?" *'*»dy. • power of 187 just split It, but 300 compie-ely divided it. The division of this star gave me ■very xrext p-r.vuurc. especially as it was accomplished «o etwilj- and so neatly.

V Cor. Bor. Very easy. 187 completely divided it; discs beautifully defined, and exceedingly minute, with 2*> hlack division was equal to 2, if not 3, diameters •afoe of the discs.

r'Bootbi. Here was indeed the crowning test, as

sent In use. tiny 1 Invite Mr. Sharpe to seud to the wiu place a pin to represent the position of an observer on the earth's surface where Mr. Beardsloy supposes him to be, anethcr lln. above It to represent the centre of the moon, and a third at a distance of 33ft. in tho dlreoiion of Mr. Boardsloy's retiring suns, and to do duty for one of them, the relative poaltlons of tho three bodies will be nearly obtained ; n thread then passed round nil will show how very nearlj; parallel the rays of light lrcm the sun to the earth and moon are, and thus prorc the contrary to what Mr. Ileardsloy puts forth as his argument ogainst ' Siglna's" explanation. The enlightened part of the moou Is directly opposite tbe sun, although the latter appears on the taugent to tho earth's surface Bhown by Mr. Beardsley.

I do not know whether Sir John Herschcl will be considered an authority with that gentlemen; he probably will be by mo<t of your readers, and 1 therefore beg to quote from his "Outlines ef Astronomy" In conclusion :—"The sun's distance belug 23»M' radii of the earth, and the moon's only 0' —the former is nearly 400 times the latter—Hues, therefore, drawn from the sun to every part of the moon's orbit may be regarded as very nearly parallel." 11. L. J.

metal, wet the list witli vinegar, reduced in strength by water if slight aclion only Is wanted, cover the wetted list with oilsklu, and fasten It with the ribbon iu desired part. About au inch of caeh extremity ol the strip ef metal, must be left uncovered at each end, and be tu contact with the skin. The cost of the contrivance must evidently be roc koned in pence. Suffolk Aiiaii.hi.

SELF-CENTERING LATHE CHUCK.

8ir,—I think that " A. It." must have made a mtstako iu his drawing of ttiu sell-conterlng lathe chuck that appeareil in your last Issue. I see that the thread Is left-handed In the chuck, where it sorews on to the latho mandrel. Now, I havo seen a groat number of chucks, aud have used them myself, but I never before either hoard of or saw eno with a left-handed thread; directly the tool in tbe rest commences to cut tho piece of work fastened between tho dogs, the chuck, instead of tightening up on the mandrel, will screw on, unless tho cut be a very small one; but 11 it was a right-hauded screw, the larger the cut, the tighter the ebuck would get. One with a leit-uanded screw might answer for a face-lathe, or more commonly called a chuck lathe, -where by twlstiug the belt, tbe lathe can be reversed, aud usiug .the sliderest at tho opposite side to where it is usually used. I think the chuck, on the whole, is a very useful ouo, it saving a great deal »t trouble iu settiug small work.

Marine.

SHODDYiMANUFACTUKE.

Sir,—Seeing "Scrutator's" remarks In No. 25?, page G10, ou tho way In which 1 endeavoured ts help one of your correspondents with regard io tho shoddy manufacture, 1 was at ouce iuduced to look further Into tho mutter, and have slnco sousulted ono of our best encyclopedias ou tho subject. In this work, 1 find that I am perfectly correct as to the machine used for tearing up old woollen rags, &c.—" Scrutator " goes further, and says, that allowing the macliiue mentioned to bo correct as 1 tako it, I have only " mentioned one of them, and therefore 13U5 is still unanswered." As far as the shoddy i'i coucerued tho willey is the only machine used to reduce it to a state fit lor spinning. To go further into the matter would be to give au accouut of the machines used in spiuuiugand weaving, &c. "W.M ," tho correspondent who asked tor the information, is apparently satisfied; therefore, what busiuess is it "of Scrutator's" to criticise that which is meant for others?

Tho tDiltey, which, he says, is used for opening rato cotton, Is quite different from the ono used in the

DR. TTNDALL'S EXPERIMENTS ON LIGHT.

Sir,—On trying tlio experiments on atmospheric dust particles, of which an account appeared In your Journal, ii occurred to mo to try tbe elfect of inorganic particles, and for this purpose apiece of brown paper was Ignited, and allowed to smouWer for a short time in the darkened room. A very small quantity of smoke (carbon particles) heightened the elicits to a romnrkable nnd unexpected degree, and on n spirit lamp being held four or Itvo inches below a beam of sunlight, the apparent cloud of smoke, before faintly seen, was now strikingly manifest. Au Iron bar, raised to a red beat, showed the same effects.

S. T. Prestos.

ARTIFICIAL ICE. Sir —In a late number of tho English Mechanic inquiry was male where ammonia apparatus for the production of ice could be obtained. I have one procured from Mignou and Rouart, lsW, Rue Oberkainpl, Paris, and havo used it with perfect success several times! Some frlsnds have had ono in constant use for four years, and it never required repair or renewal of any sort. There are two sizes for domestic use. one costing 2SII. or £11 (Is., makes one kilogramme of loe iu about two hours with the expenditure of about a double handful of charcoal, nnd J pint of methylated ssirltor less. Tho other makes 2 kilogrammes in i hours, and costs 4061., or £10 5s. There oro larger for manufacturing purposes.

J. B. Yonoe, Mebourne, Winchester.

COLLIERY VENTILATION.

Sir.—As a well-wisher of the English Mechanic. and as I desire to see its usefulness extended, I beg to suggest that colliery ventilation would bo a good subject lor discussion In Its pages. You would thereby give some of our most Intelligent workiug men an opportunity of recording their experience and giving their opinions.

To begin, would any brother reader obllgo mo with an honest reply to tho following query :-o-ll0r>"j. u the intake of a colliery should be soft, area, what should be tho size of the return—with the mode or rule by which It Is found? T- H. Thomas.

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WHEEL-CUTTING ENGINE. Sir»—Through the kindness of a gentleman at Flalifax, 1 am enabled to forward а тегу good photograph of a wheel-cutting machine for 'mall work. 1$ В is the bed, which is solid towards the left-hand end, and open to the right, to form a slide for carrying the upright part QEF, which is drawn along by a screw, of which the handle appears at II, to adjust for different diameters of wheels to be cut. Supposing the elide RK to bo taken away, and W the wheel to b • cut, the upright elide would first be brought to \U

Í>lace by turning the handle U, next the Bet screw С ooeene'd, and the peg under it slid dowu to the circle of holes containing the division to be cut, and the peg of the alidade A brought to the t-ame circle by eliding along the pin P till the Index at I ehows the right number. The stops 8 > are then adjusted, so that the lever T V, which works on a loose socket »t T, may be able to swing sideways to the amount of the augle subtended by each tooth of W; and ihe machiucbeing eet going, the first cut I» taken downwards by depressing the handle L, which may be made to rise again by hanging a weight to it over a pulley fastened to the ceiling. To move the division plate the required quantity for the next cut. the peg under С le first lifted by raising ihe end V of the spring lever T V against the pressure of the spiral spring D. and T V is then moved towards the right till it is stopped by S; aud on letting go the lever the spiral spring D forces the peg at С into the proper hole in the plate. Next, by pressing on the bead of D, the lever N prêtées against another lever (O) under the plate, lifts Л up eut of the way, and allows the plate to bo moved round till T V comes in contuct with the other stop S; utiii so on for each tooth in succession. M is a micrometer for add I eg a tooth when the division plate does not contain the precise number wanted- as, for example, in cutting 101 teeth with a division plate of 100 bolea.

For bevel wheels the quadrant Q allows tho upright elide to fall forward to any required angle, and for cutting teeth askew, as in the cape of a worm-wheel, there is a centre at К on which the vertical slide енп be swivelled over sideways. К is a hole in which a crane may be fixed to come over to steady the top of the arbor on which the woik is held.

The picture shows the arrangement for rack-cutting, tho whole length oi the elide В R being itself a rack. Into щ и id i the wheel W works. The lever L is found to work m;.re agreeably tbau the screw which ueed to be at the top of F for moving the upright elide.

J. K. P.

HOPKINSON'S VALVE.

Sir,—In reply to the inquiry of your Leamside correspondent of last week, 1 beg to inform him that the two valves act quite Independently of each other: and the small one being regulated by a dead wtigkt truide

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blades. The boss is divided into three parts (Fig. 1), the forward part carries the blades for going a-hend, the after for going asteru, Immediately upon the engines stopping or upon the vessel over-running or outsailing her screw, the blades feather themselves into a line with the keel, aud become useless. In re

§ard to Btowage of spare screw, and repairs of am aged screw, it hoe many advantages over the solid propeller, as its parts can be stowed separately, and a broken blade or other part re-supplied at once from the spare ecrew. All seamen know the difficulty of

steering a ecrew-vessel under canvas, with her preponer down, especially If it be a three bladed one. This i s entirely obviated by the Belf-feathering screw, as the blades, when feathered, offer no twist to the passlng water.

It U anticipated that, now that the opening of the Suez Canal will render auxllllary steam power of more necessity than ever, that a screw on the above

fprincipie will be found a useful invention, especially n short-handed ehips; and in all, when the expense and room of the Bcrew-well may advantageously be avoided.

The inventor bas a lOln. in dla. model in gun-metal, which can be seen on application, by letter or otherwise, to WatfonL.Crlck Station, uear Rugby. F.

HEATING GREENHOUSE BY GAS.

Sin,—In reply to D. Bothroyd (Qy. No. 2C67). I beg to stnte that the consumption of gas for heating the water-pipes in the greeuhouee alluded to in No. 170$ has again been accurately tested by a dry meter (Glover, maker), and found to be 25 to 3 I cubic feet per hour, according to pressure, which varies a good deal, and which is a very great disadvantage In the use of the most economical burner—the atmospheric—as so much depends on the proper proportions of air and gas previous to ignition so ae te eneure as complete combustiou as possible, and so prevent smoke or deposit of carbon. At 3s. per 1000 cubic feet, the cost of gas In Bath, thîe le under Id. per hour; and thie winter, with the thermometer at 19° out of doors, the temperature iuslde the greenhouse was 46, at least 6° higher than required to keep the frost out. With the apparatus of Hertsman.of Bath, it could probably have been kept steady at 40, when the expense would not have averaged ¡d. per hour. This very mu«h coincides with the result of what "Protcesional," of Brighton, gives as hie experience (see English Mechanic, page 037); only had he used 4ln. Instead of 3in. pipes, his quantity of heated water being larger, probably his result would bave been in proportion.

Where there is no regular gardener or ma'e domeatic on the premises to attend to the fire of a flue late and early—for it is hardly work suitable for a female to feed either with coal or ashes—the gas (with an atmospheric burner properly fitted, and supplied with suitable mixture) will not only be found In every way more suitable, but ehe Jt per. A man up at VI and 4 a.m. is not long fit for his nsual day's work. If the common gas-burner Is ueed, the expense will be very much greater, and the result less.

Сьп " Piofessional " give the addre°e where a simple good gas pressure gauge, to be attached to the meter, can he obtained? This would be of much value to consumere generally.

A very simple and cheap method of keeping frost out of a moderate-el zed greeuhouee is by an Arret stove of the right sort, with, an upright pipe of oto., by

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