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is holding is an extra brake, by means of wbicb the engine can be stopped at 10 yards'notice. Its greatest speed on tbe high road tt»2> miles an hour, and in town In. The coal and water are kept on either side of the boiler. Tbe carriages in construction aro just the name as any other railway carnage, with the exception of the wheels, which are as already described. It is the invention of a gentleman named Lormargot.

H. Garland.

HEADINGS FROM. THE GLOBES.—III.

S!R,--Freqoent«1!mTiOTi9 are made to the different c uKTelTirrtotw of the heavem* both by ancient and modern poets, and this arose not only from the custom amongst all natlnns of making use of figurative language, but because the sowing aud reaping of corn mid a^rb'ultiiral productions depended unon a right observation of the rising and setting of particular star*. When, for instance , the sun had reached the Tropics, or turning points, it was necessary to reckon so many days before It was advisable to undertake particular work, bat in the*e time* of accurate computations and astronomical observations, the calendar is employed, Which Is much simpler. Thu*. the poet Ili^iodlnhhr" Works and Days*' makes mention of the Tropics, and tho Roman poet has constant allusions to the* signs of tlm Zodiac.

lu order, therefore, tnenter Into the beauties of these writers we should have accurate uotious of tho coarse of the pen through the Zodiac, and it will be our business in this letter to show by the globes tho longitude aud declination of that bright luminary as he proceeds in his round. It is not our purpose now to enter into auy argument either for or against the Coperniean system, as this has been ampty discussed, but merely to Illustrate certain phenomena which can b*» explained on either supposition—namely, that the earth moves round the sun, or that the sun moves round the earth, and to illustrate the better, wo will make the latter supposition. Tbe figure is a representation of the two outer circles on the wooden horizon of the globe,

[graphic]

and the eclipse may represent the ecliptic on its surface. Tho son's longitude, then, and indeed of any other celestial «body, is its distance from the point Aries eastward. It is from this point that the reckoning begin, and it will be found from an inspection of tho figure, that he is at this point nn March 20, he then passes through the signs of the Zodiac as they aro marked in the circle, and reaches them on the months immediately above them. Thus the sun enters Cancer, June 21, and generally speaking he enters a new sign on the 20th of eaeh month ; that Is, apparently enters, for the stars being millions of ratios distant from both the earth and the sun, his entrance into them is obvious totheeye only. Concerning the origin of the names assigned to the signs of the Zodiac, there seem* to be some uncertainty, but it is generally snp[>o:fd that they are intended to denote the peculiarities of the season at abejse particular times. Thus he entered the signs of the Ram and tho Bntl, when farmers were busy In rearing domestic animals, and at his greatest height, he begins to recede, or, like acnib, to walk backwards. The Lion being a native of the Torrid Zone denotes the heat of the season, tbe llalancus when the days and nights are again equal, tho Archer, tbebnntlugseason, the Water bearer, the watery aspect of the clouds, and the Fishes that the seas were open for mans supply when the ground was bard from frost, and uncultivated. Tho declination of the sun meaus bin distance north or south of the equator, his greatest declination being 234°, when he has reached the meridian line on which the angle is measured, which the ecliptic makes with the equator, and he then shines over the North Fole, and 234° beyond it. But to tills subject we shall have afterwards to advert. If. then . we wi»h to know the sun's longitude by the globe, we have only to look on the wooden horizon fur the month, and the corresponding circle beneath will show the sign, and if we wish to know the declination we have only to bring the sun's place to the brazen meridian, ju*t as we find the latitude of a place, and immediately over it is the declination. We may remark thut the orbit which the sun makes, or the earth, for it is immaterial for our purpose, is not circular, but elliptical,and those who wish t pursue the subject will do well to study 31r. Proctor s Maps, published by Messrs. Longmans, where they will find not only the orbit of tbe earth, but the orbu (fall the planets round the sun accurately described according to scale. By a study of these maps It will be seen that the sun is in perigee or nearest to the earth in winter, ten days after ho has entered Capricorn. December Si, when, according to Mr. Proctor's scale of measurement he is 91 millions of miles from the earth, and that he Is In apogee, or at his greatest distance from us, iu Midsummer, .July 2, when his distance by the same scale is °5 millions of miles. Thus we find that we are

nearer to the sun by 4 millions of mile-* in winter than we arein summer,aud,consequently the warmth of our summer arises not from our proximity to the sun, but from the fact of hfs being longer above the horizon and his rays, owing to his altitude, darting less obliquely upon tho inhabitants of the North Temperate Zone. These m*ps show the situation of the earth in his orbit for every ten days, as well a-* what sign of the Zodiac he Is in, and should be possessed by those desirous of accurate notions upon this subject. Tims It appears tint the sun's longitude is really the distance which the earth travels in her course round the sun, which, computed in miles, will be found to be mjre than t>:>,Qou miles per hour.

T. S. H. P.S —Mr. Pyor states the following sonteuce in Letter I. to need correction: "As the globe turns on i*s axis from west to east, those who llvo in west longitude, must have their time earlier than those living in east longitude, because tbey will not come in to the enlightened hemisphere so soon." He says the word "earlier" should be " later." I submit tho sentence must stand without correction. Turn the globe on its axte from west to east, England will come into the enlightened hemisphere sooner than America, which is In west longitude; but, the time In America is earner than in England, for when it is 12 o'clock at Louden noon, it Is 7 o'clock in the morning at New York; to that we come into tbe enlightened hemisphere sooner than th'» Amertcanvand yet th«rtr time is earlier than ours. They iu tbe latter part of tho sentence must be connected with the noim^ittct of the sentence.

TO MILLBRS.

Sit.—T am glad to find «n improvement in the letters respecting mills and uiillering. The remarks of "CM.," on tho balancing of sxonefi with Clark and Dunham's patent, 1 consider good (sound reasoning, and I hope we shall hear again from him. Also J. Botting's remarks on the swish in stoues. are interesting and true. I having tried tho same remedy for similar evil aud corresponding results. It. U- Smith gives us some useful hints bo times. 1 hope all our dusty brethren, who have a little useful information, will please send it to oar Mechanic for the common benefit. I will just ask tho above, or any other, if th'jy can give me a good remedy for filling up broken joints in millstones; I have tried alum aud borax, but cannot mako a good job of it.

E. O.

Sin,—I cannot say I am prepared to dispute Mr. Evans* theory that the cause of a stoii'-'s standing and running balances differing is " The buJancc weight does not coincide with its counteracting object in its radial distance from the centre," but in my blindness I cauuot conceive why it should uot. He asks me if I cannot alter a stone's standiag balance by raising or lowering tho weights wheu runniug. I have already said I have found wheu a stone's standing and running balauce do not coincide, something was wrong, independent of tho balance; but whether it would alter or not if b >tli balances coincided, I am not propared to say, but to my unenlightened Intellect It seems itnprubablo. But in spile of Mr. Ktuus' scientific reasons why, aud my inability to dispute them, some very ugly foots (to tbe contrary) stare ino iu the face. Aud us Mr. E. savs "Facts are stubborn things," I ask botli him and Mr. Smith h >\v we managed to balaucu our stoues so that tuey "hummed like a hive of bees," made bran "like a bee's wing," and as fcooi a per centage before we had the patent balauce as wo do now with it, and why every serious fault that occurred then will now, iu spite of balance. Aud as to despising the staudiug balance as "virtually of no use, and practically non -effective," that idea has recently done serious damage. If it be of no use, It is mysterious how we managed to make our stones work aa well without the patent balance as with it. 1 repeat the fault of a stoue's dragging lies either in empty cogs or their pitch circles uot meeting, or irons not working parallel with the fa^e, or uulevel bedstone, or faulty face. <>f course the running balance will keep a stone from vibrating I.'j. I know of stones that have received a good running balance, but went empty, aud on being freed by the spoutsinan, commenced kicking violently, and before the mill could be stopped knocked off l-t mom lis' wear of skirt. Where was the ruuning balauce that it did not keep them steady? What wa* the cause of it? And why did it not occur when being balanced? The mire uutruo the face, irons, &c, are, tho greater difference there is in the two balance*—so great as to drag off the skirts when starting and stopping if the running stone has a ruuning instead of a standing balance. If, a* Mr. Evans asserts, the cause of the difference iu the two balances is owing to the unequal specific gravity of the burrs, or because the balance weight does not coincide with its counteracting object lu its radial distance from the ceutre, how i* it we often soe parties try tbe running balance of their stones every three or four dressings, and find it differ each time? To ma it looks very much like a different face to the running stone.

A Stoneman.

[Will some miller, or any oue else who may know, tell us how we can get the names and addresses of the millers of Groat liritain and Ireland ?—Ed. E. M.j

Sir,—Having in my letter in this week's Issue of the English Mkchanic endeavoured to show that a silk bolter must make a better or finer sample of flour than a wire machine, so that, after showing the advantages of said bolters, I will now describe some of their disadvantages. In the first place, owing to tbe great length they have to be made to dress from, say, even four pairs of stones is very much against; their adoption lu mauy mills because of their taking up so much room. ^ud. As they will uot, generally spoakiug, dress more than from throe to rive sacks per hour on an average, or about one-fourth tho quantity a good wire machine will dress, causes them to be worked continuously as the mill grinds, and there is often fixed over the head of the muohiue a hopper boy or cooler,

to cool the meal before it enters the bolter; it will be seen by this that a miller cannot keep bis ground meal by him for two or three days to cool, unless Indeed be has extra silks to dress it up with. 3rd. There la another matter thai Is against them, namely—not befog able to change any number of the silk and put another on in a few nlknites, according to tbo ^tufl that wants dressing aud the time of year, as of cour«e for damp wheat meal the silk should be coarser than lor dry foreign wheat meal. To obviate this I have, in conjunction with some parties in Manchester, brought out a silk machine that occupies a great deal les* space than the present silks, and any sheet may be takeo and another put in in a few miuutes. 4th. Indreseiiy With a silk bolter it is Impossible to get the brau at clean and bright us by dressing it with a wire machine Consequently I generally put a small machine to take the brau out before It enters the »>lk or after it leave* it, as by that means everything h* cleaned up aa well as possible. 3th. By dressing with the present Kilt bolters there is not: more than rme-ttalrd of the silk that actually druses, a* the rati* or ribs inside the reel as It revolve* carries the head up and drops it on to the silk near the bottom at the opposite side of the reel. It is this that force* mo»t oftbe floor through the silk-, but it at the same time gives the silk a sudden twitch, which of course Is injurious to it.

E. Davie*.

COMMERCE AND TittADB.

Sin,—As this suhject has been opened, and Is one which, directly or indirectly, is ol vast importance to us all, It may be as well to say a little more about it, especially because Its very vastness involves the consequence of very partial views being caught ef it by most minds; hence it is. that so many, even intelligent people, are cuptlvated by that delusion, "protection to unlive industry," which is now set forth under the disguise 1, but equally delusive, form of "reciprocity," or In other wonts, refusing to buy cheap what we want, because those from whom we buy have not sense enough to buy of us, what we offer them cheaper than they can otherwise obtain it. This Is the very delusion by which "F. W\ M." page 257, Is somewhat afflicted. He does not see the whole bearing of what ho Khvb. Let us accept at its worst the result he sets forth. "Our men are compelled to emigrate in large numbers." What of that f A temporary suffering to them and others—a resulting unmixed good to them and all—they leave a country overstocked with labour, where, if they cannot produce, they are n burden on the producers; they go away, and ut once become producers to their own great benefit, aud large customers to those who remain behind.

Rightly considered, the necessity for emigration is a great proof our prosperity, for "it meaus exceuot population, but an excels not due to diminished work, (for any Intelligent man knows that tbe total work of the country is vastly increased every year), but to the rapid ratio of increase of population, aud to temporary changes iu the nature of work, which at times throw out whole classes of wurkmeu, who are then unable readily to find fresh work, not from iu deficiency so much, as from the competition that involves with those already engaged in it. Now nothing is much more certain in political economy than that a rapid increase of population is a consequence of prosperity. Stagnant nations ni'Ver increase—they very commonly deorease in number, and this is only one form of Nature's universal law, that creatures only multiply where there is food for them. Kven one year's bad trade, or disaster, Is marked by diminished birth rates in the next, us well as increased death rates.

Of course it is hard to look beyond the Immediate persoual action of these laws, and hence It is that so intelligent a race as the Am^rlcaus violate them. Instead of recognising the unlimited advantages Nature gives them, as producers of raw material, tbey wish to do everything themselves, aud seek to be manufacturers, which those very advantages forbid, by their effect on the labour market. The consequence is, that while a few years ago they were daugcrous competitor- iu every market, since they adopted their tariff system, wo aro beating them everywhere, and literally driving their flag off the seas. I quote the following from tho "New York shipping aud Commercial List":—"No better commentary upon the decay of American navigation interests can be had, than is afforded in the fact that the Novelty Iron Work** have hold off most of the raachlucry and tools; the Allaire works are now occupied as a stable: the Etna Irou Works have ceased to make marine engines; the Fulton Iron Works are for sale; W. H. Webb's shtnvard is to let: Henry Steer's yard is empty; the Continental Iron Works arc almost deserted i and ^ra^s is growing in nearly all of tbo shipyards which a few yearn ago were filled with workmen."

It may be sold, that something similar applies to London, but that is only because other English ports are taking the trade. English fhipping is superaedlng American, and there is uo other reason than this —America tries to make her own iron, Ac., which she cannot, except at high price, instead of buying ours cheap with the corn and other Lhiugs who can really produoo well, and thereby indirectly throws away the friendly union with us wkich would result from being mutually necessary to, and benefactors of, each other.

Sigma.

Sth.—" F. W. M." en page Vil, neems to have fallen into the trap that *'Soul Kymea " laid. Ho believes that England is getting drained of her wealth, by an outflow of money sufficient to make our exports balance our liiports—a very c nnmon error. It Is necessary to remember that, with tho most insignificant exceptions, trade is barter; tho fluctuation in the amount of gold held by England from yar to year, may be entirely disregarded in examining her trade with the whole world, and this ono fact should be steadily kept in night, that one year with another, tho gooas we nend out to the foreigner, are cousl'J' ted by nira equal in value to what he sends us In return. Take the case that I gave, and which "F. W. M." hold? up to ridicule. 1 st-nd out a cargo, which costs me here C18.0J0, but when it reaches the foreigner, it is worth €^4,^00 to him there ; and in return he ships to me a cargo, for which he charges mc there £'-'4,000; by doing so, he balances my account, I owe him nothing, he owes me nothing; bat his cargo, when It re&cheji me, la worth £29,o5u here. I do a good trade, and can afford to pay freight, Insurance. Ac, out of the difference of the English values of the two cargoes. Where the goods arc, makes the difference in value.

Herbert.

INDUCTION COIL.

Sir.—I do not quite apprehend Mr. Cunningham's question, p. 258. Covered wire is used simply for convenience of manufacture, but any mode wf keeping the wires out of contact answers. When I first took up the itudy of electricity, I could get neither books nor materials, and bad to study out things for myself and discover them afresh, guided only by the knowledge that certain things had been done. That was in Australia, before the days of gold; It was also before the days of RbumkorfTs coil, and 1 made to all intents nu Induction coll(ouly without the condenser) without knowing that I had done something new, for my coil gave violent shocks by holding one conductor only. The reason was that I hud no covered wire, and invented a substitute, and my wire was exceed ingly fine ; in fact, I laid it very closely and evenly, separating ihe turns by a cotton thread, saturating very carefully with an insulating varnish, and divling the layers by silk saturated with the same. But such a coil, though very well for an amateur experimentalist, would be very troublesome to make, and almost Incapable of repair If Injured, and consequently would not have been made in any but such exceptional conditions.

No doubt Mr. Cunningham's coil acts as he says; the only point is, whether, with the snme battery power, much greater effect* would not be obtained from the same wire if the Interposed material were absent, lor the true point to bo aimed at is to keep fie wire as close as possible to the magnetic core, as distance has so great effect in diminishing action; the only limit to this approximation being the necessity for insulation. The moment this insulation exceeds what is necessary, it is injurious, by weakening the effect; hence the importance of using the very best insulators, and applying them on the best principles. Taking, for illustration, a coil liln. loug by 3tn. diameter, the secondary of which is an inch in depth. If wo use the common process of working the layers from end to end, the points of extreme tension are separated only by that Inch, and it being occupied with a succession of stages of wire, there is great facility offered for the spark to leap across this space, and we must resist this by extreme care in insulation; but if, beginning atone end, we work our layers in sections instead of strata, and complete the coil as we go towards the other end, the points of extreme temiou are separated by 6in. instead of one, and much less perfect insulation is needed. There is, however, a great practical diftculty in the way, and at present a compromise is effected by the process of dividing the secondary into a series of short Independent coils, each complete in itself, and connected to its neighbours.

This will reply to Mr. Forbes, .No. 3953. The connections are made alternately by passlug an end under the diso to form the commencement of next section, and soldering the outer end. The Mercury Break is only needed for very powerful coils.

Sioma

DRV PLATE PHOTOGRAPHY. Sib,—I have lately been making some experiments In dry plate photography, and I think it will not be out of place to give the renders of oui journal a few hints on this subject, which I consider ono of the most beautiful and interesting braucbes of the above art. Within the last two months I have tried three processes, tannin.'Ryley's modified Fothergill, and coilodioalbumen. but, taking everything into consideration, I prefer the tannin process, as the plates can be pre

Sared very quickly—in fact, I usually prepare two ozeu in one night, and allow them to dry In the dark room until morning, when 1 find them quite dry without the application of artificial heat. I have used Mawson's ordinary wet plate collodion, which 1 find an excellent one for the tannin process. I coat the plate and sensitise It in the usual manner, and Oh removing the plate from the bath I wash It with about a quart of water (I have tried distilled water, but I find plain water to answer quite as well). I then Immerse it in a 15 grain solution of tannin, and let it remain while another plate is being ptepared. It Is then taken out of the tannltt, and placed to dry in racks, or reared against the wall. When it is quite dry it is varnished about Jin. all round with ordinary black varnish, laid on with a soft camel hairbrush. It is then ready for exposure, which should be about 12 times as long ns for wet plates. I develope with Pyro, 2 grains; citric acid, 1 grain ; water, loz. The plrture should be brought out with the least possible

3uantity of silver in the developer, and when all the etail is out it can be Intensified In the usual manner. With Ryley's process 1 bad very little success, but with the coUodio-albumen process I have obtained some very beautiful results; but It is a very trouble»'£uc process, aud very liable to blisters, Ac.; so on the whole I prefer the tannin. I usually develope the same evening I take the pictures. I beg to *sk " Mus'* or some other of your photographic correspondents, if they will be kind enough to deFcrlbe tome the coffee and gum gallic processes, and let mo know if they have had any success in them, as I believe they are both very good processes; but I have not the formula:. I have also got an old wedding ring; It weighs just 25 grains. I shall be glad if any of our subscribers can inform me how to convert If into chloride of gold solution for toning purposes. I have tried " Mu«.*s" toning bath, and find it an excellent one, and should like to havo a few more hints from him.

Tannic Acid.

RE CAPT. PETERSEN'S STEAM LIFEBOAT. 8in,—After reading in your Journal of the 20th nit. an. appeal, on behalf of Capt. Petersen, for subscrip

tions to build a trial steam lifeboat, and among the list of subscribers Mich corporations and institutions as Lloyd's.the Salvage Institution, the Shipwrecked Mariners' Society, and the Royal National Lifeboat Institution, Ac, I was much surprised to tee la your Impression of the 27th uK. the letter from Mr. Lewis, secretary to the last-named Institution, raising such objections, many of them ridiculous on the face of them. Surely the several institutions here named, and more particularly the Royal National Lifeboat Institution, thoroughly examined the plans, models. and drawings, and fully satisfied themselves that Capt. Petersen's invention was not only practicable, but had every chance of success. 1 doubt not Mr. Lewis raised the same objections at the time his committee was inspecting the invention; and the explanations given by Capt. Pesersen were such that they were perfectly satisfied it could be carried out, and expressed themselves as such by granting £100 (about one-sixth of the amount required) towards building a trial boat.

In order to Bh jw the absurdity of these objections, I will just quote one, in which he Bays—''That the motion of boats, under such circumstances, Is so great aud violent, they sometimes standing perpendicular, with either the bow or stern uppermost, that the propeller would work at a great disadvantage, and the machinery he perhaps liable to disarrangement."

Now, it Is self-evident that when this happens, the same objection would apply to the lifeboat now in use, inasmuch as the men in the boats would, under such circuinstauc.es, be placed in an horizontal positiou; but Capt. Petersen's steam lifeboat would have the advantage, for I see by his plans that he carries two more sets of oars than the present boats, in addition to which ho has the steam propellers. This advantage Is obtained by the difference iu the formation and arrangement of the air-chnmbers, Ac.

Mr. Lewis must be wrong in sayin gthat "most of the plans merely aim at putting an engine in one of our lifeboats," for from what 1 have Men and heard of the drawings and plans of Cant. ^Petersen's selfrighting steam lifebout, I am under the Impression that it is of a very different form, construction, Ac., giving as mneh, if not more, room than those In present UBe.

I will not trespass further upon your valuable paper, as I have no doubt Capt. Petersen will fully «nswcrall the objections, to the complete satisfaction of the subscribers to the English Mechanic Lifeboat Fund; which fund I hope may be a success, and that their boat will bo built in accordance with Capt. Petersen's plans, which I believe are the very best extant.

R. S.

panded band which appears when denser hydrogen is employed." This paper is dated April 13, 1868, and I think It was after the eclipse of 1868 that the alliance referred to above was formed. But the discovery i& older yet. We must not overlook the work of .Miller, Hngglnt, PlUcker, Angstrom, Gladstone, and the host of other eminent men who have advanced the causeof spectroscopic science,

Richard A. Proctor.

REBUILDING A CHURCH ORGAN.

Sir,—I am thinking of rebuilding our church organ: adding a swell and independent pedal organ: nud I should like to kuow what the dimensions of the bellows should be for a liberal supply of wind, and yet no larger than can be helped.

I should have three Sft. stops, two 4ft., one 2ft. In great organ; two 8ft., two 4ft. in small organ ; one 16ft. tone (Sft. stopped) In pedal organ. I havo most of the pipes by me: and the pedal pipes are getlact, with a foot of 31u. diameter in the aperture for the CCC. If any correspondent would give me dimensions for bellows and reservoir I shall be extremely obliged: also, is there any real practical advantage in having three feeders worked by a three-throw crank, over the usual two feeders worked by a lever? Theoretically It seems best; but Is it?

Country Parson.

DARK LINES IN THE SOLAR SPECTRUM.

Sir.—I could hardly believe that Mr. Lockyer had not misread my letter at p. 211 ; but on referring to it I found I really had by a lapsus calami spoken of density instead of quantity, ^ ou know how much I have been overtaxed with work lately, and will therefore understand how easily such a Blip would occur. Fortunately, however, the text exhibits my real meaning very clearly; and I have written too often and too recently (see Pop. Sc. Renew for October, l$i'.<», in which I have introduced even a picture of the widened lino of hydrogen, as also my "Other Worlds." at p. 46) to a contrary effect for any question to exist as to my knowledge of the fact pointed out by Mr. Lockyer. In the letter he quotes, I was showing that the atmospheric dark lines could not change in thickness as the sun neared the horizon, notwithstanding the greater range of atmoshcre through which the solar rays then pass. No one nows better than Mr. Lockyer that this is the case. A ray from the sun might pass tangential ly through the earth's atmosphere fitly times and yet the atraospherlo dark lines would be no wider than in the spectrum of the sun in the zenith.

If "T. A.'s " view required further demolition, evidence might be urged which is even clearer than that adduced by Mr. Lockyer. I refer to the existence of dark lines In the spectra of the stars.

I take far too much interest iu Mr. Lockyer's work to overlook any of the results he has obtained. Though I do not by any means accept all his conclusions, and indeed consider some of them to be beyond all question erroneous, I none the less appreciate mom thoroughly his skill in observation, his ingenuity in devising new modes of research (witness his suggestion in 18(56 about the prominence spectrum) and the perseverance aud energy with which he carries out his purposes. If my haste in writing should ever again lead to a lapses calami let Mr. Lockyer believe the causeof the mistake to be anything rather thau forgetfulness of his work, or of any that is being done by our fellow-workers in astronomy.

May I venture, however, to point out that the discovery of the influence of density on the thickness of spectral lines belougs to an earlier date than Mr. Lockyer's alliance with Dr. Frankland. I think PlUcker would have something to say an this point as well as on the influence of temperature. I have a paper before me in which Mr. H uggins says incidentally "it seemed of importance to have proof from experiment that this line of hydrogen, when it becomes broad, expands equally in both directions. 1 made the comparison of the narrow line of the vacuum tube with the more ex

STARTLING THEORY ABOCT JUPITER AND SATURN.

Sir,—The reviewer of my work has treated me so kindly that I hardly like putting in a word in favour of the view to which ho feels compelled to *' demur at once." I object, myself, to startling theories unless put forward to explain even more startling facts. This is tbe case as regards my theory about Jupiter and Saturn.

Any theory which accounts for the apparent advance of a satellite on the disc of Jupiter and its apparent return presently outside the disc tan observation made Independently by three such astronomers as Smyth, I'earson, and Maclear) must, I apprehend, be rather strlkiug. I think mine the only available one. In fact, 1 cannot imagine any other. I by no means assert that the surface of Jupiter was necessarily changed In shape on this occasion. Clouds suspended at a great height In his atmosphere may have suddenly been dissipated. Again, Saturn's assumption of the squareshouldered aspect seems to me wholly inexplicable Oq any other hypothesis than mine.

Sir John Herschell writes to me of this theoryr •' Strange as it seemed to me at first, I cannot hut admit that the reasons you adduce are not to be lightly set aside."

R. A. Proctor.

AMALGAMATION AS A PREVENTIVE
OF RUST.

Sir,—Some time since I sent a query to our paper on preventing rust. It was not, I believe, answered. I now send a clipping on this subject, which may be interesting to others as well as myself :—" Reinsch, of Germany, proposes the amalgamation of the surfaceof the iron as the best protection against rjst, the only difficulty being to make the mercury adhere to the iron. It is well known that iron has no affinity for mercury, aud even a perfectly clean surface, when rubbed with metallic mercury, will aboorb none of it. On the other hand, gold, tin, load, and several other metals will take It very readily, and almost all metals quickly become coated with mercury when plunged into a solution of a racrcuiial salt; but iron and steel resist even this treatment, and it is only here and there that some traces of the mercury adhere. Fortunately a method of causing the mercury toadhere to the iron has at last been discovered. The iron is first well cleaned with hydrochloric acid, and then plunged into a very dilute solution of sulphate of copper, mixed with a little hydrochloric acid. It immediately becomes thinly coated with metallic copper, which copper must be removed by means of a brush, friction with paper, aud washing. The iron is then placed in a very dilute solution of bichloride of mercury, also mixed with a little free hydrochloric acid. It becomes perfectly coated with a layer of mercury, which cannot be removed by friction with, rough bodies. This mercurial covering protects the iron very well against rusting, especially if after the amalgamation it has been washed with liquid ammonia. Tbe inventor has compared the action of the air of the laboratory, continually charged, as it is, with acid vapours, on iron objects, some of which were amalgamated and some not, and found that, while the latter were rapidly eaten up, as It were, the amalgamated objects did not show a trace of rust; and ho recommends tbe process highly for the working parts of all kinds of machinery, clocks, engines, Ac. He also proposes to amalgamate the iron parts of suspension and other bridges before covering them with paint, in order to eusure a more perfect protection ■ but, as we know that mercury diminishes considerably the strength of thoselmeta Is. with which it easily amalgamates, we think it would be well to test its action in this respect on iron wire before risking the amalgamation of the wires forming the cables of a suspension bridge."

I may say that I have not practically tested the application of mercury to iron, but I see no reason why this method should not be successful.

J. H. W.

THE SAFETY VALVE.

Sir,—Having noticed many queries in our English Mechanic relative to the lever safety valve, I now offer, for the benefit of my brother readers, the following calculations thereon. Fig. 1 is a diagram of a lever safety valve of the ordinary construction in which L represents the lever, V the valve, and W the weight.

As the words effective pressure will be frequently met with In the following examples, I shall explain the meaning of this phrase before entering into the calculations.

Suppose the lever of a safety valve of uniform depth and thickness to be 24in. long, 3in. from the fulcrum to the centre of the valve, and weighing 31b. Now it is the opinion of many that a lever of this dimensions would only exert a pressure of 31b. on the valve. This, I must say, is a great mistake, for such a lever instead of exerting a pressure equal to its own weight, would exert a pressure as great as three times its own weight, or l .'lb. on the valve, and this is what Is called the effectivepressure.

Role.—If the lever be of uniform depth and thickness the effect of its weight on the valve is the same as if it were collected at its middle, but if the lever is tapered, then its effective pressure can only be found by disconnecting it and ascertaining what weight is required to balance it, making the point in a line with the centre of the valve the fulcrum. The result will be the effective pressure of such a lever.

All the examples will be worked out by two method:'.

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240800 S1W12

Ans. 77 071b. per. sq. in.

Ex. 2. The diameter of a safety valve is 25ln pressure per square inch 201b., distance from the fulcrum to the ceutre of the valve 3ln., weight on tbe end or the lever 401b, effective pressure of the lever and valve 81b. Required the length of the lever.

Let d represent the diameter of the valve, e the decimal -78*4, p the effective pressure of the lever and valve, io the weight, » the pre-sure per square inch, / the distance from A to B, and * the unknown part A

Then

d^xexs-pxt

mt j,. . Ans- 3ln.

The divisor 00150 is obtained by deducting the

S5fX* °' thc Talve and lever 'rom the direct pressure 98'161b.

Ex. 5. The lever of a safety-valve is 54ln. long, 3ln. from the fulcrum to the centre of tbe valve, effective weight of tho valve and lever 0 251b., pressure per sq. iu solb., weight on tho end of lever 69904751b. Kequired the diameter of the valve.

Let r represent the ratio of A C to A B, w the weight p the effective pressure of the valve and lever, S the pressure per sq. in. t the decimal -7854, and x the required diameter. Then

v> x r + p the area of the a

■ = valve represented . •. J x

S by a g

69 90475 weight on lever

8 ratio of A C to A B

559-13800 0 25 weight of lever and valve.

80)505-488

■7864)7-0ii8fi( 9
7 0686

^9 = 3. Ans. Sin.

Ex.6. What is the area of an annular valve whose greater and lesser diameters are 12 and 4in.?

Let d represent the diameter of the greater, and I the diameter of the lesser, c the decimal -7854 and thc required area. Then

(rf' x t) (P x i) = x

And

tan. I = tan. P Cob. (^ u-y

S y A and P can be eliminated and tbe fornra!i written In the form of

slu. \ = cos. a, sin. $ - Bin u gin. a Cob. 3

sin.,,, tan. J

tan. I = cos. („ tan.a + .

cos. a. This Is not the same formula as was given on th» page alluded to by G. Firth, but the one can be deduced from the other. I will give him the formulae proper!* corrected, and which I think is as easy a method at any other of altering co-ordinates, tan. o"

= tan. B

sin. a
B

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1005312 Ans. 100-5312 sq. In. Ex. 7. To what height must a safety valvo 3ln. In diameter be lifted to allow a free escape of steam equal to the area of the valve?

Rule.-Divide the diameter by 4. the quotient will give the height. Let d represent the diameter of the valve, c the number 4, and x tl.e required height Then

J

c Sin. -i- 4 = 0-75 Ans. Jin.

Tnos. J. O'Co.tNOR.

TO G. FIRTH. Sm,-I am afraid G Firth has attempted to study practical astronomy before making himself master of a little spherical trigonometry, or he would have no

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Cos. B tan. A sin. I = tan. \. » Tt S\Flrth wishes an example worked out in figures I shall be most happy to send it, or to give him any assistance. ^ K Omicbok

THE BOEHM FLUTE.

Cllluatrated abort.)

Sir,—The ring principle on all instruments or the nute genus, is the same, it gives the power of closing two or more holes by the action of one flngor only Ihe invention of this system belongs to a Captain (jordon. who conceived thc idea in Paris, about thirty years ago ; the application of it, however. i< Boehm-s a professor of Munich. I think a little work " The J, *,'«"" TTMnBl'ion State," by tbe Hon. and Kev H. hkefflngton, would Interest "Another Flautist" ilatchard, or Skefflngton Piccadilly, are the publishers. As a soloist I am acquainted with nearly every form of flute. For the Boehm Huto I cannot say much; It is an expensive Instrument; tbe faults I find with are, the constantly-recurring dlfflcultics in executing rapid and continuous passages, especially in the third octave, arising from tbe cross and back fingerings. To illustrate this, the B fiat, which ^produced naturally and easily on the Nicholson flute, it fingered on the Iloehm by placing the third finger ol the left hand on the ring of the G hole, and F sharp is fingered by pressing down the ring of the E hole/fe* are 3fr7 inward and unnatural flnaerlnga! added to which, tbe quality of tone In the third octave is thin, weak, and tho notes bear very ihcrp (in tbe old flute they bear/o(), and It is almost impossible for an amateur to blow them In tune. This arises from the fundamental holes being made too large, and without due attention to tho part thoy are required to take as »v>n/ holrj In the production of tho notes in the third octave. But should "Another Flautist" desire still to hear more of the Boebm , I shall be happy to give him information. Tune, tone, and facility, are the things sought for by flute players of all classes, and with your permission I will now describe as briefly as I can, a flute in which these qualities are combined to tbe fullest possible extent As a professor of the flute, having no pecuniary interest In flute manufacture, I continaed to use the Ilohin because its powers and capabilities were Bo great, notwithstanding the toil it imposed upon thc performer.especiully withtbeopenG sharp key. Nearly eight years ago, the late Mr. John Clinton showed me his flute, called tho "Equlsonant," a vast improvement on the Bobm, which I have used ever since. I subjoin a sketch of It, with an explanation In the Boehni the thumb key makes C natural, in this flute that key makes both B flat and C natural. There is a small open key at o (the C uataral key), which is keptaniit by the tbumb key, and when the latter is opened for B flat lu tbe usual manner of fingering this note, the C natural key is kept closed by the second finger of the left hand pressing down the ring surrounding the A bole. There Is also an open F sharp key at 4, which is shut by the second finger of the right hand pressing on the V natural hole ring, the third fiDger of thc same hand closes It in making D and F natural, for which latter note the " fork fingering" Is used in all rugged and difficult passages. The F natural key (short) remains. At c there Is a lever for acting rapidly upon either the B flat or C natural keys. At if la a lever for the D shake key e. Your correspondent knows not what he asks; the mechanism of flutes has been a vexed and most iutrlcate question.

Sahlk.

TOST-OFFICE TELEGRAPHS. Sib,—I shnll be glad if you will allow me. through the medium of your pages, to aak some of your able correspondente, the following questions, which are important to many in the same circumstances as myself. It is well known that the Post-Office, baa lately acquired thc telegraphs, and a great many postmasters have to waste hours dally in watching the instruments, and therefore 1 wish toknowwhetber it is possible to fix a bell, or other apparatus, to the existing single needle instrument, in such a manner, that we shall have warning without the watching now tedious to

A Poor Postmaster.

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SEWING MACHINES.

Lock Stitch. (Continued from page 232.) Sir,—Spools, or metallic bobbins, require illustration to show what has been attempted in the way of

improvement.*,The Wheeler and Wilson spool Is 1 —in.

16 5 dlam., and —in. thick outside, and holds 18J yards of

32 >To. 21 cotton, ^nd the Elliptic holds 1*J yards, 'he

1 7

diaxn. being 1 —in., and the outside thickness —in.

16 33

"Wheeler and Wilson's spool is represented at Fig. 1, a front view, and Fig. 2 u section. It is formed of a pair of thin steel discs, a A. fixed on a brass axis C, with a hole through it, which fits on the shaft of the machine to receive a rotary motion for winding on the thread, and in the space between the double couvex HpooL A very Blight variation of tension may be EiTen by windiug on the thread more or less tightly, but it is too limited to be of much nse. To provide a tension on & spool, the following plan has been adopted. A pair of flat discs, with the thread wound on the axii', which also hus a hole through It, Is placed within a case or shell, having an axis to retain the spool in place, and to allow it to unwind freely. In the rim of the case or shell, holes were made for the thread from the spool to pass through, to give tension in the same manner Ah in Thomas's shnttle. There are machines of dlffereut maker** working this kind of spool, but they are not successful compared with other machines, although it may be wrong to trace the failure to the spool only. In somenmchlues, the spool was applied horizontally, as illustrated in F*ig. 3, a plan, and Fig. 4 a sectional elevation; a is the spool, composed of two flat dines on an axis, with a hole through It. The shell or case b, has a centre pin, on which the spool a revolves. The thrend passes from it to the holes C, and is passed through one or more to obtain the tension required. The frame d d ot the machine, is bored large enough to contain the spool, and a rotating hnok e secured to the pinion f , an axis, g, passes through the pinion. The top end of the axis forms the bed on which the spool and case rests, and the lower end is secured to the frame of the machine. The pinion./*is driven by a rack motion in some machines, in others the hook receives a direct circular movement The action is ns follows: —the ascent of the needle in the needle slot, throws off the loop, the hook takes it in its rotation, or its vibratory movement, and opens It, when ou the back end of the hook, then carries it more than halfway around the spool case ; here the needle pulls the thread oft'the hook, which returns ready for the next stroke or stitch. In machines which make a complete rotation of the hook, the needle thread is le.-s liable to break, and more easily slips off the hook. When the needle thread has been passed around the spool case and released, It has to be pulled up through the needle plate at a great velocity—for every stitch from Sin. to .'"tin of thread must po pass. Th's is usually done by a spring above the needle, and the least accident to so delicate an instrument, puis the machine out of order. A thin fabric, stitched by a shuttle, compared with one stitched by a spool marhine, shows in practice what may be expected from theory—viz.. the less quantity of thread passed through the fabric, the better the stitch, and the longer the loop passed through the fabric, the more uneven it must appear, laying below the surface on the upper side, or diawu'too deeply into it, and forming too thick a ridge on the lower wide. The spool machines are not made or so.'d because of this quality, but because of certain other merits they possess. All the best qualities in the various machines have not been combined in any machine yet made, nor are they likely to be. This thought should humble some of the members of the sewing machine family, who are given to overmuch boast tug. Little remains to be noticed about the various instruments employed in the formation of the lock stitch, until the machines are described and compared.

Other contrivances have been patented, or proposed, but not of Buihcleut practical value to describe them; and some of these here noticed, serve to show the many ways proposed to do the same thins,', and to point out that the impracticable may become the stepping stone to the useful; or to show inventors what has been already done, or attempted, and to save them the trouble of re-Inventing and re-patenting. This has been done to a large extent, and many years'experience in searching the patents, and making new machines, and experiments for inventors, leads me, an a matter of duty, to waru tbo>e who are fond of new ideas to be careful. One of the most unsuccessful attempts to form the lockstitch, and one ofteu attempted, is to use a large reel of thread below in a spool, shuttle, or its equivalent, so that it will last ns long as the needle thread, and require no re-winding. One ot the most likely plans to do this, was to place the reel in a large spool case, having an external •crew, which, by rotating, carried the needle thread from its front to its back end, the holder and driver being arranged to let the thread pass over it without obstruction. The arrangement was so fur practical, but it involved long needle loops. More time is lost Vn pulling up long needle loops than in rewinding spoolt or shuttle r**els. The spool, lay ingVm its flatbed, and having a horizontal moving hook." is not so well adapted to work as one vertically placed. In the former the hook in taking the thread from the needle pulls it out of Its slot horizontally, and thus bends or •Teaks It. The vertical pull of the hook is a far better arrangement, the needle being less liable to strain or breaking; and the thread pulled more in one direction only, or chiefly dowuward instead of across theuetdle: A vertically oscillating h^k has been applied to carry the thread around the sp< ol, the hook being curved to nearly match the clrcumieiuuce of the spoul or holder. All such contrivances are more complicated than direct circular motion, and the complicated in mechanism is nearly always leading to failure ; the simple, giving less trouble to learn, or keep in order, leads to success.

It is not advisable to do more than allude to other inventions for making the lock stitch. Miuttle machines are made to work with waxed thread, kept heated by iras ; In some the loop of the needle thread is kept open by a hook, so that the shuttle passes through without dragging on the thread. In one machine working leather the shuttle is made to work

above, instead of under it. Leather machine bands, ship Bails, heavy carpets, and every kind of sewing, from the heaviest 1o the lightest, is done by the shuttle. One of the earliest Httempts at a lock stitch machine was to use n double-pointed needle with its eye in the middle, arranged to go through the fabric, and back; being pulled by mechanical fingers, and usiDg one thread, and pulling it tight at each stitch, as In hand Fewing ww Odcc thought the best way. Another inventor proposed using a long needle, and to gather the fabric on it. then pull it through, and draw out the gathers until the thread was tightened; this simple but limited appliance is often revived and sold.

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Let such be warned In time, "a stitch in time saves nine."

A Practical Man,

P.S.—" .Tacquard."3848. Notes and Queries, is advised not to be In a hurry to begin making a sewing machine. His questions show great discernment, but Involve too many considerations to be answered in a brief reply to do him service. What he requires is partly explained at page232, and the reBt will be fully dealt with when the subject Is sufficiently advanced. Machines of different classes will be fully illustrated and their qualities compared, so that ''Jacquard" and others may know how to proceed. No. 1. Some machines drive the shuttle by a crank, others by a csmwith a pause. The form of cam depends on the other parts of the machines. 2. Buy a shuttle the size you wish. The shell is stamped to shape, and the ends brazed in. 3. The stroke Is longer than shuttle from

— to $in., according to the machine and size ein

10

ployed. 4. The needle to raise about flln. and then

pause while the shuttle passes through the loop.

SHORTHAND.

Srn.—The best system of shorthand is, I believe, Mr. Pitman's (of Bath); it is not hard to learn, and any one with an hour's daily practice, should bo able to write at the end of twelvemonths, sixty wolds per minute.

G. Gregory.

The revival, and abandonment of inventions and imitations is abundantly Illustrated in the history of the sewing machine. A museum containing one machine made to work from each invention, would be verv Interesting to the mechanic; it would be a very large collection, and show each link which connects together the extremes of difference. How unlike, nt first sight, appears the Howe, and the Wheeler and Wilson machines! If they were taken to pieces, and then mixed together, we should only hud a resemblance in their needles, all the other parts being dissimilar, yet adapted in a different manner to make the lock stitch The instruments tor making the lock stitch, already described in these pages, connect link by link, with slight shades of difference, the extremes of form and action. Some of these contrivances never came in, or soon went out of use, and only served to teach what could not be done, or to suggest ideas for better Inventions. It is strange to find in "thr drrelopment of species,'' and the dewlupment of intention, there is a resemblance. The sewing machine was not a perfect and complete invention at the commencement, but was developed by successive stages of improvement; the strong and useful predominate, the weak and useless die out. Selection of the right stitch is the first thing to do by those who desire to make a sewing machine, then to copy the best mechanical contrivance to produce the stitch, to study in detail the ionnation of the stitch, and working of the thread. The workmanship should be good, but it is of less importance than correct principle and proportion. There should be harmony of movement; needle and shuttle must be in time, and woik smoothly. Further particulars will be given when the machines are completely illustrated, and at this stage many may be tempted to make sewing machines without having first paid sufficient attention to the formation of the stitch.

Sir, In answer to " Hermit," p. 255, Pitman's system is decidedly the most popular, and I think the simplest and most comprehensive. At one time I learnt Odell's, but did not somehow think very much of it. It is of course very difficult to say which Is preferable from actual experience, as to learn one system thoroughly Is not an easy task, while to mix that with others would lead to a glorious confusion. The ease with which Pitman's syBtem la attained speaks well tor it. I received from Mr. Pitman two courses of Icssous, which, of course, greatly facilitates the acquirement of the art, and can only say I was much pleased with the simplicity upon which the system is based. I would by all nutans recommend " Hermit" to join a class taken by the above gentleman, when he would see for himself how surprisingly easy it is to learn. It is perhaps scarcely necessary to add that all depends upon the time devoted, and the regularity when practising.

Harry G. Newton.

MEDICAL GALVANISM.

Sin,—Much as has been written in this valuable journal ou electricity, chiefly in relation to experiments, or to the science itself as a recreation, very little has bceu advanced on galvanism as a medical agent.

This subject appears to me of such vital importance, compared with many subjects discussed in your paper, such as electrical bells, magnetic engines, &c, Ac, that I think it is high time that the subject of medical galvanism should be better understood, and more frequently discussed than it has yet been.

I have been induced to address you because I believe there is no other Journal in this country which is better fitted to deal with this branch of electricity than the English Mechanic, both in its practical and theoretical bearings. Some of your correspondents have, I believe, objected to medical subjects beingdiscussed, and so far as physic itself is concerned. I should sa> that the inquirers themselves are not likely to suffer much iu constitution by remaining in ignorauee of some of the irrational prescriptions which arc often suggested to them. But when a subject like the present falls within the domain of physical science, and when it is remembered that electricity is both an external ami au internal remedy, which can alleviate pain and arrest disease, and this, too, iu the most rational inexpensive, and simple manner, and when the patient himself may administer the remedy without fear of adulteration or empiricsm, its importance becomes secondary to no other branch of electrical cr experimental science, and its generation and application demands the attention of any journal which is competent to deal with the subject, and professing the advancement and well-being of its readers. It is true that electricity as a remedial agrrit has worked it:* way but very slowly, but it would not be difficult to show, did space permit, how this has happened. 1 believe there are still many intelligent physicians who regard galvanism in disease as empirical simply, as I take it, because they do not see or understand why or wherefore it. being a thing intangible, and as immaterial as a shadow, should have any effect upon the nervous economy. On the other hand, others have •roue so fur as to believe this mysterious fluid to be the nervous intluenee itself. But whatever doctrine is right, it is sufficient for our purpose to know that electricity is a surprising stimulant to the nervous system; and, moreover, that it would appear to have the power of preparing, repairing, and replenishing the shattered telegraph wires of the body wit hpermanent additional energy.

Before I close allow me to Btato more particularly my object. It is to ask your correspondents to take this subject up. Amongst the mauy thousands of subscribers to the English Mechanic there will ever be a vast percentage who receive these pages every week, sufferers from some bodily derangement or disease, and who might receive incalculable benefit from a proper application of galvanism. Many, doubtless, are poor and unable to purchase the costly and often worthless apparatus, ns sold nt the instrument makers. Many do not understand the modun operandi ol the same. It would not be necessary to plunge iuto the theory of the nervous effects of electricity, but to point out the most desirable, eauy, cheap, and efficient way of constructing apparatus suitable for medical upe. V* hat is wanted, then, are plain drawings and directions with advice as to the proper administration of the current. An ordinary coil, I believe, is not suitable, the objections beinthat where there is more than one continued wire the current is completed in one, while in the secondary ■wire another current is induced which completely destroys the power as a remedial agent.

.Since writing the above I have received so many answers respecting Medical Coil (" Wanted ") as per advertisement in No. 270, that I am moro strongly convinced than ever that the apparatus and application generally are totally uosuitedto the purpose. The chief error seems to be in using" intense battery power as though the living economy were to bo relieved of pain by additional torture. Moat ot the eoils are primary and secondary wires, and most likely have no guide to the uninitiated as to which is the negatiue ami which the positive pole. It is manifest that this latter is of the greatest importance, since In many liisoasesthe current from the positive element should flow with the natural stream through the system, whilst in others thocurrent is reversed.

W. A.

LIGHT OF HEAVENLY BODIES.

Sir,—In a letter addressed to the astronomical readers of the journal in general, H. W. Bishop state* that 1 said " that the light of D'Arrest's comet was to be 0126." 1 think Imny venture to say that I never said anything of the kind. What I said Whs, probably, that the intensity of the light of the comet would be represented by that fraction.

The intensity of light of a heavenly body not eelf1 luminous is proportioned to , R and D represent

ing respectively the radius vector and the distance from the earth. At the time mentioned this fraction 1

was equal to 0-126. At the time that the comet

R2 Di

b< camo invisible to Maclear, at the Cape of Good Hope, at the last apparition, the Intensity was equal to u 190, the probability was therefore that at the date 1 mentioned, the comet would be slightly fainter than wheu lout sight of by Maclear. So far as 1 know at present, the comet has not been discovered.

Omiceon.

THE LATHE.

Sir, — Since the appearance In the English Mechanic of the description of the Saltaire Lathe, 1 have beeu troubled with a wish which I have had many a time during the last two or three years; and that is to improve the lathe in my possession, or make a new onp; I have been a subscriber from the first number, and have carefully read all the articles on the lathe, but I am undecided on many of tho different points ofa perfect amateur's lathe. Could Mr. Smith be induced to give us working drawings of an amateurs lathe, drawings to scale, say the gantry, so as to give height from floor, and a section of do., length he would recommend gantry to be, drawings of the two heads, and height of centres, together with the form and diameter of spindle of fust head—in fact, all drawings and dimensions to enable an intelligent workman to construct one, always bearing in mind all to be made so that the other tools, chucks, slides, and the different cutting apparatus belonging to a complete lathe could be added to at the inclination or convenience of the workman -, I think it would bo a very interesting subject, and one very popular. If the height of the centres given did uotsuit every one, they could enlarge or reduce the dimensions to their own. That the subject is of interest I see by the inquiries of No. 3866 and 3867 in this week's number. Some time since we had two or three promises from different parties to give us specimens, Ac, which I am sure would give pleasure to more than myself, and alBO Mr. Hunt promised drawings of his chucks. 1 am sure they will excuse me reminding them—the reason is, I am anxiously expecting them.

^" II.

^ ACHROMATIC OBJECT GLASSES.

SiR,—I f any of your contributors would kindly afford information upon the following points In connection with achromatic telescope object glasses, it would be highly esteemed. In Brewster's and other works upon optics, the secondary spectrum is described as being of a wine, claret, purple, or lilac colour, on one side, and of a green colour on the other side of the focus. Now, I have had opportunities of examining object glasses by some of our best makers, and the outstanding colours in all of them, were (to my eyes) blue and yellow. Am I to infer from this difference, that the tint ot such oustandtng colours is chiefly influenced by the colours of the two rays selected for correction, and that the extreme red and violet ones (as mentioned in the works alluded to), are no longer (as a rule) adopted for that purpose ; or is such difference attributable to the irrationality of the glass now employed! It has struck me, upon comparing one or two glasses which have come under my notice, giving a dark outstanding colour, resembling the claret, or other colour referred to, with one in the blue state of correction, that tho formor was incomparably superior in defining fine black lines, which I attributed to the contrast botwe3n black and white being much better preserved under the influence of a dark colour, than a pale bright one. The dark colour also appeared to be much less in quantity than the blue, which latter seemed to form a cloud before the object, and a wide fringe around it, while no such obstruction was perceptible or appreciable in the case of the claret or deeper-toned colour. I should like some of your readers to make the experiment upon a white enamelled watch face, at a distance not exceeding thirty or forty yards, in full sunshine, under a high power. Other states of achromatism may, by uniting more luminous rays, appear, theoretically, to offer an advantage in the shape of Increased intensity, but is it not proimble that a paramount disadvantage may accrue from the uncorrected colour being both paler and thicker, or of greater breadth, whereby the advantage which would otherwise necessarily follow the increased amount of light, would be coun

terbalanced? If the experiment suggested be made, it will be observed that uuder tho blue correction, black letters will appear absolutely blue, whereas iu the other case, the black will remain unimpaired. I should also be glad to know what effect over and under correction have apon the definition, and wh ether object glasses, in which the extreme red and violet rays are united, can now be procured, and of whom?

O. G.

GEOLOGY OF IRELAND.—WHEEL-STONES.

Sir,— The deposits of limestone pass-d over by your correspondent " F. Harwood," p. 25?, is that known to geologists as the Mountain Limestone, and occurs as the middle group iu the Carboniferous system immediately underlying the true coal measures. This formation and the carboniferous slates are widely developed in Ireland, and give to the scenery of that country some ol its peculiar and special features. J he characteristic fossils of the mountain limestone *re corals, and enermites, especially the latter, which very often mako up the entire mass of the stone; from this circumstance it is aptly termed "cuerruital limestone." The enermite, or "stone lily," so called from its elegant shape, is an animal, and belongs to the class Rualata; it Is, in fact, a star-fish set upon a flexible and jointed stalk. It is of marine habitat, and wherever fouud as fossils indicates the strata to be of marine origin. Until recently the race of Ori no ideal were thought to be extinct, and only a dozen kinds are now known to exist, one of tho dozen being dredged up by Professor Forbes iu Dublin Bay. The separated discs of which the stems of these creatures were composed being naturally perforated, were thus easily strung upon strings and used as rosaries; they were likewise called wheel-stones, and St. Cuthbert's beads, by the English peasantry, St. Cuthbert being credited with their production. Sir Walter Scott refers to this circumstance ia Marmion :—

"On a rock by Lindisfarn
St. Cuthbert sits and toils to frame
The sea-born beads that bear his name."

H. H.

THE SOLAR SPECTRUM.

Sir,—I am pleased to find Bo good an authority as J. Norman Lockyer endeavouring to remove my difficulties regarding the origin of the dark lines in tUe solar spectrum. In returning thanks, may I assure him the evidence afforded by the phenomenon referred to had beeu duly considered before 1 ventured to write the letter on p. 114; but I arrived at the conclusion that tho want of chemical power in the light of the solar prominences was the cause of some of the lines changing from dark to bright.

I cannot agree with Air. Proctor In considering Kirchhoff right in ignoring crepuscular evidence. Neither, iu my opinion, are the conditions wanting in the upper atmosphere for the chemical combination with oxygen of iiou, magnesium, &c. He is, however, undoubtedly correct in considering that employing one'stelf about photographic mauipulations does not necessarily teach anythlug concerning i he nature of light. That depends upon the character of the operator. lie may be an excellent photographer, although a poor philosopher.

In dismissing this subject, I wish to add a few wordB respecting the motive for its introduction. In the hands of the astronomer (Jut spectroscope has proved t»t great service. In the hands of the meteorologist it wouid be equally as useful. 1 hope thishiut will uqi be disregarded.

T. A.

TESTING BOILERS.

Sir,- As a rule, I do not care to call In question the replies given in good faith by your correspondents, but in your present number 1 find "Anti-Egyptian" gives a reply which is very likely, from its vagueness. to mislead many of our fellow-readers. His first remark, that it is useless to test a boiler uuless the plates and stams are visible, is quite correct; but after that he states that a single rivet ted boiler cannot be worked at a higher pressure thau 151b. on the square inch. Now, from old age or imperfect construction, it may be that his engineer forbids hfm to work his own boilers at a higher pressure, yet the statement that no boiler can be worked beyond 151b. unless it is double rivetted, is certainly a long way from the truth. I can assure him that we have three Cornish boilers constantly at work, from year's end to year's end, at a pressure varying between 351b. and 401b.. and yet they are all single rivetted. How can tho two statements be reconciled?

T. S. Conibbee.

EXAM.-QUESTION.

Sir,—'* Zeta's" query (No. 3037) is very simple. The weight of one litre of nitrous oxide at 0C. and 760 metre of mercury is 19712 grammes. 200 grammes, then, must measure 101-401 littes, and as one litro equals 61-028 cubic inches, that is equivalent to a volume of 6101 065 cubic inches. He can readily obtain the weight in grammes of one litre of any gas at 0 C. and 76 metres of mercury, by multiplying '0806 by the atomic weight of the gas, and divide by its volume: as for nitrous acids

■0806 x 44

= 1*0712 grammes.

2 Mr. Murray (No. 3961) should use gelatine as a cement for bisulphide prisms, it is the easiest, if not the best, as I can personally testify.

Urban.

ANSWERS FROM GEORGE E. DAVIS.

Sir,—A correspondent, "Another Flautist," requires the composition of German silver. I offer the following :—1st, copper, 40pts.; zinc. 2ft, and nickel, 31pts. 2nd, copper, lupts. ; zinc, t>, and nickel, ipts. The first is a good metal, which takes a high polish, and tho second is the white copper of the Chinese.

Query 8850.—" Thermo " may perhaps «oiw war -caercury iu his thermometer by giving It a aertes «f •rta.-^i jerks. See also other correspondents' acs^-r* ie "Gitcho Maui to," some time since.

Query 3975 (" Gratus ").— Bleaching powtfcer Is tmmds by placing lime (moist) in a chamber, npon p*rtwra*cr. shelves, and submitting it to the action of ehiarine. evolved by the action of hydrochloric acM ^pon ^m black oxide of manganese. Refer t*> correspofvi^^" answer- on the subject in a recent volume

Manufacture or Oxygen (3SS3.— ** M. P. O SO Oxygen may be prepared from manganate of heating it to a certain temperature in a retort -wh connected with a steam boiler, and a pump while' plies air. The retort containing the m&ngai first heated, and the tap from the steam boiler i on, when the following change takes place :—

(Naa MnU4), + (H/>1, = (NaHOH + Un aO, + <k The oxygen being liberated, is passed on to the *>:*■»_. When this part of the operation is over, the *t*e-aa * shut off, and superheated air pumped over tho sa lure in the retort for about a quarter of an hour, asm It is again ready to undergo the first operation. las air must be freed from carbonic acid by passing K through caustic soda solution before being rinmnafl over the spent manganate. The m angaria te may he made In large quantities by heating to redness mature of black oxide of manganese, carbonate asi nitrate of soda, the heat being continued for «*wat time, In order t,> decompose the sodium nitrate, whim is at first formed.

Query 3887.—The Utilisation Of SrwAOE.—* "On the Utilisation of Sewage by Irrigation and H fration," by C. E. Austin, Mem. Inst. C,E.; rt&Mau Latham's "Inaugural Address," published by >tn. and Baron Lleblg's letter on the utilisation ofti* metropolitan sewage, published by Allnutt There > plenty of Information on the subject to be font4 scattered through the Engineer* Building A'cws, *&d Gardener's Chronicle.

Water Analysis.—("Aqua" 3886).—If "AquaIs not accustomed to make analyses of different articles, and water more particularly, he should tu* Nicholson's process, which is admirably described m the Quarterly Journal of the Chemical Sorictit for December, le6"i. The apparatus required would be 2 burette*. SO cbc., divided into 500 parts, with an ErdmarjzTs float. The burettes must be provided with a gins* stopcock iu lieu of the indiaruober tube, and Mobr's clip; several small stoppered flasks, to hold about 100 cbc.; porcelain and glass evaporating basins, a few beakers, and a litre measuring flask. Solutions required are:—Standard lime solution: Dissolve o 172 grm, of selenite iu a litre of water. Soap solution; The eapo mollis of the Pharmacopoeia ia to be dissolved in a mixture of equil volumes of distilled water and methylated spirit, to be of snch a strength that ?S divisions of the burette ara required to produce * permanent lather with SO cbc. of the standard lime solution. Barium nitrate solution: 0*26 grm. in a litre of distilled water. Silver nitrate solution: *5 grms. in a litre of water. Ammonium oxalate, 0'^oa grm. la a litre ot distilled water. Chameleon eolation: Ulasolve 0159 grm. of potassityn permanganate in a litreof water. These are the standard solutions required; but for the analytical process 1 must refer " Aqua" to Nicholson's paper, the December part of tho Journal of the Chemical Society for 1802, as it would occupy too much time and space to give it In detail.

Volume or Gas (Query 3937).—" Zota" can calculate the volume of gas thus :—11 16 litres ef hydrogen weigh 1 gramme; and as the density of nitrous oxide is 'll, that is half the molecular weight, u 16 litres or that gas measured at 0°C. and ?60mra. would weteh S2 grammes. Now. as we know that ±2 gram Mips occapy 11-16 litreB, we can easily find the volume occupied by 200 grammes:— 1110 x 200

= 101-454 litres at 0°C. and 760mm.

22

Weight Of Gas (3956).— R. Terset can, by lollovIng the method here detailed, find the weight of a*v volume of gas. Ftr*t, methyl gas la known by the

formula J [j{j3 therefore, the density would be IS.

Now. a litre of hydrogen at 0°C. and 760mm. weigh-* 0i hvm grm.; consequently, a litre of methyl wouid weigh 1-344 grms. at the same temperature and pressure. But the temperature is 20°C, whilst the pressure remains unaltered. A litre at 0°C. would become 10?:i3at20oC.bytheequationl+ atorl +(0*003665x20;. We can now easily get the weight of 1 litre, for we have found that 1.0733 litres of methyl at 2o°C. weigh 1344 grammes; what will a litre weigh? By the 1344

equation: = 1*862 grms., or the weight of a

10733 litre of methyl gas 26°C. and 760mm. Aa regards the second part of tho question, bow to prepare acetic acid from the substances mentioned, he may prepare it thus:—First flack the lime with water, and boil it with sodium carbonate to form sodium hydrate. Tills must be obtained in the solid state by evaporation.;

Na,C03 + Ca(HO)3 = (NaHO)i + CaC03. The ethyl iodide Is then acted upon by the eauitic soda, and the resulting alcohol distilled off :—

CjUM + NaHO = C,H«0 + Nal The alcohol is then heated with sulphuric acid and potassium chromate, when aldehyde is formed :—

C?HcO + O = Cjfli O + HaO and this aldehyde, when fu^ed with caustic soda, forms the sodium salt of the acid, from which salt the acid ia liberated by sulphuric acid :—

aH.O + NaHO = C2H, Na02 + H3

and (tViiNal*-,), + H5SO4 = ((.^HiO.^ + Na2R0t

I hope I have made the process plain enough fur R.

Terset, and shall be most happy to help him on tbc

road to knowledge whenever he chooses to ask.

G r.oRGK E. Davis.

FLIGHT.—FIGURE OF 8 WAVE THEORY OF WING MOVEMENTS.

Sir,—Would you kindly insert the enclosed in your valuable journal; it will, I think, interest a. large number ot your readers. Aviatok.

In the proceedings of the Royal Institution of

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