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The remainder of the meeting was taken np in discussing the above-mentiuned subjects, and in bringing forward question« relative to pistons, gnidoscopee, and pedometers.

LETTERS TO THE EDITOR.

fWe do not hold ourselves responsible for the opinions of onr correspondents. The Editor respectfully requests thnt all commun i catione should be drawn up as briefly as possible.]

*,* All communications should be addressed to the Editor of the Enölish Mechanic, SI. Tavistockstreet, Corent Garden, W.C.

All Cheques and Post Office Ordere to be made payable to J. Passmore Edwards.

"I would have every one write what he knows, and аз much на he knows, but no more; and that not in this only, but in all 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 thing:*, knows no more than what everybody does, and yet to keep a clutter with this little pittance of his, will undertake to write the whole body of physicks: a vice from whence great inconveniences derive their original."—Montaigne'e Емауж*

will rise and fall with a westerly deflection, rebound through a like path, and so hop westwards round the earth. On the other hand, if a ball be dropped from a âxed point above the earth's equator it will fall castwards, rebound with eastwardly deflection, and so hop eastwards round the earth—to meet the other, belike. The case is, however, not unlike the cup in a pie-dish 32ft. high, or the problem of the elephant and the fly, where the weight of the elephant might be neglected, or (to conclude), Watson's famous problem of the man with a perfectly spherical head surmounted by an infinitely high conical cap.

Richd. A. Proctor.

*»* In order to facilitate referons, correspondents when speaking of any Letter previously inserted will oblige by mentioning the number ui the Letter, as well as the page on which it appears.

THEORETICAL.

[oo.j] Sir,—If'T. A." "fancied tVire was more of the spirit of counsel defending a client right or wrong" in my letters, he was fully justified in writing bluntly. There is no graver offence a student of ecienco can be guilty of than indulging so evil a spirit, unless it be, perhaps, purloining the work of others.

The fact is, " T. A.V reasoning never seemed to me to runder a defence of accepted theories necessary at all. He seemed to be in the dark as respect-, light.

"T. A." will get hopelessly fogged, I fear, if he attacks the question whether " the powers und activities of material substances result from tlie action of properties they inherently possess" or not. There is a volume to be written on every noun, adjective, verb, and adverb of that portentous sentence; und I would not have him be too eonñdont that he will find a prevailing belief to fight with.

Richd. А. Гкостон.

P.S.—" Urban " will notice that I have not attempted to explain '* T. A.'s" experiment. But I have no reason to doubt what" T. A." says, though I see two correspondents do so. It is a well-known f.u-t that orange-yellow glasses commonly cut off nearly ¡ill actinic light, and it is equally well known that forceч which are powerless to initiate chemical action are of tan capable of continning and modifying such action when once commenced.

CURIOUS QUESTION.

[356] Sir,—I had not noticed Mr. UsborneV rnery at p. 450 (as addressed to me), and now referred т > in letter o44,p. 593, or I should certainly not have "i„'.iored it altogether." I have, therefore, so far as hi« question is concerned, no views to revise; my silence implying no opinion either way. But as in replv to a lef'< ■ by "M. L." (23tï), p. 51H, Г had indirectly dealt wit i the question, and hud (I rind) made a mistake, I take the opportunity- of now admitting as ranch. In mv j apers on "The Earth: her Figure, Motions, Ac.," I had enunciated the first principles on which the pr.bVm is to be dealt with; butin reply to "M. L." 1 applied these principles somewhat too hastily, and so ca:ne to an erroneous result. ,ЛГу mental reasoning wa- somewhat on thin wise : a bull let fall from a great he: „'ht has a blight easterly deflection, while a ball fired veiiically upwards has a alight westerly deflection while rising; the westerly deflection during the rise of such a ball will be counteracted by the easterly deflection during th '¡ill, and there will therefore bo im deflection at allwi>r i the ball has reached the ground. The sume reasoning aromes when the hall is fired at an angle with the vertical— hence the easterly and westerly range of bulls fir«-.l at equal angles to the horizon, and with equal velocities, will be eqnal. But the reasoning is wrong, and does not apply to either case. The ball will have westvly deflection from the moment it leaves the ground' ¿\ to the moment it returns to that level; for in fulang its condition is not that of a ball let fall from a ¡ixed support, since at the moment it begins to fall it is actually travelling westwards.

I have not followed the correspondence on this matter -o cannot tell what "F.R.A.S," may have «aid. I/, answering Mr. U&borne's question directly, he has said that there is no deflection, it has doubtless been through the same inadvertence that led me wrong.

Here, by the way, is an old result of tha "curious qnestion," and the principles it depends on. If we cenceivo a perfectly elastic ball to be fired upwards Tn.m the earth's equator—the earth being conceived jor the nonce to be a perfectly smooth sphere—the boll

THE SATELLITES OF URANUS.

[857] Sir,—In reply to "Etndiant" (335, p. 501), I should be very glad to look up the papers he refers to; bnt, as I mentioned to you some time since, I am quite overpressed with work of one sort an Л another just now. Let me remark, however, that as bis notes show that the epoch 1851-2 really was in question, I have no doubt that the simple difficulty of determining the place of maximum elongation led to the seemingly contradictory result. I made the ellipse have axes as 7 to 10 by a geometrical construction ; but taking 6 to 8 "Etudiant" will see that in such an ellipse, if lines be drawn from the centre, inclined 10 or 12 degrees to the major axis, their length will differ very little indeed from that of the half-major axis. Considering the difficulty of measuring the distance of a satellite of Uranus from its primary with extreme accuracy (even when so powerful a telescope as Mr. LaeselTs was used) it may safely be held that no attempt to determine the position of the apparent major axes of the satellites' orbits could be satisfactory. Richard A. Proctob.

LUNAR COSMOLOGY.

[358] Sir,—Will "L. H. C." (248, August 26, p. 545), kindly favour your readers, including your humblo servant, with the titles of those works which contain reliable information relative to the chemical constitution of objects on the moon's surface, as well as the basis of his own conclusions thereon? We shall all be much interested in learning the chemical, or at least mineralogical character of the central mountain of "Aristarchus," which Professor Phillips compares with white trachyte, also the nature of the dark floor of *'Grimaldi." Assuming that "L. H. C." are the initials of " Littus Habet Conchas," and that the "continued series of critical observations . . -by the aid of superior telescopes and repeated examinations," are, as stated in the last letter, his own, not yet given to the pnblic, would it not be desirable to forward copies to a feelenographer able to compare them with others, and to examine them, not in connection with any given theory, but for the purpose of noting points of similarity and difference? I quite agree with "L. H. C." in the importance of "accumulating additional observations ;" for assuredly our knowledge of the moon's burlace is very limited, and the study of " Selenology," employing this term as the lunar analogue of "Geology," has as yet scarcely commenced. While, however, observations are scattered here and there, some published, others unpublished, each observer being prono t<* form and adhere to his own theory, but little progress can be made. Selenography, more than any other branch of astronomy, stands iu need of cooperation. We may pick up many shells on the seashore, and replenish our little cabinets; bnt it is the experienced naturalist who alone is capable of so studying them as to advance biological science.

W. It. Biut.

ON FEED-WATER HEATERS, AND POPULAR DELUSIONS CONCERNING THEM.

[Я59] Sir,—The total heat of steam whose pressure is 101b. per square inch above the atmosphere, or 551b. total pressure, is 1,200 degrees, counting from zero. The average temperature of river water the year round in these countries is, I believe, 40'; therefore, if a steam boiler be supplied with water at 40', and it is to be raised into steam of 55 lb. total pressure, it must receive 1,100° of heat from the fuel in the furnace; 25 per cent. or one fourth of 1,100 is 200°, and any feed waterheater which professes to save 25 per cent, of the fuel used in generating 551b. of steam from water at 40', must »end the water into the boiler at 2У0*", ami no lets. Dut that is impossible, and to profess to do it is false and absurd.

Water cannot bo made hotter in an open vessel (that is to say, one which is in free communication with the atmosphere, whether covered or open» than 212 , but 212 is only l«i per cent, of 1,100°. It appears, therefore, beyond tho possibility of donbt or dispute, that 1SJ per cent, i* the ¡jreatett Having which it is po«sibU to effect by it-arming the feed water by tlte watte steam, and that is the greatest theoretical saving which we can show even upon paper; but that is not, never was, and never wül be, effected in practice, for even if the water is got up to 212 in the "heater," it will lose several degrees on its way to tho boiler through the pump and pipes, and from this cause alone the practical effect will always fall below tho theoretical promise.

What then are we to say to those persons who persistently assert that their feed-water heaters save 25 per cent, of fuel? What can we say but that these statements originate cither in utter ignorance of the natural laws which govern that operation, or else are put forth for the purpose of misleading the public and obtaining orders for their wares "by hoolt or by crook;"

and what can we say to those persons (if there be any such) who give testimonial* that this utter imposAibiht/ is effected by tho patent apparatus of Mr. А., В., от С-, such testimonials possess no more value, and contain no more of the element of truth than would one which testified that perpetual motion had been discovered, and was to be seen at work every day at Birkenhead. Bat what can we say to Mr. S. Crompton of that place, who in the number for July 8th assures me, *■ whatever ny figures may say to the contrary, that the 'inev-itabl* U5 per cent.' saved in fuel, is not a delusion but a pra<ctvd fact with feed-water heated to 212 \" I have, says Mr. Crompton, "by careful and trustworthy experiment-, proved the truth of these statements by weighing Lot coal used, week for week doing, the same amount ci work with the engine in both experiments; that is to say, one week with the cold water, the other week wi:a the water heated to 212' with the 'économiser ;* in all cases tho statements have been fnlly confirmed." What I say to Mr. Crompton is «imply this, that he ha« bungled the matter somehow, and has succeeded in deceiving himself. It is more charitable to say that thaa to say he is trying to deceive others, but beyond t£ doubt he either has done the one, or is trying to do the other.

To show the unreliable nature of Mr. Pi ошрацм statements and experiments I 'hall examine his account of another impossibility which he relates cizeaav stantially of his "économiser."

He says that with the waste steam from а nouns/ 14-horse engine he heats 0,000 gallons of water pcxm* to 212 . Now what amount of heat is contained is lawaste steam escaping fruin a 14-horse engine in a dag* and what amonnt of heat is required to raise б.(д < gallons of water to 212' from 40"? If upon investigation we find the latter quantity to be a great deal more than the former, I suppose we shall be justified in saving that Mr. Crompton is a gentleman whose statements should be received with great caution.

A gallon of water at the average temperature of 40* is to be raised to 212', it must therefore have 172" add**!

toit.

A gallon of water in the state of 551b. sieaxxi Ьнч 1,200"' of heat in it, and in warming water to 213? it parts with 9«8J before it falls to 212е üVelf, these Stefe-an* snlficient to raise 5-74 gallons from 40" to 2123 (for if we divide 988 by 172—the difference between 40 and 212—we get 5*74 for a quotient) that is to say, b"74 gallon* of water at 40u can be raised to '¿Vi* btf tH* condensat'ton of the steam, generated from one ¡jolloi* of water umrVr a prestare of 55ïfc. per njutire inch, and the result will be 0-74 gallons of water at 21ï\ This U the tla*rctical maximum, which in practice is never reachedn a "long chalk."

A nominal 11-horse non-condensing engin*7*, working without expansion, requires 14 cubic feet of water pcr hour for stêam, but Mr. Cromptoa says his engine cuts off at half stroke, therefore it should not require more than N cubic feet per hour—for steam when cut off at half strobe has its efficiency increased from 1 to I"£P. Ihit in order to *how my generosity to Mr. Crompto», which my great advantage over him enables me to do without feeling it, I will say his engine uses 12 cubic feet of feed-water per hour or 75 gallons, or 750 gallons per day of ten hours, and that is the qnantity of water in the stiite of steam which passes into the cylinder in that tame; and as each gallon by its condensation is capable of raising 5'74 gallons from 40° to 212 , we> arrive at a total of 4,805 gaüous heated to 21Ü" instead of 6,000, as Mr. Crompton asserts, liut upon looking a little closer into the matter I find that a considerable deduction must be made from this 4,305 gallons, or else from tho temperature it is presumed to be raised to, for all the heat which leaves the boiler on its way to tho cylinder does not pass into the water-heater, about one-tilth of it at least is converted into work in th« cylinder and disappear« aï together, and not less than one-twentieth is dissipated by the exposed surface« of the cylinder and pipes, í¿c, thns no more than threefourths of the heat which leaves the boiler on its way to the cylinder ever reaches the water-heater, and the 4,54)5 gallons at» found above must be reduced to three-fourths of itself, or 3.229 gallons, to which we must add the 75(1 gallons resulting from the condensed steam, making altogether 3,970 gallons, and that is to*» greatest quantity of water which it is possible to raise from 40 to 212"" in ten hours by the waste st*aoi from a 14-horse engine, and even that require» that ететц particle of the steam tkould he conde med in the Water an<£ that not a breath of it ешеаре from the wa*U *Uam pipm (pipe G in Mr. Crompton^ drawing I, but tub condition is impossible of fulnlmeut.for aslougaspipeG is there a considerable quantity of steam will always escapo from it.

This is rather less thn;i two-thirds only of what Mr. Crompton says the heater is doing. He must therefore be mistaken in either of three ways. Eithi-r the engine must bo a great deal more powerful (SÜ-h-p. at least), and use л great deal more steam than fan supposes, or the quantity of water parsing through the heater must be very much less, or at a much lower temperature than he asserts.

Suppose his assertion is true that 0,000 gallon* do really pass through the heater per day, then the temperature of that water can be no higher than 170° (instead oí 212 ), and as Mr. Crompton feeds his boiler with this water, the saving of fuel by doing »o ia only 1Ц Vtr *"«»t., or not one-half of the •* inevitabh* •• 'i6, for which he so persistently sticks up, and this I a*o fully convinced, from my own observation and ех|*лence of feed-water heaters, ie as much as nine ont of every ten of them are doing in the ordinary way ot work.

Every nan-condensing engine should have a H#awatcr heater, it is wilful and prodigal waste to Mp¿» the boiler with cold water, when water at 1B0" wr 17* be so easily obtained for the purpose, and a saving •en 10 or 11 per cent, is not to be despised; but let lau expect ranch more, for if ho does, he will Barely inappointed.

have no hesitation in Baying that Barton's heater s good as any that can be adopted. I like the out t ©n paper very moch, though I never had the b&nre of seeing it in operation, and I feel quite sure s it is capable of doing as much as it i3 possible to i:i the direction of Raving fuel by warming the I-water, and I also freely admit I consider the L^n of it greatly superior to many others that I e seen and -which, bore a very good character as conomiserB," hnt snch wild, nnfonnded, and untent> statements as Mr. Crompton has made respecting *rt» only calcnlatod to throw doubt and ridicule in it, and to bring a very excellent article into dismte, and Mr. Barton might well cry out, "Have me m ray friends." James Baskerville,

Manager, City Foundry, Limerick.

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SPECULUM WORKING.

Ittbll SiK,—Allow me to tincerely thank Mr. arkias for so kindly coming to my assistance when I as in trouble -with my lljin. speculum, and for his alnable series of articles on speculum working. Those .rticles no doubt, thoroughly practical as they are, lave induced many to undertake the working of a .pecolum for themselves; they had the effect of stimnating me to renewed efforts after I had failed. It can »carcely be expected that all operations will go smoothly in a first attempt of snch a delicate matter as -ipeculuin working, however lucid the instructions may be ; something or other will turn up unexpectedly for .viiieh instructions make no provision; theu it is that we find our Mechanic So invaluable. It is surprising as well as pleasant, even to those not immediately interested, to see how readily and disinterestedly information is givea in " oar " pages.

Having failed in polishing my specnlum I began t^aiii, U.a Mr. Purkiss advised me to do, bnt not before I had laid the glass aside for some months, partly to attend to other matters, aud partly to allow breathing time to brace up for the renewed effort and the trouble consequent thereon. I rough-ground the glass for ;x few hours, then commenced testing the iron tool to make it coiiicitle with the curvature of the glass, which curve 1 took for granted was spherical. This testing is really a tedious process. I was about sixty hours in accomplishing this to my satisfaction, I renewed the nil and lampblack on the speculum several times, and spoiled several files; indeed, it was with great difficulty I could get the files to bite at all after a few minutes work. 1 verily believe they supplied me with a disc of bteel at the foundry instead of iron. If Mr. Purkiss will inform me if he finds a similar difficulty in filing the oiled iron, and if the time I have stated' would be considered by him much too long for the operation, I should be obliged.

Wlieu I had tested the iron tool so far that every square on it was marked by the oiled glass, I commenced the fine grinding on the polishing machine described by " Arcturns " some time ago. (Mr. Parkiss, ot course, recollects the construction of this machine. There is no side motion to it, and I should be glad to know if Mr. Purkiss considers the side motion to be indispensable.) Having completed the tine grinding, and thin I considerably prolonged, I proceeded to construct the pitch polisher. I had a die made as recommended by Mr. Parkiss, which would impress eleven grooves each way on the pitch. But in impressing the grooves I obtained an impression of the air-holesinadtlition. This appeared to bo unavoidable. However, I cutoff the projections as neatly as I wasable, :md passed on to the moulding of the pitch to the mirror, which after some trouble I accomplished, warming the pitch by the heat from a fire. By the time this was done ••-'true of tho grooves were almost invisible. I then widened the grooves with a chisel (and I will here whisper to 3fr. Purkiss that I chopped off severul of the facets). The next operation was polishing. This wa; now a moment of anxiety. I applied the rouge, and in one minute I obtained what I failed to obtain on a previous occasion in 120 hoars—a polish. But the polihb could only be observed round the edge. The hpeculnm worked somewhat stiffly, but I continued nntil it began to revolve the reverse" way to that of the tool. Ithought there was something wrong then, and remoulded the pitch and cut the facets smaller, hoping Miereby to rectily this defect. Bat I could not do it. The speculum persisted in revolving the reverse war to that desired. I continued polishing, and in 10 or 11 hours the glass was polished all over. I believe this wjuldhave been done in about three horns if I had 'ibtahwd roage of good quality. It was onlv bv using the rouge over and over again that I could advance with the polishing, avoiding adding fresh.

I now come to the process of silvering, which I have l*.en anablo to do in a satisfactory manner, and should be grateful if Mr. Purkisd can assist me in this matter. As^preliminary experiment I silwred a din. speculum, and that was everything that could be desired; the -iher adhered tenaciously to the glass, and would bear Mubiug perfectly well. With exactly the name Nation* I could not obtain an adhesive deposit on my HJ Bpecalum. Nor have I been able to produce a silver

surface that will bear washing after the deposit is effected. I have manipulated principally with the tartrate of Simiu process, requiring heat and sunlight combined. I have tried this process, with aud without sunlight, with aud without heat, aud with weak and strong solutions. Lastly, I tried Browning's formula, as given in his "Plea for Reflectors," and allowed the speculum to rest iu the solution »J hours. The result, to my surprise, was a bright deposit of silver, only a portion of the surface being slightly clouded with a bluish film, which I thought would instantly disappear on the application of a little rouge. I was extremely careful iu washing the silvered surface, pouring the water on from a cup—yet a portion of the surface was torn. I used hard water in washing, as I did on the other occasions, and distilled water for the solutions. The water was of the same temperature as the speculum. After I had been pouring for a few minutes a portion of the silver surface began to assume a dull appearance. On close examination this proved to be clusters of small blisters. (I should be glad to hear the explanation of this phenomenon.) Though the blisters flattened as drying took place, they left their outlines: but, on the whole, there is a capital polish, though I have administered no rubbing whatever; indeed, to do Bo would be to rub the silver off altogether. I want a surface that will bear polishing when tarnished, and should be rejoiced to hear of a certain process, or of Mr. Purkiss's method of procedure. I cleaned tho speculum before immersing in the silvering solution with whiting, theu with strong nitric acid, washed well with common water, then with distilled water, and finally with alcohol, and immersed the speculum wet with the alcohel. I have frequently obtained a deposit that would adhere well in patches, and in one instance at least, towards the edge of the speculum, I obtained a ring about £th of an inch wide, that I could not rub off with my fingers, even when wet. I am quite unable to account for this.

I should like to know what I ought to be able to accomplish with my specnlum, supposing it to bo a good ouc; it is 11 Jin. in diameter, and iHft. focus. Will Mr. Parkiss be kind enough to tell me? I wish to test with terrestrial objects, that method being most convenient to nte at present. I can command space of about a mile, and at that distance the objects may be buildings, a turret clock, or hnman beings. I did not construct the testing rod described by Mr. Purkiss for ascertaining the figure of tho speculum, or I might perhaps have known what figure I had obtained with little trouble. I have never had the pleasure of looking through a good telescope of large diameter, but as far as I can judge I have reason to be satisfied with mine. I by no means regret the tronble I have bestowed on it. I cau nse the whole diameter of the speculum, even with what I believe to be a high power, on terrestrial objects. I have temporarily mounted it in the Herschelian form (atrial). Mr. Purkiss questioned the advisability of my making a speculum of such a long focus; but I may ineution that I can adopt an arrangement of my own, possibly akiu to the pancratic eyepiece, whereby I can use the speculum at any length within the focus, and which is convenient for terrestrial purposes. May I not expect good results in viewing planets by using a cap next the eye with a very small hole in it?

As it is my intention to work another specolnm of the same diameter and focus, I must plead this as my reason for detailing my experience, hoping that Mr. Purkiss will favour ine by explaining my seeming misadventures, and giving a hint or two on those points he may think needful, as he seems so well ablo to do. Perhaps I am the only one of "our " speculum workers who lias met with difficulties in his labours, as we hear little or nothing about them. Are there any who have began aud have given up iu disgust—banished tho subject from their minds for ever '.' Let us hope not; something is to be accomplished, even by inexperience, assisted by "our" kind contributors, and our own energy, as mylljiu. mirror proves. J. P.

SPECTRUM ANALYSIS.

Sir Isaac Newton, Peok. Robcoe, Mr. Norman Lockyer.

[362] Sib,—It is very desirable that those who assume the duty of popularizing science should, in doing so, confine themselves to facts and avoid all misrepresentations and erroneous statements. I regret to find that this is not always the case. In a course of lecturos on Spectrum Analysis, delivered in the year 106$, before the Society of Apothecaries, by Prof. Roseoe, and published by Macinillan & Co., it is stated at page 2*2, by way of apology for Sir Isaac Newton not having discovered the spectral lines, that "Newton did not observe them; and for the f*ood reason that he allowed the light to fall on the prism from a round hole iu the shutter. In this way he did not obtain what is termed a pure spectrum, but a series of spectra, one overlapping the other, owing to light coming through different parts of the round hole. If he had allowed the light to pass through a fine vertical slit; and if this slit of liglit—if we may use such a term—had then fallen on the prism, placed so that the edge of the refracting angle is parallel to the slit,he would have observed that the solar spectrum is not continuous bnt broken up by permanent.dark lines. Dr. Wollaston, making nso of a tine slit (l-20tb of an inch wide) of liglit, discovered these fixed lines in tho solar spectrum." Before proceeding to show the erroii-ousnesfl of these statements of Prof. Roseoe, it may be well to point ont similarly erroneous statements made by Mr. Norman Lockyer, in a coarse of lectures, delivered by him before the Society of Arts during the winter of Itiii'J. In Mr.

Lockyer's first lecture, delivered on the oth of December, 1809, and published in the Journal of the Society of ArU of January 14th, 1870, page 153, it is stated, " Newton made a ronnd hole in a shutter for his experiments; but we now know he ought not to have done that; fie ought to hare mad* a slit. But this did not come out till 1812 (1802 ?) when Dr. Wollaston, by merely using a slit instead of a round hole, made a tremendous Btep in advance." Other statements of the same import were frequently made by Mr. Lockyer in his coarse of lectures.

Are these statements of Prof. Roseoe and Mr. Lockyer conformable to facts? Did Newton in his experiments confine himself to n round hole as alleged by those savant* f The statements not only are not conformable to facts, but are directly opposed to facts. We have the refutation of them in Newton's own words, by which it plainly appears that he used in his experiments not only a round hole, but "an oblong slit l-20th of an inch wide," and also a triangular hole. In Newton's " Optics" (edition of 1704), page 41>, it is stated " Yet instead of tho circular hole, 'tis better to substitute an oblong hole, shaped like a parallelogram, with its length parallel to the prism. For if this hole be lin. or 2in. long, and but a 1-I0th or l-20th of an inch broad or narrower, the length of the imago (spectrum) will be as simple as before, or simpler, and the imago will be much broader, aud therefore more fit to have experiments tried in its light than before." Hence we find that Newton used the very width of slit which Prof. Roseoe and Mr. Lockyer assert he did not use. And, further, they allege that if he had awed the narrow slit, lie would have discovered the spectral lines. He did, however, use the narrow slit, but did not discover the spectral lines.

It is quite evident that if Prof. Roseoe and Mr. Lockyer had read Newton's "Optics," they wonld scarcely have ventured to make such palpably erroneous statements. Indeed, if they hod even read more recent works on optics, such as "Priestley on Light, Vision, and Colour," and the "Treatise on Optics" published by the Society for the Diffusion of Useful Knowledge, they would have found that Newton, in his optical experiments, used the narrow slit, which they allege he did not use.

Throughout the wliolo of Mr. Lockyer's first lecture, several very erroneous and inaccurate statements are made. For instance, in the Journal of the Society of Arts, for January 14th, 1870, page 155, he states, "You see, therefore, that our spectroscope depends first of all on Newton's discovery of the prism in 1075." iMr. Lockyer ought to know better. He might have known that tho prism had been used in optioal experiments by the Italian philosopher, Grimaldi, long before the time of Newton's optical experiments. This is evident from Newton's "Treatise on Optics" (edition 1704, page 28), where Grimaldi's experiments are specifically referred to by Newton himself. Hence the prism was not discovered by Newton as alleged by Mr. Lockyer. Dr. Priestley states in "Light, Vision, and Colour," that Grimaldi perceived that when a beam of the sun's light was transmitted through a prism, instead of coming out round as it entered, it made a very considerable oblong imago (spectrum) of the sun on the opposite wall, by which he proved that colours are produced by the refraction of Light without any reflection; and, farther, that Grimaldi's "Treatise on Optics," giving a detailed account of his experiments, was pulvUshed before Newton commenced his optical experiments. E. Nucjent.

ME. DAVIS'S ARTICLES ON CHEMISTRY.

[363] Sib,—Will you allow me in all kindness to offer a suggestion to Mr. Geo. E. Davis, the writer of tho chapters on " Chemistry," in tho Mechanic? I am afraid ho has mistaken the class of readers for whom he is writing. He promised at the outset that the chapters shonld be of a thoroughly practical nature, and at once he plunges into theory. Under the head of "Atomic Weights," why not have given the method of finding them from the specific heats of the metals referred to—lithium us unity,—and the real method of finding the exceptions, such as carbon. Instead of saying " It has been found by experiment," give the experiment in full aud simple language, and in giving typical examples of the classes of compounds, take the simplest and not the more complex? The whole return* on "Crystallography" might have been advantageously omitted. The question of isomerism might very easily have been explained without any reference to the elaborate memoirs -of Roseoe, Berzelius, and Raninielsberg on vanadinite, apatite, and mimetite ; this kind of discourse and reference to Nature and the Chemical Xeics, will only tend to disgust and dishearten and not instruct. One of two things is obvious—if the articles are intended for snch as read Nature and the Chemical News, then they are superfluous in the Mechanic; if, for more elementary readers, then they are too loose and diffuse. I would not have Mr. Davis sacrifice anything of scientific language and accuracy to anything like a popular discourse—that would be going to the other extreme. With every good feeling let me advise Mr. Davis to take up definite points and explain them fully, but on strictly scientific principles, defining everything sharply and. accurately, then bis chapters will be of great value both to the lone plodding individual and to the classes rending "Chemistry" throughout tho country during the winter. But if the teacher has to warn his pupils against such and snch statement in the Mechaktc it becomes awkward. Prom what I know of Mr. Davis I think we may expect good things from him, as his experience increases. In conclusion, let me remind him and your chemical readers.

of one serions oversight—the word radical, throughout both chapters is spelled radicle; the latter would be correct enough in botany, bnt net in chemistry.

BOBERT E. H. GOFFIN.

MARANTA ARROWROOT, Etc.

[864] Sib,—The representation of Maranta arrowroot which appeared in the English Mechanic, p. 498, was drawn from a sample, given me by a niicroscopist аз genuine maranta, and as the pbite was drawn by camera Incida there is no error I am convinced. On turning toBrande's " Dictionary of Chemistry" I find a representation of West Indian arrowroot, and that differs only from mine in having the rings much more strongly marked; the shape is the same. Hassal] in his "Adulterations Detected," gives a similar plate. The Marauta arrowroot, plate, p. 493, is magnified 200 diameters, that power being ample to delineate most starches.

I enclose a drawing also magnified 200 diameters of

sentid to the eye is tolerably level, but at the points marked A A, there are considerable elevations, as if for the attachment of muscles; and the external borders of the bones, extending from A to the extremities, are strong, the maximum thicknees being about 8in. ihe bones somewhat resemble in form the supposed opcrcul* of the supposititious fish AnhUhthL They are not much nnlike the maxilla of MegalieMhy, (the Parabatraehu, of Owen), but thev differ from both these in the curve of the inner edge, in the greater thickness of the bone, in the solid protnberances near A A, and in the longitudinal striations which cover the surfaces of the bones in a manner resembling the striations on the carpal bones of (¡yracanthu,. Upon the slab bearing the two nnknowu bones there are scales of MegalieMhy, and Rhitodo,,,i., but to neither of these fish can the remains be ascribed.

I forward this communication in order that anv coal measure paleontologist who peruses vonr periodical may be enabled to compare the specimen with the lossils in his possession.

T. P. Babeas, F.G.S.

[graphic]

SQUARING THE CIRCLE. [368] Sir,—In the correspondence on the "Rectification of the Circle," allnsion is made to the ratio 22-7, which, like 81416, I suppose is the result of a process of approximation, and I would, therefore, wish to ask:

1. Would the result of any geometrical demonstration be objected to simply because it did not exactly agree with the latter, so universally recognized as true viz., 3-1418?

2. Has any one ever attempted to subject the excess of the square over the circle to a similar process of approximation, and if so, with what results?

My reason for putting such questions is, because I have a method of solving the problem geometrically, but the result is in excess of 31416 and less than 22-7, and if the answer was favourable I might bo induced to publish it, as I feel convinced it is the true geometrical ratio. It would, though, put me to a little trouble to reduce to a system, 4c. G. H.

a sample of pure mustard, showing wheat starch (b) and turmeric [a).

Will " J. C." kindly mention the works, the plates of which I might possess or procuro, from which he states he drew his information 1

I am afraid if " Young Photo" tries to precipitate an ordinary old cyanide plating solution with common salt (T. W. Boord's process) he will not meet with much success, sodium chloride will not precipitate silver in solution as the double cyanide.

Hydrogen Lamp.—"F.R.M.S." can purchase for Is. the platinum at Griffin's, to fit in the thimbleBhaped socket. If nothing else is wanting, when the hydrogen is turned on to the dry platinum it ought to ignite.

Nitrate Of Silver (4676).—" Paddy" must mix the precipitate with carbonate of soda, and strongly heat either in a crucible or in a deep cavity on charcoal; the resulting silver must bo dissolved in nitric acid and crystallized.

Science And Art Examinations (4699).—" CrowTrees " must be thoroughly acquainted with the organic portion oí Fownes and Miller, and the contents of Bnnsen's " Gasometry," Liebig's " Organic Analysis," and Ganot's "Physics." The Government examinerrecommends that the candidate should digest the contents of Watts's " Dictionary " in addition.

(470ÍÍ).—Has " In a Fix" got any cyanide in the water? Salt will not precipitate silver in the presence of an excess of cyanide. George Е. Davis.

AN UNKNOWN COAL-MEASURE FOSSIL. [805] Sin,—The pages of Vols. I., П., and III. of Scientific Opinion contain many letters and papers from my pen on the subject of Coal-measure Palteontology, and especially on that department of paleontology which refers to fish and reptile remains found in the collieries of Northumberland.

You will perhaps permit me to direct the attention of your readers to two bones which I have recently obtained from the Northumberland Low Main Cool

[graphic]

stratum. Thev are, I believe, new to science, and whether new or not.they are as yet uninterpreted 'by the best pahvoutologists inEngland, several of whom have examinod them.

The forms of the bones and the positions in relation to each other in which they lie upon the slab of shale to which they are attached will be best understood by reference to the annexed outline sketch, which is onehalf the natural size of the specimens.

The length of each bone is 5jin., and the width of • ach at the broadert part is 2Jin.: the surface pre

[367] Sir,—In one of vonr June Nos. a method is propounded of squaring a circle, which has been sufficiently anatomized and is clearly wanting in exactness. Had the writer, however, made the basis of his triangular sections infinitely small it would have been correct theoretically, as the following calculation may tend to prove :—For, let А В С be ono of these sections, with base В С infinitely small. The circle will then be

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smith. It is the invention of Mr. J. B. Smith, the foreman of the smith department of the Harlem Railway Works (U.S.).

It is made from gas-pipe, and of size according to the work required. The process is as follows :—One end of the pipe is mode solid for tho thread, while the other is prepared for the bottom end of the rod, and several holes are made in the side of tho pipo in which to insert the lever for turning the buckle. Fig. 1 represents the outside view and the holes for lever; Fig. 2 gives a sectional view. Compared with the usual buckles, which are expensive, and require the hand of an excellent workman, it is very cheap, its cost being at least C2J per cent, less than the ordinary method.

Карга.

IRIS DIAPHRAGM.

[869] Sir —Three weeks since I wrote un arruto query No. 4498, but as the letter has not been шffSSPÄiT!!* ш,ге? *° " u°ät," that he will find an

Ins Diaphragm » described in the A'otiW« of the Jf„. ? H" T^ztm11 and РЫмрЫеа! Society, and also uiMr. Darby s ¿»tronomifo! ОЬиттда; in both «rticU» drawings are given of the details of construction I noticed that "F.RA.S." (317) describes the principle of the diaphragm, hut I may tell "Unit" that Su figure is alnmt perfect, and that in the telescope ne rays are caused when bright stars are observed

The instrument described by Mr. Revelcy is ver» ingenious, but I fear would be quite useless for astro'nomicalwork, as the image of the object оЬеелна would probably take the form of the aperture which is pentagonal. The principle involved in any diaphrwir, has been long known ; but I am not aware of anv o*fcV descnptiou of the adaptation for scientific pnrp^« than those I refer to. Messrs. Beck may hare adopted it to their microscope, but I fancy the* have superseded its use by another similar in mm ciple ta that shown in (259), and made of metal The blades are numerous, and the figure is goal bnt not so regular as that formed by india-rnbfcer and wonld, most likely, cause ravs from bright starinstead of clear discs.

The only disadvantage of india-rubber is that it cecays in conrse of time, but can of course be renewtd и required. The diaphragm should never be allowed •-> become frozen, and the more it is used the longer I will last. The full aperture may be used or ma- i closed entirely, and by the simple machinery I ha« (► scribed may be worked from the eye end of the teîescc-к. and this is the chief advantage of the apparatus.

х- 1 1 тт. , A- Bbothebs, F.RA.S.

Fnlshaw, Wilmslow.

SCREW PROPELLING. [370] Sib,—I hope the Rev. E. Kernan will not feci vexed if I add to the names of those connected with the history of the propeUer, one not less deserving than the rest to be mentioned amongst the '• certain facts" Imean that of Josef Ressel, then Imperial Woodward a i Trieste. In his treatise on the Archimedean screw, as annexed to his petition on 28th November, 1826, for the grant of a patent, showing, I believe, an earlier date than that of Captain J. Ericsson, and declaring a new invention consisting in propelling vessels at sea, and even in rivers, by means of a wheel resembling an endless screw, he says " the troublesome noise cansedby tb paddle-wheels of steam-boat«, and the nnpleasatí oscillation inherent thereto, and also the waste d power and time in working the same, induced me to contrive an apparatus free from such defects. I adopted the theory of the screw and gave the motor of the vessel the form of a screw, . . . and I think it advisable to fix the wheel at the stern, since the back water of the ship might promote its action." May the al>ove suffice to explain the claim!

A Member Of The English Mechanics' Scientific And Mechanical Societt.

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I trust may prove useful :—A A A is a frame made o! beech wood, with a bead run all round the inside front edge. There is a slot in one of the uprights A for tbp bar D to work in. В В inside frame, with two iron rods С С working through guides to keep the frame true; D is a bar of beech hinged at E; F is a piece of iron to lift the frame up and down; G is a thnmb-scrtrw to tighten saw; H, saw; I is a connecting rod from tt* bar D in the crank J. On the mandrel there is a table to lay the wood on, not shown. The whole is well pnT together, and stained over the вате as a new plane. К is a bolt to bolt it to the luthe-bed.

W. GREENWOOD.

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"Ready Reckoners M for the above. In return I will enclose in an envelope for him, on application to the Editor of the English Mechanic, the numbers of a few of my plates. People do not always see that very different numbers may be desirable according to peculiar requirement, or to the diameter of plate to be dealt with, or to the breadth of the flat portion thereof (as iron pulleys are commonly hollowed towards the centre in front), and that it requires some judgment to Relect from a really large variety of good working numbers those that are not too crowded, or those that may contain the greater number of prime factors, or those that contain decimal numbers or multiples of 5, and so on, according to the individual requirements of the person using them.

An amateur using an excentric chuck, for instance, may consider a pattern containing 12 circles to be rather coarse, and one of 14 as much too fine, and for his use it is requisite to supply a circle containing a multiple of 13, and in the same way one of 11, 17. and 19, and thus it occurs that on Holtzaptfel's lathe you sometimes find 209 and 221, upon which two unhappy numbers I let off a stream of abuse in my answer to query on p. 572.

J. K. P.

ACCESSORIES TO THE MICROSCOPE.—No. JX Light And Condensers.

[878] Sir,—Light. Gentle reader, do not think that I am going to attempt a learned dissertation upon the wonderful properties of light, all I intend doing is to treat, as briefly as possible, light in connection with the microscope. "Wo may possess a very beautiful instrument, have the best object glasses, and splendid specimens, but unless we have the required light all is in vain.

Light.—Daylight and artificial light are both requisite in studying the microscope. We presume most of our readers will have to work at this pleasant subject when the labours of the day are done; so we will describe the use of artificial light first, as what we say under this head) with some little modification, will apply to daylight.

The first thing necessary will bo a lamp. Most opticians supply a vory useful lamp upon a sliding rod, so that it may be raised or lowered as desired, the price of which is from half a guinea upwards. Another handy and inexpensive arrangement is to have an ordinary lamp, and a solid block of wood, longer one way than the other, so

that for transmitted light the lamp may bo placed upon the side of the block, and for rellected light upon its end.

Many kinds of lamps are 'recommended according to the fancy of the microscopist; but for ordinary work, a parafllne lamp with a good base, costing from Is. to Is. 6d., will answer every purpose. In trimming the wick it will save much trouble and vexation of spirit if the corners be cut round; and when it is lit it should not be turned up as high as it may be required, until the chimney i3 quite warm ; by so doing many glasses will be saved, and expense spared.

The first accessory necessary when proceeding with this delightful study, will be a condenser, upon the successful use of which will depend the beauty of the object shown.

A simple condenser consists of a plano-convex lens fitted into a framework of brass, which is attached to a steel rod, and so arranged that it can be turned in any direction. The cheapest is the stage condenser, as shown at Fig. 7; perhaps the next least expensive and most useful are those shown at Figs. 4 and 5. Figs. 1, 2, and 8 are large stand condensers, and suitable for the more expensive microscopes.

The parabolic or silver side reflector has a very useful illuminator; by its aid some very beautiful effects are produced. Professor Smith, of America, has invented an illuminator for high powers. It is so arranged that it has to be placed immediately above the object-glass, through which the light is thrown on to the object.

It will be observed that the condensers already described are for opaque objects; for transparent objects many beautiful contrivances have been adapted, and known as achromatic condensers. Most of the best microscope makers supply different arrangements with their microscopes to effect the same object, and it is only just to those gentlemen that their condensers should be made known to every scientific man who wishes to make the microscope his companion. Messrs. Powell 4 Lealand, Mr. Thomas Ross, Messrs. R. & J. Beck, &c., have each done much to bring, not only the optical, but also the mechanical part of the microscope, to its present high state of perfection.

A short time since Mr. James Swift, of the Cityroad, presented to the Royal Microscopical Society an achromatic condenser he had just completed, which has the advantage of containing more accessories than any other piece of apparatus, as Figs. 8 and 9 will show:—

A, optical combination.

B, rack adjustment for focussing.

SUMMARY OF OBSERVATIONS.—SPOTS ON PLATO, AUGUST, 1870.

[874] Sru,—The observations of the floor of Plato during the lunation of August, 1870, were much more numerous than those of the two lunations Juno and July of the same year, the number in June and July being 89 and in August 125. Theso numbers not only indicate that an improvement in the state of our atmosphere occurred in August, but they afford the moans of a very fair comparison of the degrees of visibility of the spots observed (19)—viz., Nos. 1, 8, 4, 5, 6, 7, 9, 11,13, 14,16, 17, 18, 19, 22, 23, 25, 30, and 31. So far as the number may be regarded as a measure of tho state of our own atmosphere it would be rather better than in April when observations of 17 spots were recorded. We, however, meet with the same irregularity which has'characterized these observations throughout, for the two sets do not consist of the same spots. Those common to the twot series are Nos. 1, 3, 4, 5, 6, 9, 11, 14, 16, 17, 22,25 and 30; those seen in August, but unseen in April, were Nos. 7,18,18, 19, 23, and 81; while those seen in April, but unseen in August, were Nos. 24, 32, 83 and 35. The two last mentioned series consist of spots of low visibility.

In the summary for Jnne and July, 1870 (english Mechanic, No. 282, August 19, p. 518), allusion is made to the decline of visibility of spots Nos. 4, 14,10, and 5, to which may be added No. 9. Of these Nos. 4 and 9 have exhibited pretty nearly the same degrees of visibility at they did in April and May, No. 4 rising to 1-000 and No. 9 falling to '200, the values in April and May of No. 9 being *210, and of No. 4 '978. No. 5 has risen from *514 to '650. No. 14 has not exhibited similar phenomena, although its visibility has increased it still falls short of that which it exhibited in April and May—viz., -482, being in August *250, and the visibility of No. 16 has continued to decrease. Fixing our attention on these five spots we find on examining the spat ledger that the decline of visibility of No. 4 is most probably duo to "personality " for although the spot was recorded in June by Messrs. Elger and Gledliill, it was on Mr. Gledhill's returns for July that it was absent, being recorded that month by every other observer. The observations of Nos. 5 and 9 do not appear to be affected in the same manner, being distributed as usnal. Nos. 14 and 16 have certainly decreased, especially the last, daring the lunations April to August, and although No. 14 has increased during August it has not regained the degree of visibility it exhibited in April and May. On the 6th of August Mr. Elger remarked that the western part of the floor of Plato from a point just east of No. 14 and west of No. 16 was of an even light colour, and it is just on this region that the spots the visibility of which has of late been decreasing are situated. Of these we have No. 22, seen twice only in the four lunations April to July, on April 12 and May IS. During the August lunation it was seen on four occasions by Mr. Pratt, who thus records hif observations :—" Spot No. 22 according to my own observations has manifested a remarkable increase of brightness, and those parts of the shaded portions of the floor of Plato which are nearest the rim have come out more conspicuously darker than tho rest than I remember to have previously noted. Tho tint of floor, too, has progressively paled. These three phenomena may possibly be connected by a common cause, for certainly in this lunation there is somewhat of a coincidence amongst them for instance, spot No. 22 is intensely bright at the time the marginal portions of the shaded parts are more « mspicuously dark and these too again coincide with the time when the gem-ml tint of floor was at its very darkest. Again, after August V2 and 13, spot No. 22 decreased in relativ? intensity although I am not ready t » hazard the assertion that it hud on August 10 positively declined to its usual intensity as it was not seen. Two similar instances, 1 believe, I have noted before when Bput No. 22 manifested а singular brightness ai ifimriie. lint the connection between the visibility of the deeper tinted margina and the general deepening of colour ie perhaps closer still, as both certainly paled after August 13. The perplexity seems te be that the variations in intensity of the margins are r§latiee in respect of the general colour, and if differences of angles of illumination and vision do affect the general tint it might be supposed that they would in the same measure affect the margins and so produce no relative variation of intensity." On this point we may remark that the passage of a thin cloud beforethe moon greatly intensifies the darker tint*. Returning to the spots which are situated on the western part of the floor wo have No. '¿, seen once only in the five lunations АргЦ to Anglist, on May 13 ; No. 18, twice only, on May 13 and August 12 ; No. ID, the same; No. 16, which was ьееп on fourteen occasions in April and May has beeu declining since; while No. 1ft, which is alee on the western part of the floor, has been increasing the spots which have been most frequently observed are situated upon and near the central part of Plato, and it is worth inquiry as to whether the localities of those which are most intermittent are situated in low portions of the floor. There is great reason to believe that while the sites of the markings are permanent, the manifestations of the markings themselves are variable. There is another feature connected with the spots which mnst not be overlooked. The oWrvations of the eeeond year have now proceeded sufficiently far to allow of the projection of the earlier portions of the curves; those of No. 17 and Sform a group po*-essing high but decreasing visibility; those of Nos. 5, 14, 25 and У another, the first member manifesting a visibility above its ordinary value of about *500, the degrees of the remaining members are lower. The curve of No. 16 is descending throughout, and the spot being in the neighbourhood of Nos. 13 and 19, it appears as if it were undergoing a similar change to that which has impaired their visibility, and which may be temporary or permanent. The spots Nos. 30 and 31, which have been seen as minute craterlets, have been increasing in visibility, not so much perhaps from an improved state of the earth's atmosphere, which has been of the opposite character during the period of their increase as from a probable action in the group to which they belong, the principal member, No. 3, having also increased in visibility at the same time. Taking into consideration the whole of the phenomena presented botween April, 1K61», and August, 1870, inclusive, it would appear that the visibility of the craterlets is not so intermittent as that of the spots, and both seem to bo affected by agencies that occasionally obscure them, and in some instances render them invisible for considerable période. W. U. Birt.

BICYCLE RIDING.

f 875] Sin,—We arc sorry to differ with Mr. Reveler in bis opinion of bicycle riding. Mr. Reveley states that no man can make more than sixty strokes per minute; this we can prove to be incorrect. A mile has often been done in less than five minutes, but wo will take five minutes for example, ко that there may be no doubt about it. This wa-¡ doue on a circular course of nine laps to the mile, so that it could not have been all down hill. This, supposing the wheel to be 3ft. diameter (which is about the average) would be 117 strokes per minute, or about double what Mr. Reveley states to bo posssble. We do not pretend to say that this could be kept np for any great length of time ; but still it is sufficient to prove that at least two strokes per second can be attained. We are sorry to observe that Mr. Reveley thinks fit to disbelieve all statements made ti the effect that more than neveu miles has been done in one honr. We ourselves have done forty miles in rive hours. If Mr. Reveley can find an inclined plane forty miles in length, where a bicycle will run at that rate without any motion of the rider, we will believe that the journey we performed wa-i flt that very place.

Two Bicycle Riders.

[376] Sir,—I have beon very niuoh surprised lately on reading letters written against the bicycle, and I rejoice to see the sensible answer of "Souex." I aleo may call myself an elder : I am now- too delicate and nervous to attempt horse riding, though accustomed to it all my life. I require exercise, and with my doctor's advice I tried the bicycle. I cannot walk more than four miles, and yet I have several times enjoyed rides of from 14 to 1H miles on my delightful .machine. It is a shame for persons, without any object that I can see, to try and prejudice young men against the most delightful and health-giving exercise that has ever been invented. I can only hope that the writers of such letters have either had bad roads or l-ud machines to warp their better jndgmeut.

An Invalid Clergyman.

thought at the time would prevent me ever going any great distance. Bnt now to show the ad vim tage of bicycle riding, I left Leeds last Saturday (along with my two sons, one 17 years and the other It years, all npon ordinary bicycles) for Sonthowran, near Halifax, upon the worst road imaginable for bicycle riding —there is not 20 yards of level road in the whole distance of 1С» mites ; this we travelled against a strong wind, and returned to Lends a^ain in the evening less fatigued than I should have been with walking six miles. Although I am a year older since I learnt to ride, I feel 10 years younger in health and strength.

James Smith.

[The above letters are specimens of many others we have received on the same subject. We are sorry we cannot afford space for those written by O. Nash, "Motive Power," "Ajax" and "W. T.;' All the writers are unanimous in favour of bicycle riding.— Ed.]

BICYCLE RIDING AND WALKING.

[$7b] Sir,—The remarks of Mr. R. G. Bennett in yoar August l'2th number, do not fairly represent what can be done by ordinary bicycle riders, and are, therefore, calculated to mislead. Mr. Bennett's "twelve months' continual practice" has had a most disheartening result, if it only enables him to go four miles an hour for five hours. The writer, who has never run in a race, and whose walking powers are certainly not more than at the rate of four miles an hour, considers himself an "ordinary nmatenr bicycle rider," and does not make the following statement boastfully, but simply as an instance of what can be done without traininn and without over-erertion. On 0th August I started from Leeds on the bicycle at 2 a.m., and arrived hi Stoke-ou-Trent at 1 p.m., thus riding a distance of seventy-five miles in eleven hours. As to the им of bicycles for practical purposes, I may say that myself and a friend havo travelled on them upwards of a dozen times during the prcsont summer distances varying from twenty to fifty miles, without any signs of exhaustion. I need scarcely remind your correspondent that alight well-made bicycle, and the knowledge how to use it are as necessary to ensure success as moderately strong limbs. W. B.

[The above letter is a samnle of a dozen letters on the same subject which have come to baud in answer to Mr. R.G. Bennett.—Ed.]

fore, a source of heat, and that we may find in the •■> a.

Let the liquid acid be confined in a suitable vcs*el or boiler, and thence made to pass (by a force-pomp) through a «oil of copper piping casing the ship externally. After passing through these pipe«; let it retara to thu boiler. It is evident that circulating through these pipes it will absorb heat from the external sea, and maintain itself at the кате temperature. This Li all that U requisite to furnish a gas with au expansive force of some HUOlb. per square inch. Not only maj carbonic acid be nsed for this purpose, but also any other gas which liqúenes nnder pressure. I prefer the above though, for several reasons.

1. Its cheapness, as it can be made by compressing the ordinary gas by a series of pumps, one working into another, suitable means being taken to absorb the heat so developed.

2. Its absence of action on metals.

8. Some will call the high pressure te be obtained an objection ; but I say that a pressure of 1,0001b. La quite under control with modern appliances, and if all the plant is conscientiously and well put together it would be much safer than many iron-works of the Black country. Feu Follet.

[377] Sir,—I wish to say a few words in favour of

bicycle riding. lam in my COth year of age, 5ft. (»in.

in height, and weigh more than 12 stone. I learned to

ri le a little last autumn, but could not go more than

jtutf a m^c before I mnst stop to take breath, this I

[graphic]

A BICYCLE STEP.

[Л79] Sir,—I send yon a sketch oí a simple contrivance of mine—a step for the bicycle—which will be found very useful to riders thereon. I have been using one for about two months, and can therefore recommend it with every confidence, believing it to be superior to any patent device for the same purpose. It only weighs, when complete, l^lb., and being in one piece, it makes no noise in travelling along the road. Any blacksmith can make one, and fit it to the bicycle for about two shilling*.

The sketch requires little explanation. A -Jin. ronnd bar of iron is bent into the shape shown; the step, S, is about one foot from the ground: through the top of N the bar a gin. pin, P, is passed, on which it should swing, pendulum fashion. To fasten it to tin; bicycle, remove the pin which carries the brake, enlarge the hole to ijiu., then insert the pin, P, and fasten securely with the iint, N. This pin, P, will then carry the brakeaswellasthestep. When travelling, the step is held up parallel with the hind fork by means of a spring made of a piece of ¿in. india-rubber tubing, one end of which is fastened to the hole, O, in the bar, and the other to the top of ono of the upright springs which carry the saddle spring. To nse the step, move the bicycle till the off treadle or crank points upwards, then press the stop down to the ground, place the foot thereon, and quietly throw the other over the saddle on to the off treadle, press upon it, and when the bicycle moves forward (not before) remove the foot oft* the step, which will then spring back and upwards to its placo, parallel with the hind fork and quite out of the way of any obstacle. John C. Frank.

CARBONIC ACID GAR AS A MOTIVE POWER.

[380] Sir,—The use of carbonic acid in one shape or another has often been snggested as a motive power, and for some years I havo been working on its application in the liquid state as a motive power for submarino vessels. Illness now prevents me continuing them, and I fear I ehall not again be able to go on with them. May some reader of the Enolish Mechanic carry out what I am unable to do; but for a nobler purpose than that of warfare—viz., the lifeboat. Anybody will understand the advantage of placing in a lifeboat a means of propulsion of'20 to30 horse-power, and this is qnite practicable by using carbonic acid. This gas under aprensare of about 8001b. per square inch becomes a liquid. This liquid, the same as water, requires a certain amount of caloric to reconvert it into gas. If it gets this heat.from the atmosphere its evaporation will soon become comparatively slow, and the tensile force of its vapours low and useless. We want, there

FREE-REED ORGANS r. HARMONIUMS. [Я81] Su,—Mr. Nash in his letter {No. 811, Sept. 2), hopes I shall not be angry with him for his defence of the harmonium against the objections raised in my note (239). He writes so temperately that in my turn I have to bespeak his courteous attention to the fact that he merely replies to my objections without answering them.

He admits the first, viz.:—the rcediness of tone of the harmonium, only urging that in this respect some harmoniums are not qnite so bad as others, which I readily admit. He says himself, " this doubtless is the chief objection to the harmoninm." I thought so too. and hence placed it iirst. But since he so entirely agrees with me on this subject it seems strange he did not concede the point without hesitation.

Mr. Nash think?, my second objection, about the bass overpowering the treble, unfair, yet doe* not scruple to reply to it in a much more unfair manner. He says, "on the large harmonium the division of the stops into treble and bass renders it entirely the player's own fault if he suffers his right hand to be overpowered by his left." From which it is to be presumed he means that by using two or three treble stops against one bass the treble may be made predominant. Does not Mr. Nash see that his defence is based upon admitting the very indictment I make against the harmonium? Why is it that two or throe sets of reeds in the treble require to be used against one set of reeds iuthebass? Is uot this the bt»t р/хыЫе proof that the bass does overpower the treble? Bet no stratagem of the kind really overcomes the radical defect of the inequality existing in any single set of harmonium reeds all through the register. It is easy to makeup a quantity of tone in the treble by employing more stops than in the bass, but in that case the quality of the treble and bass are different. Yet more, as regards the treblo itself, the objection is uot removed bnt only carried an octave or two octaves higher; the alto overpowers the treble and the tenor overpowers the alto, for the simple reason that every note down the scale is relatively more powerful than the one above it. Mr. Nash concedes that the objection is jnst against any harmonium with a single set of reeds. It must therefore necessarily exist against any larger instrument combining a number of such sets of reeds. That a good player on a large harmonium with a number of stops at his command will do his best to hide the radical defects of his instrument and sometimes almost succeed, is not to be denied, but in such a case the credit is entirely the player's. In the Mason & Hamlin organ there is no such defect, and consequently no such need for strategic skill in manoeuvring, every note in the register being of eqnal power and volume, and of л tune so pure and round that comparison is terribly odious. Mr, Hermann Smith, in his admirable scries of article* on the harmonium, does uot attempt to palliate the defect. In the very number of your journal which contains Mr. Nash's reply, Mr. Smith savs (p. 556): "No doubt л smoother bass would be gained anil better balance tone " (that is, were reeds employed of such a size a* would necessitate a case almost as large an organ'. "for at present the tenor and Ьава are «horn of their j**' proportion*, in order to economize space, and hence the roughness aiul reedinetB preponderates, drowning th* treble, unless great skill is exercised to restrain it within doe bounds."

Mr. Nash's answer to my third objection respecting the sluggishness of articulation of the harmonium rendering it only adapted for the slowest of music. I may dismiss very briefly. He retorts that he has both heard brilliant operatic music performed and played such music himself upon the harmonium, and that (speaking modestly of the first case only) it was a very creditable performance. For my own part I have heard the "Dead March in Sanl" played upon а Jew's harp, and I thought it a " very creditable performance," without believing any the more that the music was adapted to the instrument. Mr. Nash does not go so far as to say that brilliant music is adapted to the harmonium. * He is, I feci sure, too good a musician to assert anything of the kind, and too honest to deny that the harmonium is not slnggish of speech. I do not wish in anywise to exaggerate the defects of harmoniums, but I think most musical people will agree with me that the three objections I have urged against them are both writ-founded and weighty, and that an instrument like the Mason 4 Hamlin organ.

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