338 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



the water now have it in their houses ; in fact, in some of the very poorest 

 there may be seen in the corner of the room a small cask, with ball-cock, 

 to receive the supply, and Ihey invariably speak (if it as being the ;;reatest 

 comfort, and as well vvorlh (he money it costs Ihcm. Nolwitlistanding this 

 apparently very favourable statement, there is yet much room for improve- 

 ment; for if we assume, in the absenceof any such statement in the census 

 of 1841, that the number of families in Exeter are in the same proportion 

 to the population as they were in 1831, we state them to amount to 7000, 

 so that there are only, of the whole population, about one-lhird who have 

 it in their houses, and only about half who are consumers of it.— Second 

 Report of I he Health of Towns Commission. 



ON LOCOMOTIVE AND ATMOSPHERIC RAILWAYS. 



Sib,— Before proceeding to a consideration of the comparative merits of 

 the Locomotive and Atmospheric system of railways, it may be as well to 

 mention that the chief causes of loss of power in general may be classed 

 under three different forms. The first au<l most obvious cause, and that 

 which is the most generally known, is friction : the second, loss of heat : 

 and the third, the escape of an elastic air, gas, or vapour from a greater to 

 a less density; the two last causes not being so universally known as the 

 first, and often overlooked. One obvious defect of the Locomotive system 

 is tlie force which is required to move the engine and tender, which in 

 light trains amounts to a considerable share of the actual duty performed, 

 a defect which cannot be classed under the head of loss of power, but is 

 more properly power employed unprofitably When we compare their 

 merits as regards friction, we shall find the Locomotive system vastly 

 superior: first, there is the friction of the different parts of the engine, 

 which is common to both, but would be found perhaps rather more in the 

 Locomotive than iu the Stationary ; but, in addition to this, the Atmo- 

 spheric system has the friction of the air punijis, and last, though not 

 least, the friction of the travelling piston in the long pipe. But the fric- 

 tion of the pumps is proposed to be done away with by the patent plan of 

 Messrs. Nasmyth and aiay ; but when I come to a consideration of the 

 second and third causes, I think I shall be able to prove that their system 

 has defects as great as tliose it proposes to remedy. 



As regards their merits under the secoud cause, or loss of heat, the 

 Atmospheric system has the advantage, in consequence of our being able 

 to use condensing engines, and thus save a great quantity of heat which in 

 the Locomotive is blown away into the atmosphere in a latent stale. But 

 according to Messrs. Nasmyth and i\laj's plan there will be a loss of heat 

 in two ways; first, by having such an extensive surface exposed to the 

 atmosphere, as there would be by having four enormous cylinders 150 feet 

 in height and 10 feet in diameter; certainly they propose to counteract 

 this disadvantage by having the cylinders coated inside and out with wood, 

 which they describe as a very simple afiair, but which I am inclined to 

 think would not be so easy in practice but that some of the heat would 

 escape. The other way in which heat is lost is one which I believe has 

 been entirely overlooked, or at least has not been adverted to either by the 

 patentees or any one else that I am aware of, and it is, that when they 

 admit the steam into the cylinder it is full of air at the temperature of the 

 atmosphere ; now it is asserted by the patentees that the steam will gra- 

 dually force out the air, but it appears to me that when the steam conies 

 in contact nith air so much colder than itself, instead of forcing it out 

 ■would be itself condensed, giving to the air its latent heat until it was 

 raised to a temperature corresponding with that of the steam, which, sup- 

 posing it to be the case, would be no small loss of heat, and consequently 

 of power. 



Under the third cause, or the escape of an elastic air, gas, or vapour 

 from a greater to a It-ss density, we siiall find that both the systems are 

 subject to defects, the amount of which would not be easy to calculate 

 R'ith exactness, at least until it is understood better than at present, la 

 the Locomotive system it acts by means of contracted steam passages, and 

 the great velocity of the piston, causing the pressure in the cylinder to be 

 less than in the boiler, and the steam that is blowing out of the cylinder to 

 be at a higher pressure thiin the atmosphere, even when it is worked ex- 

 pansively, and when that is not the case the difference will be still greater. 

 In the Atmospheric system the stationary engine is subject to it in a less 

 degree, but it comes under its influence by the leakage of the long valve ; 

 but Messrs. Nasmyth and iSlay propose to remedy that, in a great degree, 

 by means of a large store vacuum, so that when a communication is opened 

 with the pipe it may close the valve at once, and by that means diminish 

 the leakage; but the employment of a store vacuum is attended with a 

 very great loss of power, to prove which I will suppose that it is uf the 

 same internal capacity as the pipe which it has to exhaust, then, if the 

 store vacuum be perfect, it will reduce the pressure in the pipe to 71 lb. 

 per inch ; but suppose the store vacuum to be made into the form of a pipe 

 of the same length and diameter as that which it has to exhaust, and that 

 it has an air-tight piston at that end near the pipe to be exiiausted, then, 

 when a communication is opened between the two pipes, the piston will be 

 driven onnard with a force of 15 lb. per inch at first, which will diminis-h 

 to TJ lb. at the last, therefore it follows that there is a loss of power which 

 is sutiicient to drive a piston with the above-mentioned forces,— no small 

 matter indeed, — greater I should thiuk than the loss from leakage which 

 it is proposed to remedy. 



A Working Mechanic. 



Newcastle-on-Tyne, Henl. 2G. 



DREDGE'S SUSPENSION BRIDGES. 



Sir, — Notwithstanding the evident proofs I gave in my last letter of 

 having nothing to do with Mr. TurnbuU's treatise, Mr. Bashforlh still en- 

 deavours to make me responsible for it, and then by specious reasoning on 

 an erroneous proposition in the argument, he concludes my principle is 

 wrong. He has in consequence of Mr. TurnbuU's papers voluntarily come 

 forward to attack my invention. But because those papers are not mine, 

 nor the invention a consequent of them, I do object to his reasoning, for it 

 is fallacious ; and will not allow sophistry to confuse the ohject, for as it 

 bears no reference to the subject it would be averse to a fair spirit of 

 scientific enquiry, and only tend to prevert the truth. I beg Mr. Bash- 

 forlh will therefore at once set aside this treatise, and discuss the subject 

 on its own merits. 



To the best of my belief, t have not referred any one for answers to 'heir 

 objections to Mr. TurnbuU's papers since they were published, and there- 

 fore the charge of making them an authority is unfounded. The letter I\Ir. 

 Bashfurth quotes was I think printed about the same time as the foot note, 

 and previous to the publication of the papers, both were announcements 

 that such a work was about to appear, but I do not conceive that either 

 of them makes me responsible for it.* 



The oblique direction of the suspending rods necessarily involves much 

 complication in the investigation, but not sutiicient to place it beyond the 

 resources of analysis. 



Mr. Bashforth complains that I unfairly charge him with misquoting, — 

 I gave an example, and placed the full senteuce by the side of ids quota- 

 tion ; as I read them, they have a distinct meaning, and how can he 

 assume that a portion of the sentence gave my real meaning, if the whole 

 conveyed a different one. 



I thought I had set aside the objections which were so distinctly pointed 

 out by disavowing the treatise that contained them ; but this does not 

 satisfy Mr. Bashforth, he wishes me to answer objections to proposi'ions 

 manifestly impossible. I can only reply to this by saying they are erro- 

 neous ; and if the author were really as ignorant as the proposiiions con- 

 sidered by themselves lead us to suppose, he would deserve the foil cen- 

 sure Mr. Bashforth has measured out to me. But in my opinion he was 

 not ; and my reason for this is obtained from the 4lh page, part of which 

 was quoted in my last letter. I had not any idea of attempting by spe- 

 cious reasoning on this quotation to prove an impossibility, though I did 

 mean to say that in my opinion it clears the author from the charge of 

 such gross ignorance. 



A\'hat are we to understand when a person tells us that " most unfortu- 

 nately not one of these writers even professes to have any knowledge of 

 mathematics, and consequently all their opinions are worthless," but this 

 — that every opinion except that founded on mathematical experience is 

 worthless? This deduction is perfectly fair and obvious, and how can Mr. 

 Bashforth presume to say that " the authors of the letters could not have 

 had time and opportunities for gaining practical experience." Is he per- 

 sonally acquainted with each? 



Mr. Bashforth now alludes to the bridge that has lalely fallen in India, 

 and passing in this case the opinion of the mathematician by merely ob- 

 serving it was a laudable desire to ensure success, doubting tlie express 

 statements which have accompanied all the accounts that have reached 

 England, he comes to the conclusion that there was another weighty 

 reason. I presume he has premises for this opinion ; will he slate them ? 

 This would be a legitimate object of discussion. I can tell him the reason. 

 The parties erecting it did not sufficiently understand what they were 

 about, and if Mr. Bashforth had been contractor, a similar accident would 

 have happened to him, and then no doubt he would have been enabled to 

 appreciate the value of practical experience. 



1 gave the aulhoiity of my statements respecting the Mtnai bridge ; the 

 quantity of iron 1 should require I obtain by calculation. 



I see nothing very surprising in the fact that Telford did not adopt the 

 modification of the oblique bars, nothing very extraordinary in his not per- 

 ceiving it, but I am astonished at Mr. Bashforth's surprise. 



If it be impossible to obtain the solutiou of a problem without data, Mr. 

 Bashforth's remarks were worthless ; for he did think he had written 

 enough " to show the principle was wrong," and with acknowledged in- 

 sufficiency of data; but, as he says, " I do not see the force of his ergo," 

 let us analyze the construction of his opinion. In a treatise entitled " The 

 Mathematical Principles of Mr. Dredge's Suspension Bridges,'" a funda- 

 mental proposition is erroneous, aud such of the argument as depends on 

 that proposition is also erroneous, but Mr. Bashforth carries his reasoning 

 further, and concludes that the principle is wrong. Before by such argu- 

 ment he can do this he must accomplish two impossibilities : first, he must 



* The first intimation I had of Jtr. Turnhnll's treatise was in June, 18-11, when I rt. 

 ceived a let'er from the author asltiog me to purchase some papers he had just compiled, 

 which I decl ned. A few weelis alter ttiis he called upon me in London, and told me Mr. 

 Weale wo'ild buy them provided hecould obtain aarawing and specification of some pr.ac- 

 ttcal example, tie had his MS. with him, I ut I did not read it, though after a few 

 minute:,' coiiversalion I promised to give him what he required, and caused an announcfi- 

 nieiit to be printed at tlie foot of some of mv detached circulating papers. As nearly as I 

 ollect about the same lime I was engaged in controversy Willi Mr. Fordham, and 



Itnowing that P.lr. TurnbuU was a teacher of mathema! 

 illnstratiiijf mechanical problems algcbraicary, 1 had £ 

 that gentleman to the work that was about to appear 

 sponsibility of what I had cot seen upon myself. Vi 

 judge hiiw far Mr. Bashforth will be justified if he : 

 grounds. 



nd had written several tracts 

 lent confidence in him to refer 



had no idia of tailing the re- 

 ■eaders will now be enabled to 

 nues the argument on these 



