^0 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Januart> 



This piston must be further moved through the remaining I of the length 

 of the main (ore) against an opposing pressure of 151b=15 c, the sum of 

 the opposing force is therefore .i- 4+ 15 c. 



To assist its motion there would be a pressure of 51b throughout the 

 length of the main a = 5 a. The general expression of the resistance of this 

 piston is therefore x A+15 o— 5 a ; and for both pistons (15 — x) b + 

 (i 4+ 15 c— 5 a) or 15 (i + c)— 5 a: But (i+e) being equal to a, therefore 

 15(4 + c) — 5a = 15o — 5a = 10a. 



The power operating upon the train piston is (15 — 5)a = 10a; therefore, 

 (leakage and friction excepted,) no power is lost. 



It may be thought necessary to enquire, whether, if an air-pump be used, 

 this reasoning still applies. 



To talie the secondary exhaustion first, I think that Mr. Haydon's formiila 

 ;s just as applicable to the supposed piston, as it is to that of the air- 

 pump ; as each stroke of the pump would be merely a diminished represen- 

 Jation of the one long stroke of the supposed piston. 



In respect of the preliminary exhaustion, it is most easy to form a judge- 

 ment by making a calculation of the effect of one or more strokes of a pump 

 of dimensions assumed with reference to [convenience of calculation only. 

 For instance ; we may assume that the pump has twice the capacity of tlie 

 main, in wliich ease it would effect the preliminary exhaustion by one vptvard 

 stroke. The power necessary to effeiit this stroke may be estimated as 

 follows : The opposing power is 2 ox 15. The necessary power is therefore 

 (2 ax 15) — (2 a j) = The assisting power 2 a .»-, (15 a). 2 a. And as a = 



3 * ,, • /,r X (3 4x2) ,,. 



— ' the power IS (15 — x). — - — -' = (15- 



-x). 3 b. 



Now as upon the other supposition the preliminary exhaustion of the main 

 •was found to require a power=(15 — ,j) 4, there would he a loss in using the 

 pump of (15=ij) 2 i ; but this amount represents exactly the powercon- 

 sumed in exhausting the cavity of the pump itself, which by the supposition 

 is twice the size of tlie main. 



In my last communication I stated that the only loss (disregarding leakage 

 and friction) was that of the power employed in exhauting the pumps and 

 branch pipes j and tlie above reasoning confirms that view of the matter. 



I may observe lastly that, with small pumps and branch pipes, this loss 

 cannot bear more tlian au insignificant proportion to the whole power 

 expended. 



It has been argued that atmospheric traction is like traction by means of a 

 liighiy elastic rope, which requires to be stretched, at the expense of much 

 power, before it becomes rigid enough to overcome the resistance of the load, 

 and which being so stretched, is assumed to afi'ord no advantage over an 

 ordinary rope. 



Now it would be easy in this case to adopt an arrangement which would 

 recover all the power employed in stretching the ropef. And in the case of 

 atmospheric traction a mere glance at the subject shows that such a recovery 

 of power must take place ; for otherwise, the preliminary exhaustion being 

 analogous to the stretching of the rope, the subsequent exhaustions, to 

 complete the analogy, must require as much power as would be sufficient to 

 drag along the train piston by means of a rigid medium ; i. e. a power equal 

 to that of the train-piston itself! a proposition all hut self-evidently erroneous. 

 The power acting upon the train-piston as previously shown being constantly 

 15 — 5 = 10; while the resistance to the air-pump piston is 10 through one 

 third only of its stroke, with an average of about 3 during the remaining two 

 thirds of its stroke, making a total resistance equal to about one half oi the 

 power acting upon the tram-piston. 



E. H. 



[We have to thank our correspondent for taking great pains to investigate 

 the subjects of certain papers in this journal, and for communicating his 

 observations in a most obliging manner. We cannot, however, carry our 

 courtesy so far as to assent to his conclusions. His arrangement of a 

 supposititious " exhaustion maintaining piston," " some little in advance of 

 the tiain-pislon," is wholly different from anything that occurs in practice. 

 It docs not appear how this exhaustion maintaining piston is itself to be 

 moved — not by stationary; air-pumps, for the degree of vacuum in front 

 of it is !uppnsed to constantly diminish, until the air actually recovers its 

 natural density ; and this cinikl only ^take place on the supposition that the 

 air-pump ceases wurkinrj directly the train starts. 



The mere consideration, that in the air-pump the air is alternately dilated, 

 and condensed, ought of itself to shew that some power is lost, independ- 

 ently of leakage and friction. If the particles of air be at constant intervals 



t Suppose a weight is to be raised from the bottom of a well 100 feet deep, by means 

 of an elastic rope loij feet long, passing over a puily at the top of the well, aud being 

 drawn along hurizontally by a man. Suppose the man found the rope to stretch to 



■ double its lvn!,'tli ; i. e. to UOl) feet, by the time it began to lift the weight. Being BO 

 -stretched, lei its upper end (the 100 feet from the well head) be secured upon the ground, 



(by spiking or otherwise) ; and let the man then take hold of the rope close to the well 

 head; the slightest force applied there will make the weight begin to rise; because the 

 man has in his f.ivour the reaction of the half of the rope which lies behind him, but as 

 the weight riKea, this half of the rope will shorten, and its reaction will diminish constantly 



■ till it ceases just when the man has moved iO feet from the well head, raising the weight 

 50 feet. 



In this oper.'ition however one half of the power, originally employed in stretching the 

 rope, will have been recovered. 



In like niniiner let the man make fast upon the ground the part of the rope which he 

 now holds,— go back to the well head, and draw the rope forward !'."» feet, raisijlg the 

 weight '27i feet ; and he will ftn'ther get back one quarter of the said power. 



He may regain ^ ~q t^V &c. so long as he goes on 3 whereby he may regain all b 

 lufinitesimal fraction. 



moved closer together, and farther apart, than they are in their naturel 

 state, it is clear thit force must be absorbed for the purpose. For to sup- 

 pose that an elastic fluid has the power of contracting and dilating itself i» 

 to indue inert matter with an inherent power of motion. 



The principles assumed in Mr. Haydon's paper were so entirely in the 

 ordinary routine course of mathematical investigation, that no mathemati- 

 cian would dream of disputing the nature of the loss exhibited ; all that 

 can possibly be done is to question the amount. If our correspondent 

 wish to do this, he must do it not as here, indirectly, but b y directly show- 

 ing some error in Mr. Haydon's paper. He must make no suppositious 

 arrangements of pumps and pistons, but must follow themethod of thepaper 

 in question, by taking the dimensions of the pump and tube, and the 

 number of strokes to be exactly what they are in practice. 



The illustration of the man drawing a weight from a well is accurate 

 enough, except that the man's journeys would be less and less eachtime, while 

 the stroke of the air-pump is always of the same length ; Jthe consequence 

 would be that in the former case the loss of power might be made much less 

 than it could possibly be in the latter. Our correspondent himself says, 

 that there would be some loss in the former case, — that the power usefully 

 applied would never exactly equal the power expended ; the question there- 

 fore is, as we said, not one of principle, but a question of mere amount.] 



THE PROPOSED EXPOSITION OF MANUFACTURES AND 

 INDUSTRIAL ART. 



The preparations for the proposed exposition of Manufactures and Indus- 

 trial Art, in connection with the Manchester School of Design, under the 

 direction of our talented townsman, Mr. George Jackson, are in a forward 

 state, and the exhibition will be ready for the Christmas holidays. Mr. 

 Jackson and his colleagues are gentlemen fully competent to the task they 

 have undertaken, and their success so far has even exceeded their antici- 

 pations. It is now expected that every nook and corner of the Royal In- 

 stitution will be required for the display of the articles, so numerous will 

 they he. From the answers already received, it is expected that the contri- 

 butions from the Potteries will be rich, indeed magnificent — several houses 

 of eminence in the china trade having promised contributions of the highest 

 class, and of considerable amount. From Stourbridge and Birmingham they 

 have promises of an extensive collection in glass, comprising chandeliers and 

 articles of general utility ; and from the latter place there will be a consi- 

 derable quantity of japanned goods, brassworks, and manufactures of other 

 descriptions peculiar to the town. There will also be some very brilliant 

 specimens, both in gold and silver, of the electro-deposit. From Coventry 

 splendid specimens of ribbons and gauzes will be forthcoming, and watches 

 equal to anything of foreign production. From Leicester a display of ho- 

 siery ; and from different places in Nottingham a splendid collection of laces 

 are promised. Sheffield is to contribute specimens of its steel and other 

 goods, comprising grates, fenders, fire-irons, cutlery, &c. Glasgow is under 

 contribution for carpets, and some magnificent articles of large dimensions 

 woven in one piece are amongst the specimens. From Paisley, shawls, 

 scarfs, and other woven fabrics peculiar to the district are bespoken. From 

 Ireland specimens of linen are to be exhibited, manufactured from Irish- 

 grown flax. London is to send silversmiths' goods, including splendid spe- 

 cimens of plate ; and some magnificent collections of testimonial plate are 

 on their way to Manchester from thence. Among the manufactures of our 

 town an effort is making, and the contribution will include a large collec- 

 tion of printed cottons, de laiues, and silks. The exposition of the latter 

 description of goods will also include silks from Macclesfield, Spitalfields, 

 and other places. The Council, we learn, have not restricted the exposi- 

 tion to articles of home manufacture only, but have induced several foreiga 

 houses to exhibit and forward specimens of their manufacture. As a favour- 

 able sample of the spirit in whit-h they have been met on this point, we 

 may mention that one house in London declined answering the application 

 till a partner in Paris bad been consulted. His reply was not very favour- 

 able, but be added, that in his opinion the Council had adopted " a prin- 

 ciple with regard to this exposition that ought long ago to have been adopted 

 on the continent, in allowing the foreigner to exhibit in competition, and 

 that the exoosilions on the continent would never be successful till they 

 adopted the same principle." The Council, however, confine the exhibi- 

 tion to the production itself, and do not contemplate exhibiting, as at the 

 Parisian exposition, the means of production. With regard to the School 

 of Design itself, it is expected that on this occasion it will fully bear out 

 the »>vticipations which the public have been led to draw, and that the spe. 

 cimens will show the utility of such institutions like this, where they cau be 

 properly conducted. We are also glad to find that the prizes offered by the 

 Council, to stimulate designers of the town generally, have had a good 

 effect ; and that the drawings sent in for competition are numerous, and 

 many of them talented. — Manchester Times, 



