372 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Dkoembkr, 



Detailed statement of the length, diameter, and cubical contents of the 

 main tube aud branch — {Beri^in.) 



Contract and present prices of th» iron tube. Length and weight of each 

 pipe. Mode of supportinj; aud fixing the tube — (Samuda.) 



" The contract prices we entered into for the Croydon Railway were 

 71. 15s. per ton for the pipe. The length of each separate pipe is ten feet 

 in the clear ; it weighs I cwt. a foot. A foot is cast on the pipe, under- 

 neath the socket, and that is laid on the same transverse sleeper which 

 supports the chairs of the rail at the spicket end of the pipe ; the pipe is 

 turned, and is fixed into the board part of the next socket, so that the pipe 

 is supported every 10 feet." 



The distance of the engine from the tube on the Dalkey line is a great 

 disadvantage. It is more prudent, perhaps, to begin with sections of three 

 miles, but the alternate engine at six miles could do the work. If the 

 leakage could be prevented there would be no loss of power in a section of 

 six miles, but more time would be expended in producing a vacuum." 



Vacuum. — Economical vacuum to work at is at a pressure of 71b. to the 

 square inch — (Gibbons.) 



Explanatiun of the process by which it is proposed to produce a half- 

 Tacum in the pipe by means of a large tank rapidly emptied. There 

 would be no danger of the tank falling in from the pressure of the atmos- 

 pheric — {Brunei.) 



" I propose to build, in masonry, an air reservoir, about equal to the 

 contents of the whole section of the pipe, and to pump the water up into 

 that reservoir by the engine power, which I have, and then allowing that 

 reservoir to empty itself with a bead of 20 or 2.5 feet of water, \\'ill, of 

 course, exhaust the pipe at once to half-vacuum, or seven pounds upon the 

 inch, and that will enable roe to fill the pipes more quickly, and will save 

 a great deal of the first leakage; it will enable me to fill the pipe more 

 quickly, and will also enable me, if I wish to work it again at the same 

 time as the engine, to exhaust with twice the rapidity that I could do with 

 the engine aloue. If the reservoir were equal to the size of the pipe, the 

 emptying of the reservoir would produce what is commonly called half- 

 vacuum, supposing there were no leakage. I should make the reservoir 

 a little larger. I propose making the reservoir a third larger than the pipe. 

 A reservoir of about 26,000 cubic feet. Of very rough materials ; the 

 sides of rough masonry, and the top of brick work. There will be no 

 danger, from the pressure of the atmosphere, of its falling in; that pressure 

 is equal at the most to 30 feet of water, and there will be no ditliculty in 

 making a reservoir that will stand a pressure of 30 feet outside of it. I 

 think it is an affair of 300/. or -100/. All the engines upon the Hlymouih 

 line have also small auxiliary engines, of lO-horse power ; our engines 

 consisting, in fact, of a pair of 40 and a pair of 10 ; all those lOhorse 

 engines might be at work constantly, both pumping the water for the 

 boilers and pumping the water up into the reservoir, and doing the ordi- 

 nary work at the station." 



■Witness found his objections to the asmospheric system from the fact of 

 the power required toobiain a higher vacuum increasing in a much greater 

 ratio than the vacuum produced — {Stephenson.) 



Have you drawn up any table upon that subject? — The number repre- 

 senting the power would be 4 J-horse power upon the piston. What horse 

 power on the engine? — The pressure would be in proportion to the horse 

 power; when the barometer stands at 9 inches the mean resistance upon 

 the air pump is 4 1 pounds ; I may state that that is not a theoretical cal- 

 culation, but one made from actual experiments; therefore there is no 

 hypothesis involved in this. From practical experiments we are told that 

 57-hurse power is necessary to produce that result; viz. to raise the 

 vacuum 9 inches?— It is very probable. Take 18 iuches, what is the re- 

 sistance in that case '—There the average mean pressure is 6-3, therefore 

 the horse power will be m the ratio of 4 5 to 03. Eighty-seven is the 

 horse power that is put down in the table which has been given in, there- 

 fore you double your power without doubling your horse power? — Yes, 

 up lo that point. There is no loss there ; on the contrary there is a gain. 

 If you go on to 20 inches; the resistance of the air-pump is 9 7. There 

 again you have acquired a greater result with a less proportional exertion 

 of power ?— Ves, that is the exertion of power upon the pump, and so you 

 go on till yiiu get near the end of the scale ; you gain in point of power, 

 so far as the mere working of the pump is concerned, but the power ex- 

 pended in the pump is not the power given out to the train, and that brings 

 me back to the same position which I took originally ; this is not the 

 quantity of power that is given out to propel the piston in the tube which 

 draws ihe train along, this is merely a representation of the quantity of 

 power necessary to drive the pump ; but the second question follows, w hat 

 proportion of that power is given out to the train, and that is the whole 

 question, upon which I believe I differ essentially from other engineers. 

 I think the difference arises from confounding the power generated with 

 the power given out. By the power given oui you mean the power use- 

 fully exerted?— Yes, the proportion betwren the power necessary to work 

 the pump and the proportion of that power applied to the train gets worse 

 and worse as you go on increasing your vacuum. Between the vacuum 

 pump aud the train all the leakage exists, aud there it is that I dill'cr from 

 those who maintain that ihey gel all the power generated for the propell- 

 ing of the train ; it is perfectly clear that, if there be any leakage at all, 

 the power represented by the air-pump does not give you any true repre- 

 sentaiiun of the power given out to the train, because the whole length of 

 the valve exists between the one and the other. The 18 inch vacuum 

 will carry double the load that the nine inch vacuum would ?— That is the 



difficulty. If the pipe lying along the railway were hermetically sealed, 

 and the train put alongside of it, and no leakage took place, that position 

 would be perfectly clear ; but inasmuch as between the power exerted by 

 the engine, that is, the air-pump, and the power given out to the train, 

 there is a valve, which is the prominent imperfection of the system ; how- 

 ever beautifully in theory it may be overcome, still there it stands as the 

 prominent imperfection, and one which affects all calculations made upon 

 this; and any opinion therefore based upon the power given out by the 

 pump must be fallacious, because it takes no account of the leakage. 

 Supposing the valve to be propped open just before the train, the pomp 

 would require the same power to work it, and ihe train would not move at 

 all ; the same power, therefore, would be exerted upon the pump, and the 

 train would not move an inch. Supposing yon had a train 10 tons weight, 

 and a 9-inch vacuum was sufficient to carry that train forward at the rate 

 of 10 miles an hour, if you increased the weight to 20 tons and obtained 

 a vacunin of 18 inches, would not the sustained vacuum of 18 inches con- 

 tinue to carry forward that train at the same speed per hour? — If yen 

 sustain the vacuum at 18 inches it would, but the difficulty is in sustaining 

 that vacuum, because as the vacuum rises the leakage is increased. 

 When Ihe barometer is standing at 9 inches the leakage is a small amount, 

 but when you get it up to 24 or 25 it requires the full power of the engine 

 to maintain that vacuum ; it requires 1000 per cent, more than it requires 

 at a smaller vacuum. According to this scale it requires 56-horse power 

 to produce and maintain nine inches of vacuum, and it requires 87-horse 

 power to produce and maintain 18 inches of vacuum ; you have there 

 double power produced in spite of the leakage of the valve, without a 

 doubling of the horse power being requisite ? — Yes, but that double power 

 would not produce the same velocity, because the leakage has increased. 

 Though the same amount of power is expended in the air-pump it is not 

 given out to the train ; for as you increase the vacuum, you, as it were, 

 sever the connexion between the pump and the train. You mean to say 

 this, that if you can maintain in the atmospheric main a vacuum measured 

 by 18 inches of mercury, that gives double the power to Ihe train that you 

 have when the gauge stands at nine inches ? — Clearly. But you say that 

 that increased vacuum cannot be maintained without an exercise of more 

 than double the power, inconsequence of the increased leakage? — Pre- 

 cisely. The leakage, as 1 understand, remains the same; that is, the 

 number of cubic feet of air that you get into the main in a certain time 

 remains constant? — As the density of the atmosphere. But owing to that 

 air expanding you have a loss of power from a certain quantity of air 

 of tlie atmospheric density entering into the main, which quantity rapidly 

 increases as the mercury rises in the gauge? — That is precisely my view. 

 As 1 understand, the main objection which you have to the economical 

 working of the atmospheric power is this, that when the vacuum was at 

 that degree of height which is necessary for moving large trains, the ex- 

 penditure of power would be so great that the system would become very 

 expensive? — Just so'. Otherwise, the power applied very directly to the 

 work to be done, and is economically aud usefully exerted? — At low 

 vacuums it is." 



It is difficult to say at what vacuum it is most economical to work an 

 atmospheric line ; it is not yet ex ictly known — {Cubitt.) 



Valves. — Statement of the dimensions and of the outlet valve in the 

 engine on the Dalkey line — {Bergin ) 



In consequence of the large size of the outlet valves, when there is any 

 degree of exhaustion within the pump tliey are composed lo so much 

 pressure from the external atmosphere, that their edges are forced into very 

 close contact with the plate of leather on which they ilrop, the effect of 

 which is, to require a force equal to about 1 I lb. on every square inch of 

 the air-pump piston to lift these valves, in adodilion to the force necessary 

 to drive the air out; the final result is, that a force of about 8 J hor»e 

 steam-engine power is absorbed in opening those valves a! Tt. in addition 

 to the power necessary to expel the air. In the Dalkey pu. :p:> there are 

 two very large valves, 20 inches in diameter, opening by a hinge at one 

 side, which must be forced up a considerable distance to let the air out ; 

 the Croydon valves are several in number ; 1 do not know how many, but 

 they are small valves, seated in the covers of the air-pump ; they have to 

 rise but a small space, and when down form a completely smiioih surface, 

 both above and below ; they have not an edge resting on a plate of leather, 

 to which they would adhere ; they are simply conical valves, seated in the 

 air-pump cover, counterpoised, so thai 1 expect they will present only a 

 portion of the resistance of which I have spoken. So that two or three 

 horse power, instead of eight, will overcome that resistance? — Probably; 

 certainly much less than in the Dalkey pump. There is another evil ao- 

 companying those large valves, arising from iheir gieal magnitude ; they 

 shut with enormous violence, so great, that upon the very first evening we 

 started, some of them were broken within three or four strokes, and they 

 shut with such violence as to shake ihe whole mass of the puiup and its 

 framework, which weighs se\eral tons. Either Mr. Clegg, or Mr. 

 Samuda, or both, were present at the time, and saw this efi'ect, and they 

 added an ingenious apparatus at the back of the valve, which has dimi- 

 nished the violence of the shock, at the same time that it has increased 

 somewhat the resistance of wliicli I have spoken. The valves which were 

 originally constructed, we have been obliged to replace by others two or 

 three times, and subsequently we have removed them altogether, and put 

 very much stronger, made of Iron. Those valves we have now worked 

 for five or six nioiiihs without any inconvenience. The steam-engine which 

 is now working, is that which is known as a high pressure expansive and 



