1845. 



THE CIVIL ENGINEEIl AND ARCHITECT'S JOURNAL: 



205 



THE PROPERTIES OF AIR AS A MECHANICAL AGENT ; 

 Conaidered more particularly in reference to Atmospheric Railwayi. 

 " Mr. P. Barlow presented, as an appendix to his paperon the atmospheric 

 system, the results of a series of experiments upon the force employed in 

 drawing carriages up an incline plane of 1 in 43 by a stationary engine and 

 rope traction u[ion the Canterbury and Whitstable Railway. From these ex. 

 periments it appeared that the stationary engine of 2j h. p., witli a rope, 

 would produce an useful mechanical effect, equal to the engine of 100 li. p. 

 on the Dalkey Atmospheric Kailway— thus proving by direct facts the 

 deduction of Mr. Stephenson as to the amount of lost power hy the latter 

 system. These statements were ordered to be printed with Mr. Barlow's 

 paper." — from the Proceedings of the Institution of Ciml Enyineers. 



It seems almost incredible, but it is yet true, tliat in all the nnnip- 

 rous discussions and disquisitions wliicli tlie introduction of atmo- 

 spheric railways has produced, notice lias not once been taken of the 

 propriety or improprirty of employing an elastic agent for communi- 

 cating motion. In a former number of this publication it was rigo- 

 rously proved that the communication of power by the agency of 

 elastic air was attended with an enormous waste quite independent cf 

 the losn by leakage : that paper was copied into many other periodicals, 

 and lias not yet met with contradiction. It has been cuiisiiicred indis- 

 putable that'in atmospheric railways there was a great waste of power, 

 but unfortunately the waste lias been uniformly supposed to be occa- 

 sioned by friction and leakage. Now it will be our present object to 

 show that the greatest loss of all lias been hitherto entirely overlooked, 

 that this loss is irremediable, arising as it does, not from imperfections 

 of machinery, but from the fundamental unalterable properties of mat- 

 ter, that it is therefore beyond our control and ingenuity, and mould 

 exist in an atmospheric railway nholly and perfectly free from leakage 

 andfrictwn. 



We may state, as a general axiom, that elastic substances are unfit 

 agents for communicating the mechanical eft'ect of a prime mover, and 

 in showing how this general truth bears upon the particular case of 

 atmospheric railways, we cannot do better than view that invention 

 liistorically, and we shall see that in the very first instance in which 

 the air was used for the transference of mechanical power, the cause 

 of failure arose from no diftcls in the mechanism employed; and we 

 must conclude that had the laws offeree been as systematically estab- 

 lished then as now, this failure, being as it was a clear deduction from 

 those laws, would have been at once conclusive against the hope and 

 possibility of ultimate success. 



The original idea, then, of the atmospheric railway belongs to 

 Papin ; and we entreat particular and very careful attention to the 

 following exposition of the experiment made by him, because an at- 

 tentive consideration of that experiment will, we are certain, unravel to 

 the reader all the mysteries of the "atmospheric system," and enable 

 him to grasp accurately and philosophically the whole subject. 



Papin's experiment was on this wise. He wanted to pump water 

 out of a mine, and the only motive force for the purpose which he had 

 at his command was a water-wheei turned by a neighbouring stream. 

 He was desirous of making this wheel work the pumps which were to 

 draw oft' the water of the mine; but he laboured under this disadvan- 

 tage, — the distance from the mouth of the mine to the nearest point 

 ■where he could erect his water-wheel was upwards of two miles. 

 His object, then, was to transfer the force of the wheel to a point two 

 miles off; and the agent which he used for this purpose was the aik. 

 He caused a continuous air-tight tube or pipe to be laid down from 

 the w-ater-wheel to the mouth of the mine. At the extremity of this 

 pipe next the wheel a piston was placed, and worked backwards and 

 forwards in the tube by a crank connected with the wheel : at the 

 other extremity of the tube a similar piston was placed, and this 

 piston was connected with the lever of the pump of the mine. From 

 this arrangement Papin expected to be able to communicate the pov^er 

 effectually through a distance of two miles. He anticipated that as 

 the piston next the wheel was worked backwards and forwards, the 

 piston next the mine would move backwards and forwards through the 

 name distance. What, however, really took place was this— the 

 piston at that mine did in truth move to and fro, but the extent of that 

 motion was much less than that of the Jirst piston; in the language of 

 the narrator of the event, the extent of motion which it was necessary 

 to give the first piston in order to work the pumps was "preposterous?' 



We repeat that a clear apprehension of this experiment will put 

 the reader in possession of the whole case of the atmospheric rail- 

 ways. 



Here was, it will be observed, an atmospheric tube without the lon- 

 gitudinal aperture — an ATMOSPUErac railway without leakage— and 

 yet the waste of power was " preposterous." 



Now, we wish to show bow the loss of power arose, and we intend 



also to explain a simple method by which the amount of the loss may 

 be calculated with all the precision of a mathematical investigation. 

 In the first place, then, we will consider what took place in the tube 

 while the piston next the water-wheel— the prime moving piston — was 

 worked backwards and forwards. When it advanced, the air through- 

 out the two miles of pipe was compressed until the second piston 

 could no longer withstaiul the compression; ami when the first piston 

 receded, the air throughout the two miles of tube was dilated till the 

 piston connected with the pumps was moved bv the pressure of the 

 external atmosphere. We will, however, confine ourselves to the 

 second case, namely, where motion was produced by the dilatation of 

 air, because this is the case of the atmospheric railway. 



When the prime moving [liston receded, the air in the tube wa,'* 

 dilated and consequently pressed with diminished force on the second 

 piston, which therefore was moved by the prepomlerating force of the 

 external air. Let us suppose, for clearness sake, that the atmosperic 

 pressure is 151b. to the square inch, and that the pressure to move the 

 second piston was 10 lb. to the square inch. Well then, in order that 

 the external atmosphere might press on the second piston with an 

 ejjflclive pressure of 10 lb. we mu.st have the pressure on the tube di- 

 minished to 5 lb. to the square inch, because the real atmospheric 

 pressure of 15 lb. being opposed by an internal pressure of 5 lb. the 

 efftctive pressure of 10 lb. would be the result. 



Simple as all this may seem it is very necessary for our purpose 

 that it should be clearly laid down. We have then the internal air 

 pressure reduced by dilatation to 5 lb. to the square inch; its pressure 

 undilated being 15 lb. Now, to reduce the pressure in the pro- 

 portion 5 : 15, or 1 : 3, the extent of dilatation must be in the same 

 proportion, or in order that the air may press with only one-third its 

 original force it must occupy three limes its original space. If then 

 it had been requisite for Papin's purpose to have on his second piston 

 a pressure of 101b. to the inch, it woii,d be necessary that the air in 

 his tube should occupy three limes its original space — that is, should 

 occupy a tube G miles instead of 2 miles in length! 



Let the reader carefully review this argument, for it is one which, 

 as far as the writer is aware, has never been offered except by him- 

 self, and then proceed to the application of it to the "atmospheric 

 system." First, however, to fix the idea more clearly, let us suppose 

 one or two variations of the problem. Suppose, for instance, M. Papin 

 wanted only an effective pressure of 7i lb., then he would have to 

 reduce the internal pressure in the proportion 74 : 15, or 1 : 2, or the 

 internal air would have to occupy four miles instead of two. If he 

 wanted an effective pressure of 5 lb. only, the internal pressure must 

 be 10 lb. instead of 15, and the reduction must be in the proportion 

 2 : 3, or the air instead of occupying two miles of tube would occupy 

 three. Or tabulating the results — 



For a pressure ot 12 lb. the space occupied by air = 10 miles, 

 „ 10 lb. „ = 6 „ 



,, Ti lb. „ = 4 „ 



.. 5 lb. „ = 3 „ 



In the 1st case the dilatation is five-fold, in the 2nd triple, in the 

 3rd double, in the 4th one and a half. 



To apply these results to the case of the atmospheric railway, it 

 will be seen that to obtain at starting a working power of ten pounds 

 to the square inch on the travelling piston, the preliminary dilatation 

 must remove two-thirds of the pressure in the tube, or two-thirds of 

 the air must be pumped out before the train is set in motion. Now, 

 if we exclude the idea of leakage altogether, it will be clearly seen 

 that the whole quantity of air pumped out of the tube from first to last 

 is exactly the quantity occupying the whole tube at the ordinary den- 

 sity of the atmosphere: for as the travelling piston successively oc- 

 cupies every portion of the tube, it must, between the beginning and 

 end of its journey, displace the air in every portion of the tube, and 

 as by our supposition this air passes out through the pumps onlv, it 

 follows that the amount of air pumped out is exactly the quantity con- 

 tained in the tube before the pumps began to work. Now we have 

 shown that in order to start with a pressure of 10 pounds to the inch 

 two-thirds of the air in the tube must be pumped out; it therefore 

 follows that of the whole quantity of air pumped out of the tube, two- 

 thirds are exhausted before starting. 



Now if we can show that this extraction of two-thirds of the air 

 contributes nothing to the subsequent motion of the travelling piston, 

 we shall have arrived, by a process to all intents as indisputable as a 

 mathematical investigation, at the inevitable conclusion that two-thirds 

 of the motion of the prime mover are expended niltiout producing 

 motion in the train of railway carriages. 



In the first place, then, we have to consider what takes place in the 

 air-tube after the preliminary exhaustion has been accomplished. 



We suppose that the working power of 10 lb. to the inch is main- 

 tained throughout the journey, and that consequently the rarefaction 



27 



