PXKr. MATH'S. 



37 



Fig. 17. 



mgly been devised to obviate thig. The principle on 



merely that a conical valve is moved \ty the 



pressure BO as to close to a greater or leas extent the pipe along 



in a small but useful piece of apparatus, known as the 

 Fig. 17), which may be explained here, as it acts in 



a Minil.ir v ;iy to the pressure-gauge just mentioned. In many 

 chemical experiments in the laboratory, as well as in the manu- 

 factory, a large amount of gas is evolved by the 

 changes taking place within some closed vessel. 

 If no escape be allowed for this, the pressure may 

 increase to such an extent as to burst the vessel ; 

 while, on the other hand, it is desirable not to 

 allow the gas to be lost. A safety-pipe, similar to 

 that shown in the annexed figure, is therefore 

 introduced. This allows a portion of the gas to 

 escape when the pressure reaches a certain limit. 

 It is, in fact, a safety-valve of low pressure. A 

 glass tube has a bulb B blown near the middle, 

 and each end is then bent back upon itself. The 

 upper end is also shaped into a funnel, which 

 should be rather larger than the bulb. Water or 

 mercury, according to the pressure required, is 

 now poured into the funnel, so as to fill the bend 

 and part of the bulb. If the pressure inside the 

 vessel becomes too great, the liquid will be forced 

 iuto the part c of the tube, and any excess of gas 

 will then bubble up through it, the funnel prevent- 

 the escape of the liquid. If, on the other hand, 

 the pressure inside becomes less than that without, 

 owing to the absorption or condensation of the gas, the pressure 

 of the air will force the liquid into the bulb, and air will then 

 bubble up through it. In this way the tube prevents the 

 difference between the pressures from becoming 1 dangerous, and 

 at the same time, under ordinary pressure, excludes all air. 



We have seen that power may be stored up in compressed air ; 

 hence it is sometimes employed to drive an engine in place of 

 steam. Of course some power must be first employed to com- 

 press the air, and therefore in ordinary circumstances no ad- 

 vantage will be gained by the substitution, but in many special 

 cases it may be and is employed. If steam has to be conveyed 

 to any great distance, there is a considerable loss by condensa- 

 tion in the pipes, and in some places it is inconvenient or 

 impracticable to have the boiler near the machine. In such 

 cases, therefore, the steam may be employed in the compression 

 of air, and by this the power may be transmitted to the place 

 where it is required. 



In mining operations this is especially advantageous. A 

 narrow seam of coal, in which there is no room for an engine, 

 has sometimes to be cut out by a machine, and even if the 

 engine could be placed there, the steam and smoke would pre- 

 vent a man being by it ; such machines are therefore driven by 

 compressed air. The same remarks apply to a narrow tunnel, 

 as, for example, that which was driven through Mont Cenis ; 

 and here, too, compressed air was used instead of steam. 

 There is also this further advantage attending the nse of air, 

 that the machine can be more easily moved, for a portion of the 

 pipe may be made flexible, which cannot well be done with 

 steam-piping. 



The most important application of the pressure of air to 

 driving machinery is seen in the atmospheric railway. At 

 present this has not come into practical use, but it appears 

 probable that the principle will ultimately be adopted in our 

 underground railways, as it will effect a saving in working 

 expenses as well as in construction, to say nothing of the much 

 greater safety which would be ensured by its use, and the 

 greater purity of the air in the tunnel. 



The original plan proposed, and actually carried out on a 

 short piece of line near Paris, was somewhat as follows : A 

 large iron tube, having all along the top an opening which was 

 closed tightly by a flexible lid, was laid along the middle of the 

 line. This tube was made uniform in size, and a pair of pistons, 

 made to fit it, were fixed one to each end of a little carriage 

 which travelled along in the tube. From the middle of this 

 carriage rose an arm which projected through the slit, and was 

 attached to the carriage on the line. A coulter-shaped piece of 

 metal was placed on each side of the arm, so as to open the slit 

 for it to pass along, and the aperture closed of itself as soon 



as the arm had pawed. The piston* were also attached to short 

 arms, so that the valve admitted no air in front of them. At 

 each end of thin tube was fixed a powerful double-acting air- 

 pump, and whenever it was required to start the train, the 

 pump at* the end to which it was going was set to work. It 

 soon produced a vacuum in the tube, and the pressure of the 

 air behind the piston was sufficient to drive the train. There 

 were, however, many practical difficulties in the carrying oat of 

 this plan. The valves could not be got to close well, and hence 

 there was a considerable leakage of air which greatly diminished 

 the power. All the strain, too, was transmitted through the 

 arm, and thus there Was danger of breakage. From these 

 and many similar causes the design was not carried oat 

 elsewhere. 



More recently, however, an altogether different plan was 

 tried with much greater success. In this the tube was built of 

 brickwork, and made of such a diameter as to take in it an 

 ordinary-sized railway carriage. A trial line, of nearly a mile 

 in length, was constructed in the grounds of the Crystal Palace 

 at Sydenham. The line was made with steeper gradients and 

 sharper curves than any line yet worked, so as to give the 

 system a full trial. Tho tunnel was carefully constructed, so as 

 to be of uniform size, and one end of the carriage was made 

 nearly to fit it, an aperture of a few inches being left all 

 round. A brush fixed round the carriage nearly filled this, 

 and was found to exclude the air sufficiently. The ends of the 

 tunnel were closed by air-tight doors, and in a building near 

 one end was fixed a large fan, constructed somewhat after the 

 plan of a centrifugal pump, and so arranged that by causing it- 

 to rotate in one direction it exhausted the tube, while on re- 

 versing it the air was condensed. The pipe leading from this 

 entered the tunnel at a little distance from the end. The car- 

 riage being now placed just in the mouth of the tunnel at the 

 further end, the engine was set to work, and, as soon as a slight 

 amount of exhaustion was produced, the pressure outside forced 

 it rapidly along. As the whole area of the carriage was exposed' 

 to the pressure, it was found that only a very small degree of 

 rarefaction was required, a pressure of a few ounces to the inch 

 being quite sufficient to impart to it a great velocity. As soon 

 as the carriage had passed the portion of the tunnel where the- 

 exhaust pipe entered, it ceased to be carried forward by 

 the pressure of the air, but it had acquired an amount of 

 momentum sufficient to propel it with considerable violence 

 beyond the end of the tunnel. The doors at the end were, how- 

 ever, closed by powerful springs. The air, therefore, enclosed 

 between them and the carriage became more and more com- 

 pressed, until the pressure was sufficient to open the doors v 

 and allow the carriage to run slowly out. The air acted, in 

 fact, as a buffer, and brought the carriage to rest with scarcely 

 any shock. When the carriage was to be sent back to the other 

 end, the engine first exhausted the tube until the carriage passed' 

 the opening, the doors were then closed, the engine reversed, and" 

 then the air behind was condensed, and drove the carriage to- 

 the other end on the same principle as a boy drives a pea 

 through his pea-shooter by the pressure of his breath. 



The experiment appeared satisfactory, through no practical 

 use has yet been made of it. The carriage, with passengers in 

 it, could be started from one end, driven round the cnives, and 

 up and down the steep inclines, and yet stop at the other end, 

 nearly a mile off, in the space of about one minute. The system 

 appears to possess many advantages. Much greater inclines 

 can be allowed, and all danper of the carriage running off the 

 line on sharp curves is avoided. There is also much greater 

 safety from accidents. Collision is impossible, for two car- 

 riages can never be travelling in opposite directions at the same 

 time, nor can one overtake the other. The boiler, too, being 

 away from the train, cannot injure the passengers if it explodes, 

 and the only inconvenience then would be that the passengers 

 would have to walk along the line to the nearest station. 

 Further, as the trains would travel very rapidly, one line would, 

 in most cases, be sufficient, and the additional expense incurred 

 by the careful building of the tunnel would, in many places, be 

 compensated for by the smaller amount of land required. The 

 tunnel, too, unlike our present ones, would be well ventilated, as 

 the air would be entirely changed each time a train passed 

 through. There are, of course, many practical difficulties which 

 might occur in the actual working, but the plan seems to 

 promise well, and to be worthy of a thorough trial. 



