476 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



the employment of compressed air in a steam locomotive presented 

 a certain number of drawbacks. It is expedient that the air should 

 issue from the cylinder under the least possible pressure, in order that 

 refrigeration may be reduced to a minimum ; for it is known that the 

 expansion of gas is accompanied by a loss of heat which increases 

 with the pressure. 



On the other hand it is necessary that the air should arrive in the 

 distributing apparatus with the least possible pressure, for it is in this 

 apparatus, in the slide-valve, that the greatest losses take place, and 

 these losses increase in proportion to the pressure. 



M. Kibourt, the engineer of the tunnel, has devised an arrange- 

 ment which allows the compressed gas to flow at a fixed pressure 

 whatever may be the pressure in the reservoir. The gas in escaping 

 from the reservoir enters a cylinder B (Fig. 321), over a certain extent 

 of 'the walls of which are openings m m, that communicate with 

 another cylinder c, which surrounds it to the same extent, and which 

 is connected with the slide-valve by which the air is distributed, or, 

 more generally, with the space in which this air is to be utilised. On 

 one side moves a piston, E, which shuts the cylinder and hinders the 

 escape of the air. This piston carries externally a shaft, r, which 

 supports externally a spiral spring, H, the force of which is regulated 

 by means of a screw. Internally it is connected by another shaft, L, 

 with a second piston, N, which bears a cylinder, M, movable in the 

 interior of the principal pump, and forming thus a sort of internal 

 sheath. This sheath presents openings, n n, which may coincide 

 exactly with those already referred to, and in that case the gas passes 

 without difficulty from the reservoir at the point where it is to be 

 employed. But if the sheath is displaced, the openings no longer 

 correspond, there is resistance to the passage, and consequently 

 diminution of the quantity of gas which flows out, and hence lowering 

 of pressure in the exterior cylinder. By making the position of the 

 sheath to vary continuously we may make the pressure of exit constant, 

 notwithstanding the continuous variation at entry. But the apparatus 

 is automatic. In fact the part of the cylinder B comprised between 

 the bottom and the piston N communicates by openings, p (which are 

 never covered with the escape-tube of the gas), in such a manner that 

 upon its posterior face the piston N receives the pressure of the gas at 

 the moment when it flows, a pressure which it is sought to render 



