476 THE POPULAR SCIENCE MONTHLY. 



should issue from the cylinder under the least possible pressure, in 

 order that refrigeration may he 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. This condition was satisfied by 

 causing the air to act under restraint; that is, by allowing the com- 

 pressed air coming from the reservoir to enter during only a part of 

 the course of the piston. But the admission of the air ought to vary 

 if it is desired to obtain the same final effect, since the pressure in the 

 reservoir diminishes continuously ; and as the apparatus which regu- 

 lates the admission was arranged to correspond only to determined 

 fractions, but not to vary in a continuous manner, it followed that 

 there was a greater expenditure of air than was necessary, and con- 

 sequently a diminution in the length of the course over which the 

 locomotive could run. 



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. No means could, 

 however, be thought of for diminishing the pressure in the reservoirs, 

 which would have reduced considerably the work which the machines 

 were capable of doing, unless by augmenting considerably the volume 

 of the reservoirs, the dimensions of which were already unusually 

 large. 



At this stage M. Ribourt, the engineer of the tunnel, devised an 

 arrangement which allows the compressed gas to flow at a fixed press- 

 ure, whatever may be the pressure in the reservoir. The gas in es- 

 caping from the reservoir enters a cylinder B (Fig. 1), 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 utilized. 

 On one side moves a piston J, which shuts the cylinder ai d hinders 

 the esoape of the air. This piston carries externally a shaft F, which 

 supports externally a spiral spring JET, 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 in- 

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

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

 actly 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 dimi- 

 nution 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 con- 

 stant, notwithstanding the continuous variation at entry. But the 



