COMPRESSED-AIR LOCOMOTIVE. 477 



apparatus is automatic. In fact the part of the cylinder jy comprised 

 between the b< i m and the piston JV communicates by openings p 

 (which are ne 1 . covered with the escape-tube of the gas), in such a 

 manner that vpon its posterior face the piston JV receives the pressure 

 of the gas at the moment when it flows, a pressure which it is sought 

 to render con tant. The piston E receives on its anterior face the 

 action of the spring which can be regulated at pleasure. As to the 

 other faces of the two pistons, they are subjected to equal actions 

 proceeding from the pressure of the gas at its entry, actions which 

 thus cou. '-a-'t each other; so that the forces which determine the 

 position movable system are, on the one hand, the tension of the 



spring, a tant and determined force, and, on the other hand, the 

 pressure o '' flowing gas ; and thus equilibrium cannot occur unless 

 the two forces are equal. If the gas should flow in too great a quan- 

 tity, the pressure increases on the posterior face of the piston A 7 ", the 

 spring is overcome, and the movable system advances a little toward 

 the left ; but then the orifices are partly covered and the flow dimin- 

 ishes. If the pressure then becomes too weak at the exit, the spring 

 in its turn prevails, pushes the sheath toward the right, uncovers the 

 orifices, and consequently a greater quantity of air may enter. 



The machines which are now used at the St. Gothard Tunnel, gen- 

 uine com) 'fesed-air locomotives, are furnished with M. Ribourt's ap- 

 paratus. They consist of the following parts : A sheet-iron reservoir 

 to contai ' the compressed air is mounted on a framework quite like 

 that of s 'ocomotives, and carrying glasses, cylinders, distribut- 

 ing appa . , etc. The tube for receiving the air possesses, within 

 reach of t e driver, the automatic valve of M. liibourt. The screw 

 being eaj ' gulated, the air can with certainty be made to issue 

 from the -atus at a determined pressure. This air then passes 



into a sm ervoir (about one-third metre cube), intended to deaden 



the shod sh are always produced when the machine is set agoing 



or stopj: . stly, this small reservoir communicates with the cylin- 



ders, an t r which reaches them acts in the same manner as steam 



in ordina Dmotives. 



The 1 e in the principal reservoir at the point of exit depends 



on the pi f the compressing apparatus; at St. Gothard it may 



attain fo f een kilogrammes per square centimetre, but is ordinarily 

 about 1A > kilogrammes. The pressure in the small reservoir is arbi- 

 trary, depending on the regulation of the screw; at St. Gothard it 

 has a mean of 4.20 kilogrammes. The entire machine weighs about 

 seven tons. Nature. 



