PNEUMATIC TELEGRAPHY. 
55 
separate the piece t from the stop a. As the exhaustion no longer 
continues above the piston of the cylinder D, that piston falls by 
its own weight, and closes the communication with the vacuum 
reservoir. At the same time, the clack-valve v having been 
opened by the movement of the lever p , the air enters the 
cylinder h and the underground pipe ; at this instaut the 
cover of the box e, which was kept closed by the atmospheric 
pressure, falls by its own weight. The apparatus is thus 
brought to its usual condition, and is ready to act again. 
'V'-mN V 
In despatching a carrier it is placed in the tube s, and 
then the stud b is pressed ; the compressed air then enters the 
cylinder l, Fig. 19, by the tube l and urges the piston forward; 
this closes the valve m, which in that position stops the 
extremity of the subterranean tube. When the piston has 
gone beyond the opening b of the pipe d 3 the compressed air 
rushes through cl into the cylinder n, the piston of this 
cylinder being thereby pushed to the bottom of its course 
(Figs. 20 and 21) is maintained in that position in a manner 
similar to that which we have indicated above for the 
cylinder d : the compressed air rushes into the cylinder H 
and the underground tube, and the carrier is driven to the 
extremity of the latter. On the arrival of the carrier, the 
