PNEUMATIC TELEGRAPHY. 
57 
forwards. Its impetus causes it to open the clack-door c, and 
its velocity having thus been destroyed, it is drawn against 
the opening o of the tube s. As soon as the person ap¬ 
pointed for that purpose sees the door c fall, he closes the 
cock Y, and then the carrier, being no longer kept up by 
the external pressure, drops out of the tube t by its own 
weight. 
A carrier is sent in the manner following :— It is placed in 
the tube t, and the slide formed by the rods g and the cross¬ 
piece d is drawn out by the handle m ; the cross-piece is thus 
Fig. 22 . • Fig. 23 . 
brought against the stop / fixed on the rod b and brings this 
with it. The sliding door k attached to b then advances and 
closes the end of the tube t. As soon as this is done, the 
inclined plane h fixed to the rods g meets the roller j and 
pushing against it, opens a valve placed within the cylinder L 
and thus connects the reservoir of compressed air with the 
pipes M and t. The carrier is thereupon sent through the 
subterranean tube, and w'hen its arrival has been announced 
by the ringing of the electric bell, the slide is pushed into its 
normal position. 
If the rod b were invariably connected wfith the cross¬ 
piece d, a certain effort would be required to push back the 
slide, on account of the friction produced by the excess of 
pressure on the upper surface of the door k. To obviate this 
inconvenience, the rod b is made to slide in the cross- 
