58 
THE TELEGRAPH. 
piece d within the limits given by the stops / and l. The 
stop / acts in the w T ay we have already explained ; the stop l 
comes into use when the slide is pushed in, the rods g, and the 
cross-piece d first slide alone, while the rod b remains 
stationary ; during this movement, the inclined plane h leaves 
the roller j, and the compressed air ceases to enter the tube t ; 
then the cross-piece d meets the stop l , and the rod b carries 
the slide-door easily along v r ith it. 
Most of the pieces forming the valves are of brass, and are 
fixed against two stout planks (Figs. 19, 20 and 21), one of 
which is vertical, and the other horizontal. The latter forms 
a table on which to place the carriers about to be sent, and to 
receive those that arrive. 
In all the terminal offices the arrangement is the same, 
whatever may be the system of valves employed. The end 
of the subterranean pipe opens into a cubical wooden box 
measuring 14 inches each way. The front of this is provided 
with a door having a glass pane, and turning on two hinges 
fixed on the upper horizontal edge, the door opening towards 
the inside. 
This box rests on another of larger dimensions lined in the 
inside with lead. A leaden pipe, communicating with the 
town drains, enters the lower part of this box. A cast-iron 
grating placed in the panel separating the two boxes allows 
the air to pass from one to the other, and prevents the carriers 
from falling into the lower receptacle. 
When a carrier has to be sent, it is placed in one end of the 
tube, and the central office is signalled to exhaust. 
When a carrier is received, the air it drives before it closes 
the door of the box and escapes through the grating without 
inconvenience to the operator. 
The communication with the sewers is necessary to carry 
away the water with which the tubes may have to be flushed, 
in order to drive out carriers that have accidentally stuck. 
These cases are extremely rare \ but special arrangements have 
nevertheless to be made to get rid of such obstacles. For this 
purpose, a pipe brings the towm water into the room containing 
the apparatus at the central office. When the tube is blocked 
with a carrier, it is filled with water, and the compressed air is then 
made to act upon the water at the highest obtainable pressure. 
