PNEUMATIC TELEGRAPHY. 
65 
When the carrier comes into this tube it compresses the air 
contained in it, gradually losing thereby its own velocity ; but 
the compressed air, being allowed to escape by the opening at 
the end of R, never acquires sufficient tension to drive the 
carrier back again. When the carrier reaches R the passage of 
the air-current through R is of course intercepted. But a side- 
pipe d allows the current to pass, and when the apparatus is 
brought into the receiving position, a valve v is made to open 
the tube d by the action of the moveable frame on the lever l. 
Similarly the moving of t into the circuit closes the valve v. 
Two iron rods q limit the turning movement of the frame, 
and firmly connect the two fixed parts of the apparatus. 
The touching surfaces of the fixed and moveable parts are 
fiat, and they must be perfectly fitted to each other, so that no 
air shall pass between them. To render the movement easier 
a pedal may be added to the apparatus. The reception box R 
is for a portion of its length provided with a flat glass-cover 
which can be removed at pleasure in order to take out the 
carriers. The person in charge thus at once sees when a 
carrier arrives. He thereupon pulls the moveable frame 
towards him, and by pushing the handle/he causes a rod 
sliding in the guide-piece G to enter the tube R and expel the 
carrier. 
When a carrier has to pass an office without being stopped, 
the clerk, having been warned by electric signal, places the 
frame in the same position as for transmission ; and the 
carrier in passing touches a spring within the tube by which a 
bell is struck, so giving a signal that the carrier has passed. 
