PNEUMATIC TELEGRAPHY. 
71 
between the appearance of these two marks can be estimated, 
it is easy to see that the distance between the membrane and the 
obstacle can be calculated. 
The chronograph used by M. Bon temps has three tracing- 
points, acted upon by electro-magnets. The first is placed in 
the circuit which is formed by the alternate risings of the 
india-rubber. 
The second tracing-point corresponds with an electric 
regulator, marking seconds of time on the cylinder. 
The third tracing-point subdivides the space of the second, by 
means of the vibrations of an electric trembler. 
The following is an example of a practical determination : 
An obstacle is placed on the line, at a distance of 185 feet. 
The trembler executes 83 oscillations per second. 
The space on the strip of paper covering the cylinder between 
two consecutive marks of the india-rubber, corresponds with 11 
oscillations. 
The distance of the obstacle is calculated by the following 
formula :— 
D=| x 1074 x ^ J = 179. 
The approximation is therefore within 6 feet, and the 
obstacle can be reached by a single digging up of the tube. 
In the current practice, this method is used without employ¬ 
ing a registering chronograph and an india-rubber membrane. 
An ordinarv chronometer measures the interval with sufficient 
accuracy. The return of the wave is made perceptible to the 
ear by the sound of a whistle, or to the eye by the sudden 
rising of a little water manometer fixed in the end of the tube. 
Various Accidents. —We shall mention some other in¬ 
stances of derangements that have occurred. 
1. The piston forgotten .—The air then acting directly on the 
carriers, finds its way between the covers and tends to separate 
them. The carriers become separated and reach the extremity 
of the tube one after the other, generally open and with the 
papers in disorder. 
2. A train of carriers forgotten .—Though almost incredible, 
this certainly has happened. The reception signal had been 
given before the arrival of the train, and when another was 
sent it pushed the former one along and took its place. The 
