PNEUMATIC TELEGRAPHY. 
69 
law to the results of this experiment is capable of indicating, 
to thirty yards, the place where the tube must be taken up. 
Constant observation of the manometer enables the employes 
to make a sort of a practical graduation of the instrument, by 
Fig. 31. 
which they can follow the progress of the carriers, and by 
availing themselves of these indications they may infer the 
approximative position of an obstacle causing the sudden 
stoppage of a carrier. 
M. Ch. Bontemps has applied the mode of experiment used 
by Regnault for the determination of the velocity of sound in 
