PNEUMATIC TELEGRAPHY. 
63 
time pass by every section of the tube, the volumes, and 
consequently the velocities, are in inverse proportion to the 
densities, in accordance with Mariotte’s law. 
We have said that the same reservoir and the same 
machines are used for tubes on Siemens’ system and for those 
on Clark’s system. As an account of these has been given 
above, we have only to add, that Messrs. Siemens’ aspirator 
having been finally adopted for a part of the system, a set of 
the apparatus has been fitted up at each of the extreme offices, 
and the circuit has been divided into separate and parallel 
lines, traversed by air-currents in opposite directions, produced 
Fig. 26 . 
by exhaustion only; the use of compressed air having been 
entirely dispensed with. 
The pipes forming the conveying tube are made of wrought- 
iron. They are 3 inches in internal diameter, \ inch thick, 
and 18 feet 6 inches long. They are bored out internally. 
The junctions are made by means of cast-iron sockets with 
their internal cylindrical portion bored so that the ends of the 
two tubes may exactly fit one against the other. The connec¬ 
tion is completed by a packing of hemp and lead. The 
direction is changed by means of curved tubes having a 
minimum radius of 12 feet. 
The pipes are buried in the ground to a depth of about 1 
foot. These are laid inclining towards wells placed at intervals, 
