PNE UMA TIC TELEGRAPH Y. 
61 
communicating with the underground tube, draws out the air 
and tends to produce a vacuum. The useful effect depends 
upon the section of the opening from which the steam issues, 
on the length and section of the tube in which the air and 
steam are mixed, and upon the pressure of the latter. Messrs. 
Siemens have found that by this aspirator a vacuum corre¬ 
sponding with 23 inches of mercury can be produced with less 
expenditure of steam than with a piston machine. According 
to them the first cost and the working expenses are only one- 
twentieth of those of an ordinary steam-engine. Besides this 
advantage, the apparatus is remarkable for its simplicity, the 
small space it occupies, and the facility with which it can be 
worked and kept in order. 
We shall now describe Messrs. Siemens’ system of tubes :— 
As we have already mentioned, there are, between the 
terminal offices to be connected, two tubes forming a complete 
circuit (Fig. 24). 
Intermediate offices may be interposed along the circuit. A 
current of air is constantly circulating through the tubes in 
the direction shown by the arrows. To send a carrier from 
any point of the line, it is only necessary to place it in the 
tube. This is done by valves of a particular kind (which we 
shall presently describe), and the office for which the carrier is 
intended having been w r arned by au electric signal, stops the 
carrier in its passage by means of these same valves. 
If none of the intermediate offices intercepted the passage of 
a carrier sent from the Central Office, the carrier would return 
again to that office after having traversed the whole line. 
Instead of specially warning each office when a carrier is 
sent to it, the precise time at which each of them should have 
its valve in the receiving position, might be fixed beforehand. 
