62 
THE TELEGRAPH. 
This arrangement would have to be so made that there should 
not be any delay, on account either of the insufficient number 
of carriers, or of the inopportune intrusion into the circuit of 
carriers from any particular office. 
Several similar circuits, each comprising a larger or smaller 
number of intermediate offices, may have their terminations at 
the office provided with the machinery. The power of the 
machines will of course depend, other things being equab upon 
the length of the circuits they supply. 
One of the extremities of each circuit is in permanent com¬ 
munication with a reservoir of compressed air, and the other 
with a reservoir of rarefied air. The speed of transmission by 
this arrangement is evidently greater than that which would 
be obtained if the compressed or the rarefied air were used 
alone. 
It should be observed that I he internal pressure of the air 
varies throughout the tube. When no carrier is passing, this 
pressure, at its maximum near the reservoir of compressed air, 
continues to decrease as the distance increases, diminishes to 
the pressure of the external air at a certain point, and beyond 
that continues to fall lower as the vacuum reservoir is 
approached. The position of the point at which the pressure 
is that of the external air varies with the relative powers of the 
pressure and the vacuum, and is modified by the windings of 
the tube as by an effect of friction. In consequence of this, 
the velocity of the air-current varies at different places. For 
as the same quantity of air in weight must during a given 
