POPULAR SCIENCE .REVIEW. 
464 
through the out-draught pipe Q Q', and the grating G behind 
the carriage D", which is thus blown rapidly on, driving out 
the air in the despatch-tube before it through the distant 
grating H. We shall now see the necessity for closing the 
tube at both ends. As soon as the carriage (D") has passed 
the distant grating at H, the wind behind it ceases to propel 
it, for it at once escapes by the grating H. So, as the car- 
riage has no means of maintaining its motion beyond the 
impetus it has received, its speed will slacken. But this is not 
all. There still remains in front of it, in the closed extremity 
of the tube (H V') a cushion or buffer of air. This the close- 
fitting carriage compresses more and more, meeting with more 
and more resistance the more the cushion of air is compressed, 
until when its motion is nearly arrested, it passes over the 
trigger n } when the far- terminal valve V' opens, and the carriage 
trundles quietly out and brings up on the short railway out- 
side. Directly the train has passed the grating at H, the 
engineer shuts off steam from the stationary engine (fig. 2) . How 
he knows it has arrived there is very simple too. With the 
despatch-tube is connected a pressure-gauge in the engine- 
room, and while the train is in transit, the atmospheric 
pressure is increased, and the gauge shows four or five ounces 
to the square foot — that is all. It is the rate of exhaustion 
of the despatch-tube, or the rate of the propulsion of air into 
it, that gives the velocity to the trains, not the pressure per 
square foot. The pressure per square foot gives momentum 
to the inert weight of the train, not speed of transit. The 
moment, then, the carriage has passed the grating H, the 
atmospheric pressure in the despatch-tube is released by the 
free vent given to the air, and the mercury or water in the 
pressure-gauge falls to zero. The engineer instantly knows 
that his duties are over. 
Having now blown our carriage through, we want to get 
it back, or, otherwise, the other station wants to send a train. 
They have no engine and fan-wheel there, so we must suck 
it through for them. They telegraph to us that they have 
a train to send, and our engineer turns on steam. The fan- 
wheel revolves, we close the out-draught valve at Q, and the 
in-draught valve at It, at the same time opening the valves 
S and T. The exhaustion of the despatch- tube now com- 
mences, and the free atmosphere rushes in through the open 
end of the tube at V', and through the grating H, and 
pressing on the superficial area of the end of the pellet- 
carriage D'", forces it along through the tube as fast as the 
exhaustion proceeds, until the carriage has run over the 
grating G, when the exhaustion of course ceases, and the 
engine, if kept at work, merely sucks air through the then 
