462 
POPULAR SCIENCE REVIEW. 
the valves of the feed-pipes no air can fly off at the periphery 
of the fan-wheel, because no air can get in to fill up the 
vacuum which would arise, and the natural pressure of the 
atmosphere has thus the power to keep the air within the 
interspaces of the fans. Consequently, if with closed feed- 
valves the fan-wheel be set in motion, no matter how rapidly 
it may be twirled, the enthralled particles of air will acquire 
exactly the same amount of motion as the fan-wheel itself, 
and both will revolve together as if the whole were one 
solid circular plate of rigid iron. When the rotation of the 
fan-wheel is made to take place within the inclosing case of 
the fan-wheel thus the result is identical with working in a 
vacuum. A very little power of the steam-engine will start 
the wheel and put it in slow action. As the wheel acquires 
motion the steam is released of resistance in its work, and the 
engine will acquire greater and greater speed, until the wheel 
be driven round with such velocity as to equal the rate of 
generation of steam — the only practical limit to this accumu- 
lation of force. The utmost speed of rotation is thus gradually 
acquired, power is accumulated in the wheel — -just as it is in 
the child's humming-top — and if the feed-pipes be now opened 
the air will instantly rush off from the periphery of the fan- 
wheel, at the rate of forty or more miles an hour. Indeed, so 
great is the accumulated power in the wheel that its rotation 
alone, unassisted by the steam-engine, would be sufficient to 
blow or suck the usual train of carriages through the whole 
length of the despatch tube — something more than a quarter 
of a mile. 
There is great economy in thus accumulating power, and it 
is consequently adopted in practice. The engine is started 
with a scanty supply of steam shortly before a train is sent, 
and the wheel put in motion and driven in this practical 
vacuum until extreme rapidity of whirl is acquired, when the 
train is despatched; but, as the surest means of avoiding 
dilemmas, the engine is continued to be worked at full speed 
during the transit. The carriages do not, however, shoot out 
of the despatch-tube like darts from a blow-pipe, or peas 
from the blower. They do not crash up against the end of 
the station, or knock over porters waiting to receive them. No 
such thing. A little before their arrival, the sound of their 
rumbling alters, the tube draws in a long breath, or blows a 
deep sigh, there is a click and a clatter of rods, the flat- valve 
at the orifice of the despatch opens of its own accord, and the 
pellet- carriage comes forth from the dark interior of its under- 
ground passage, runs itself clear a few feet on the rails, and 
stops without either break or touching. A toddling baby 
might pull it up as it comes forth to the day. How this is 
done is very simple — very simple, indeed ; but marvellously 
