THE PNEUMATIC DESPATCH. 
461 
wheel inside is made to rotate rapidly, and in its revolutions 
throws off by centrifugal force a continuous stream of air 
which issues from the nozzle. If a pea or a pellet were 
inserted in this nozzle it would be blown out, just as the pea is 
ejected from the pea-blower, or the dart from the blow-pipe. 
If the nozzle were extended for a mile, the same ejectment of 
any pellet within it would equally take place. Suppose it so 
extended and you have the rudiment of the Pneumatic Despatch. 
We shall make this clearer by describing briefly the apparatus 
which for tsvo years has been in work at Euston Square. 
At the side of the railway station we enter a very unattractive 
engine-house. Within, on one side, is the end of an iron tube 
two feet six inches in diameter, closed by a flat iron valve. 
Beyond this is a short line of rails — some fifteen or twenty 
feet in length — upon which are some low iron carriages, like 
round mummy-cases with the middle part of their lids cut out, 
and running on four wheels closely indented into their sides. 
The bottoms of these carriages are flat, almost on a level with 
the lower periphery of the wheels, and close to touching the 
ground ; the section of the carriages being indeed coincident 
in form with, and only about three quarters of an inch less 
in dimensions than the section of the tube itself. A padding of 
leather or india-rubber round each carriage nearly fills up the 
interval between it and the internal 1 sides of the tube. On 
the opposite side of the house is the steam-engine, and 
in the centre is a great iron box or semicircular case, within 
which a large fan-wheel twenty-two feet in diameter rapidly 
revolves, like the fan-wheel of the fire-bellows. On either side 
of this case there is a tube opening into the interspaces of the 
fans round the axle of the fan-wheel. As the wheel revolves, 
the particles of air in these interspaces between the fans have 
the wheel’s motion communicated to them, and then like water 
drops from a twirled mop have a stronger and stronger 
tendency, in proportion to their velocity, to fly off. As they 
stream away at the periphery of the rapidly whirling wheel, 
the air is sucked in as fast at its centre, from the air-feeding 
pipes at the side, to supply the vacuum created by their loss. 
Thus a violent stream of air is thrown off from the outer 
margin of the wheel, while a suction is established in the feed- 
pipes. We have thus a supply of wind at increased atmo- 
spheric pressure to blow, or propel from behind, the carriages 
through the pneumatic tube from its near end, and oppositely 
an exhaust through the feed-pipes to suck them through from 
the far end, as one would suck up water through a straw. It 
is a very small engine that does duty. And there is a curious 
fact connected with the work it does. It can do what steam- 
engines very seldom can — take its work easily. If we close 
vol. hi. — NO. XII. 2 i 
