466 
POPULAR SCIENCE REVIEW* 
first practically manipulated tlie machinery entrusted to his 
charge. For more than two years the tube at his station has 
been at work, and more than a hundred mail-trains are daily 
blown or sucked through it. 
If the form of the fan-wheel as seen in section may appear 
singular to observers, there is again a very simple reason for 
it — a reason apparent on the face of the plan. The inter- 
spaces of the fans being triangular, the air coming in at the 
centre would have room to spread as it passed outwards, and 
therefore would pass out in a broad, gentle current. What 
is wanted is high velocity of the wind thrown off, and equal 
distribution of it along the whole margin of the wheel. This 
is got by gradually diminishing the width of the wheel from 
centre to periphery, so that the broad central mass of air 
becomes more and more narrowed as it attains velocity and 
occupies the wider parts of the interspaces. Every particle is 
thus ejected with the highest possible momentum. 
Nor is this mail- line the only example of the practical working 
of the system. It has been even still longer employed at the 
Central Telegraph Office in Moorgate Street, for the trans- 
mission of messages from and to the outer city stations, thus 
avoiding the delivery by messengers, and being more facile 
for such short distances than manipulating by the electric 
wires. Moreover, it possesses this by no means unimportant 
advantage, — the actual message, as written by the sender , is 
itself transmitted. The pipes at the Central Telegraph Office 
are all small — 2f inches in internal diameter — and double, 
being worked entirely by exhaustion. The pellets are cylin- 
ders made of felt or flannel, and about six inches long. The 
object of the double pipe is, that one pipe being always ex- 
hausted, the station at either end of the other pipe can, by 
turning on the connection, despatch a message. When one 
has to be sent, the electric telegraph bell is rung, calling the 
station at which it is to be received. The pellet containing 
the actual written message is then inserted into a slit in the 
despatch-pipe, and the covering-valve of the slit pressed to. 
The connection with the exhaust is then made, and the pellet 
sucked through. As it arrives at its destination, the exhaust 
is stopped, and the air in the terminal extremity of the pipe 
compressed by the motion of the pellet, opens the valve which 
the pressure of the atmosphere had held closed during the 
exhaust, and the pellet-parcel falls out. 
But the latest phase of the utility of the Pneumatic Despatch 
has yet to be witnessed by the public, and will possibly have 
been seen by hundreds before these lines are perused by our 
readers. A large tunnel, 9 feet high by 8 feet wide — capable 
of containing the largest carriages of the Great Western 
