§ 94. _Application to the Case of Pneumatic Tubes for the Transmission
of Messages._--In Paris, Berlin, London, and other towns, it has been
found cheaper to transmit messages in pneumatic tubes than to
telegraph by electricity. The tubes are laid underground with easy
curves; the messages are made into a roll and placed in a light felt
carrier, the resistance of which in the tubes in London is only ¾ oz.
A current of air forced into the tube or drawn through it propels the
carrier. In most systems the current of air is steady and continuous,
and the carriers are introduced or removed without materially altering
the flow of air.
_Time of Transit through the Tube._--Putting t for the time of transit
from 0 to l,
_
/l
t = | dl/u,
_/0
From (4a) neglecting dH/H, and putting m = d/4,
dl = g d[Omega]²p dp/2[zeta]W²cr.