n its drum. This method is sufficient in the case of most
road vehicles, and is largely used on railway vehicles. But the power
thus available is limited, and becomes inadequate for heavy vehicles
moving at high speeds. Moreover, on a train consisting of a number of
vehicles, the hand brakes on each of which are independent of all
others, either a brakesman must be carried on each, or a number of the
brakes must be left unused, with consequent loss of stopping power;
while even if there is a brakesman on every vehicle it is impossible to
secure that all the brakes throughout the train are applied with the
promptness that is necessary in case of emergency.

Considerations of this sort led to the development of power brakes for
railway trains. Of these there are five main classes:--

Railway power brakes.

(1) Mechanical brakes, worked by springs, friction wheels on the axle,
chains wound on drums, or other mechanical devices, or by the force
produced when, by reason of a sudden checking of the speed of the
locomotive, the momentum of the cars causes pressure on the draw-bars or
buffing devices. (2) Hydraulic brakes, worked by means of water forced
through pipes into proper mechanism for transmitting its force to the
brake-shoes. (3) Electric brakes. (4) Air and vacuum brakes, worked by
compressed air or by air at atmospheric pressure operating on a vacuum.
(5) Brakes worked by steam or water from the boiler of the engine,
operating by means of a cylinder; the use of these is generally limited
to the locomotive. Of this kind is the counter-pressure or water brake
of L. le Chatelier. If the valve gear of a locomotive in motion be
reversed and the steam regulator be left open, the cylinders act as
compressors, pumping air from the exhaust pipe into the boiler against
the steam pressure. A retarding effect is thus exercised, but at the
cost of certain inconveniences due to the passage of hot air and cinders
from the smoke box through the cylinders. To remedy these, le Chatelier
arranged that a jet of hot water from the boiler should be delivered
into the exhaust pipe, so that steam and not the hot flue gases should
be pumped back.

Power brakes may be either continuous or independent--continuous if
connected throughout the train and with the locomotive by pipes, wires,
&c., as the compressed air, vacuum and electric brakes; independent if
not so connected, as the buffer-brakes and hand-brakes. Continuous
brakes may b