this energy, expending it in overcoming some external
resistance. If the energy be great in proportion to the usual resistance
tending to stop the body, the motion will continue for a long time, or
through a long distance, before the energy has been completely expended
and the body brought to rest. But in certain cases considerations of
safety or convenience require that this time or distance be greatly
shortened, and this is done by artificially increasing the external
resistance for the time being, by means of a brake.

A simple method of obtaining this increased resistance is by pressing a
block or shoe of metal or wood against the rim of a moving wheel, or by
tightening a flexible strap or band on a rotating pulley or drum. In
wheeled road vehicles, a wheel may be prevented from rotating by a chain
passed through its spokes and attached to the body of the vehicle, when
the resistance is increased by the substitution of a rubbing for a
rolling action; or the same effect may be produced by fixing a slipper
or skid under the wheel. Other forms of brake depend, not on the
friction between two solid bodies, but on the frictional resistance of a
fluid, as in "fan" and "pump" brakes. Thus the motion of revolving
blades may be opposed by the resistance of the air or of a liquid in
which they are made to work, or the motion of a plunger fitting tightly
in a cylinder filled with a fluid may be checked by the fluid being
prevented from escape except through a narrow orifice. The fly used to
regulate the speed of the striking train in a clock is an example of a
fan brake, while a pump brake is utilized for controlling the recoil of
guns and in the hydraulic buffers sometimes fitted at terminal railway
stations to stop trains that enter at excessive speed. On electric
tramcars a braking effect is sometimes obtained by arranging the
connexions of the motors so that they act as generators driven by the
moving car. In this way a counter-torque is exerted on the axles. The
current produced is expended by some means, as by being made to operate
some frictional braking device, or to magnetize iron shoes carried on
the car just over, but clear of, the running rails, to which they are
then magnetically attracted (see TRACTION).

The simplest way of applying a brake is by muscular force, exerted
through a hand or foot lever or through a screw, by which the brake
block is pressed against the rim of the wheel or the band brake
tightened o