continuously, since one of the four pistons is always at work.

While this takes long to describe, the motion is faster than the eye can
follow, and the "phut, phut" noise of the exhaust sounds like the tattoo
of a drum. Almost every gasoline motor vehicle carries its own electric
plant, either a set of batteries or more commonly a little magneto
dynamo, which is run by the shaft of the motor. Electricity is used to
make the spark that explodes the gas at just the right moment in the
cylinders. All this is automatic, though sometimes the driver has to
resort to the persuasive qualities of a monkey-wrench and an oil-can.

The exploding gas creates great heat, and unless something is done to
cool the cylinders they get so hot that the gas is ignited by the heat
of the metal. Some motors are cooled by a stream of water which, flowing
round the cylinders and through coils of pipe, is blown upon by the
breeze made by the movement of the vehicle. Others are kept cool by a
revolving fan geared to the driving-shaft, which blows on the cylinders;
while still others--small motors used on motor bicycles, generally--have
wide ridges or projections on the outside of the cylinders to catch the
wind as the machine rushes along.

The inventors of the gasoline motor vehicles had many difficulties to
overcome that did not trouble those who had to deal with steam. For
instance, the gasoline motor cannot be started as easily as a
steam-engine. It is necessary to make the driving-shaft revolve a few
times by hand in order to start the cylinders working in their proper
order. Therefore, the motor of a gasoline machine goes all the time,
even when the vehicle is at rest. Friction clutches are used by which
the driving-shaft and the axles can be connected or disconnected at the
will of the driver, so that the vehicle can stand while the motor is