h (fourth) is compressed by the
following-up movement and is ready for the next explosion. This is
called a two-cycle motor, because two complete revolutions are necessary
to accomplish all the operations. Many machines are fitted with heavy
fly-wheels, the swift revolution of which carries the impetus of the
power stroke through the other three operations.

[Illustration: A GIANT AUTOMOBILE MOWER-THRASHER
This machine cuts a swath 35 feet wide and thrashes and sacks the grain
as it moves along. Seventy to 100 acres of grain a day are harvested by
this machine, and 1,000 to 1,500 sacks are produced each working day.]

To keep a practically continuous forward movement on the driving-shaft,
many motors are made with four cylinders, the piston of each being
connected with the crank-shaft at a different angle, and each cylinder
doing a different part of the work; for example, while No. 1 cylinder is
doing the work from the force of the explosion, No. 2 is compressing,
No. 3 is getting a fresh supply of gas, and No. 4 is cleaning out waste
gas. A four-cylinder motor is practically putting forth power
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 ha