itself and is closed by a spring. By an ingenious contrivance
the valve is opened when the piston moves out of the cylinder,
and a vacuum is created behind it and in the carburettor. This
carries a fine spray of petrol to be sucked up through the
nozzle. Air is also sucked into the carburettor, and the mixture
of air and petrol spray produces an inflammable vapour which is
drawn straight into the cylinder of the engine.

As soon as the piston moves back, the inlet valve is
automatically closed and the vapour is compressed into the top of
the cylinder. This is exploded by an electric spatk, which is
passed between two points inside the cylinder, and the force of
the explosion drives the piston outwards again. On its return
the "exhaust" or burnt gases are driven out through another
valve, known as the "exhaust" valve.

Whether the engine has two, four, or six cylinders, the car is
propelled in a similar way for all the pistons assist in turning
one shaft, called the engine shaft, which runs along the centre
of the car to the back axle.

The rapid explosions in the cylinder produce great heat, and the
cylinders are kept cool by circulating water round them. When
the water has become very hot it passes through a number of
pipes, called the "radiator", placed in front of the car; the
cold air rushing between the coils cools the water, so that it
can be used over and over again.

No water is needed for the engine of a motor cycle. You will
notice that the cylinders are enclosed by wide rings of metal,