part of the internal-combustion
engine, as used on the motor-car, is the radiator, which prevents the
engine from becoming overheated and thus ceasing to work. The higher
the speed at which the engine runs the hotter does it become, and the
greater the necessity for an efficient cooling apparatus.

But the motor on an aeroplane has to do much harder work than the motor
used for driving the motor-car, while it maintains a much higher speed.
Thus there is an even greater tendency for it to become overheated; and
the great problem which inventors of aeroplane engines have had to face
is the construction of a light but powerful engine equipped with some
apparatus for keeping it cool.

Many different forms of aeroplane engines have been invented during the
last few years. Some inventors preferred the radiator system of cooling
the engine, but the tank containing the water, and the radiator itself,
added considerably to the weight of the motor, and this, of course, was
a serious drawback to its employment.

But in 1909 there appeared a most ingeniously-constructed engine which
was destined to take a very prominent part in the progress of aviation.
This was the famous "Gnome" engine, by means of which races almost
innumerable have been won, and amazing records established.

We have already referred to the engine shaft of the motor-car, which is
revolved by the pistons of the various fixed cylinders. In all aeroplane
engines which had appeared before the Gnome the same principle of
construction had been adopted; that is to say, the cylinders were fixed,
and the engine shaft revolved.

But in the Gnome engine the reverse order of things takes place; the
shaft is fixed, and the cylinders fly round it at a tremendous speed.
Thus the rapid whirl in the air keeps the engine cool, and cumbersome
tanks and unwieldy radiators can be dispensed with. This arrangement
enabled the engine to be made very light and yet be of greater
horse-power than that attained by previously-existing engines.

A further very important characteristic of the rotary-cylinder engine
is that no flywheel is used; in a stationary engine it has been
found necessary to have a fly-wheel in addition to the propeller. The
rotary-cylinder engine acts as its own fly-wheel, thus again saving
considerable weight.

The new engine astonished experts when they first examined it, and all
sorts of disasters to it were predicted. It was of such revolutionary
design that