s led to the construction of engines having as many as eighteen
cylinders, arranged in three parallel rows in order to keep the length
of crankshaft within reasonable limits. The aero engine of to-day may,
roughly, be divided into four classes: these are the V type, in which
two rows of cylinders are set parallel at a certain angle to each other;
the radial type, which consists of cylinders arranged radially and
remaining stationary while the crankshaft revolves; the rotary, where
the cylinders are disposed round a common centre and revolve round
a stationary shaft, and the vertical type, of four or six
cylinders--seldom more than this--arranged in one row. A modification of
the V type is the eighteen-cylindered engine--the Sunbeam is one of the
best examples--in which three rows of cylinders are set parallel to each
other, working on a common crankshaft. The development these four types
started with that of the vertical--the simplest of all; the V, radial,
and rotary types came after the vertical, in the order given.

The evolution of the motor-car led to the adoption of the vertical
type of internal combustion engine in preference to any other, and
it followed naturally that vertical engines should be first used for
aeroplane propulsion, as by taking an engine that had been developed to
some extent, and adapting it to its new work, the problem of mechanical
flight was rendered easier than if a totally new type had had to be
evolved. It was quickly realised--by the Wrights, in fact-that the
minimum of weight per horse-power was the prime requirement for the
successful development of heavier-than-air machines, and at the same
time it was equally apparent that the utmost reliability had to be
obtained from the engine, while a third requisite was economy, in order
to reduce the weight of petrol necessary for flight.

Daimler, working steadily toward the improvement of the internal
combustion engine, had made considerable progress by the end of
last century. His two-cylinder engine of 1897 was approaching to
the present-day type, except as regards the method of ignition; the
cylinders had 3.55 inch diameter, with a 4.75 inch piston stroke,
and the engine was rated at 4.5 brake horse-power, though it probably
developed more than this in actual running at its rated speed of 800
revolutions per minute. Power was limited by the inlet and exhaust
passages, which, compared with present-day practice, were very small.
The heavy c