show, however, that
prophecy in any direction is most unsafe.

V. THE HORIZONTALLY-OPPOSED ENGINE

Among the first internal combustion engines to be taken into use with
aircraft were those of the horizontally-opposed four-stroke cycle type,
and, in every case in which these engines were used, their excellent
balance and extremely even torque rendered them ideal-until the
tremendous increase in power requirements rendered the type too long and
bulky for placing in the fuselage of an aeroplane. As power increased,
there came a tendency toward placing cylinders radially round a central
crankshaft, and, as in the case of the early Anzani, it may be said that
the radial engine grew out of the horizontal opposed piston type. There
were, in 1910--that is, in the early days of small power units,
ten different sizes of the horizontally opposed engine listed for
manufacture, but increase in power requirements practically ruled out
the type for air work.

The Darracq firm were the leading makers of these engines in 1910; their
smallest size was a 24 horsepower engine, with two cylinders each of 5.1
inches bore by 4.7 inches stroke. This engine developed its rated power
at 1,500 revolutions per minute, and worked out at a weight of 5 lbs.
per horse-power. With these engines the cranks are so placed that two
regular impulses are given to the crankshaft for each cycle of working,
an arrangement which permits of very even balancing of the inertia
forces of the engine. The Darracq firm also made a four-cylindered
horizontal opposed piston engine, in which two revolutions were given to
the crankshaft per revolution, at equal angular intervals.

The Dutheil-Chambers was another engine of this type, and had
the distinction of being the second largest constructed. At 1,000
revolutions per minute it developed 97 horse-power; its four cylinders
were each of 4.93 inches bore by 11.8 inches stroke--an abnormally long
stroke in comparison with the bore. The weight--which owing to the build
of the engine and its length of stroke was bound to be rather high,
actually amounted to 8.2 lbs. per horse-power. Water cooling was
adopted, and the engine was, like the Darracq four-cylinder type,
so arranged as to give two impulses per revolution at equal angular
intervals of crankshaft rotation.

One of the first engines of this type to be constructed in England was
the Alvaston, a water-cooled model which was made in 20, 30, and 50
brake hor