number of cylinders, in order to give working impulses at equal angles
of rotation and thus provide even torque; this angle is determined by
dividing the number of degrees in a circle by the number of cylinders
in either row of the engine. In an eight-cylindered Vee type engine, the
angle between the cylinders is 90 degrees; if it is a twelve-cylindered
engine, the angle drops to 60 degrees.

One of the earliest of the British-built Vee type engines was an
eight-cylinder 50 horse-power by the Wolseley Company, constructed in
1908 with a cylinder bore of 3.75 inches and stroke of 5 inches, running
at a normal speed of 1,350 revolutions per minute. With this engine, a
gearing was introduced to enable the propeller to run at a lower speed
than that of the engine, the slight loss of efficiency caused by the
friction of the gearing being compensated by the slower speed of the
air-screw, which had higher efficiency than would have been the case if
it had been run at the engine speed. The ratio of the gearing--that is,
the speed of the air-screw relatively to that of the engine, could be
chosen so as to suit exactly the requirements of the air-screw, and the
gearing itself, on this engine, was accomplished on the half-speed shaft
actuating the valves.

Very soon after this first design had been tried out, a second Vee type
engine was produced which, at 1,200 revolutions per minute, developed 60
horse-power; the size of this engine was practically identical with that
of its forerunner, the only exception being an increase of half an inch
in the cylinder stroke--a very long stroke of piston in relation to
the bore of the cylinder. In the first of these two engines, which was
designed for airship propulsion, the weight had been about 8 lbs. per
brake horse-power, no special attempt appearing to have been made to
fine down for extreme lightness; in this 60 horse-power design, the
weight was reduced to 6.1 lbs. per horse-power, counting the latter
as normally rated; the engine actually gave a maximum of 75 brake
horse-power, reducing the ratio of weight to power very considerably
below the figure given.

The accompanying diagram illustrates a later Wolseley model, end
elevation, the eight-cylindered 120 horse-power Vee type aero engine
of the early war period. With this engine, each crank pin has two
connecting rods bearing on it, these being placed side by side and
connected to the pistons of opposite cylinders and the two cylin