s. An exceptional type of American design was the Kemp
Vee engine of 80 horse-power in which the cylinders were cooled by a
current of air obtained from a fan at the forward end of the engine.
With cylinders of 4.25 inches bore and 4.75 inches stroke, the rater
power was developed at 1,150 revolutions per minute, and with the engine
complete the weight was only 4.75 lbs. per horse-power.

III. THE RADIAL TYPE

The very first successful design of internal combustion aero engine made
was that of Charles Manly, who built a five-cylinder radial engine in
1901 for use with Langley's 'aerodrome,' as the latter inventor decided
to call what has since become known as the aeroplane. Manly made a
number of experiments, and finally decided on radial design, in which
the cylinders are so rayed round a central crank-pin that the pistons
act successively upon it; by this arrangement a very short and compact
engine is obtained, with a minimum of weight, and a regular crankshaft
rotation and perfect balance of inertia forces.

When Manly designed his radial engine, high speed internal combustion
engines were in their infancy, and the difficulties in construction
can be partly realised when the lack of manufacturing methods for this
high-class engine work, and the lack of experimental data on the various
materials, are taken into account. During its tests, Manly's engine
developed 52.4 brake horsepower at a speed of 950 revolutions per
minute, with the remarkably low weight of only 2.4 lbs. per horsepower;
this latter was increased to 3.6 lbs. when the engine was completed
by the addition of ignition system, radiator, petrol tank, and all
accessories, together with the cooling water for the cylinders.

In Manly's engine, the cylinders were of steel, machined outside and
inside to 1/16 of an inch thickness; on the side of cylinder, at the top
end, the valve chamber was brazed, being machined from a solid forging,
The casing which formed the water-jacket was of sheet steel, 1/50 of an
inch in thickness, and this also was brazed on the cylinder and to
the valve chamber. Automatic inlet valves were fitted, and the exhaust
valves were operated by a cam which had two points, 180 degrees
apart; the cam was rotated in the opposite direction to the engine at
one-quarter engine speed. Ignition was obtained by using a one-spark
coil and vibrator for all cylinders, with a distributor to select
the right cylinder for each spark--this was be