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 horse-power at a speed of 950 revolutions per minute, with the remarkably low weight of only 2·4 lbs. per horse-power; 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. [Illustration: Cross Section, Manly’s 5 Cylinder Radial Engine.] In Manly’s engine the cylinders were of steel, machined outside and inside to 1/16 of an inch thickness; on the side of the 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 before the days of the high-tension magneto and the almost perfect ignition systems that makers now employ. The scheme of ignition for this engine was originated by Manly himself, and he also designed the sparking plugs fitted in the tops of the cylinders. Through fear of trouble resulting if the steel pistons worked on the steel cylinders, cast-iron liners were introduced in the latter, 1/16 of an inch thick.