Necessity for increasing power and ever lighter weight in relation to the power produced has led to the evolution of a number of different designs of internal combustion engines. It was quickly realised that increasing the number of cylinders on an engine was a better way of getting more power than that of increasing the cylinder diameter, as the greater number of cylinders gives better torque--even turning effect--as well as keeping down the weight--this latter because the bigger cylinders must be more stoutly constructed than the small sizes; this fact has led to the construction of engines having as many as eighteen cylinders, arranged in three parallel rows in order to keep the length of crank-shaft 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 of 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. [Illustration: Two Cylinder Daimler Engine, 1897.]