An internal combustion engine divides the heat energy obtained from its fuel into three fairly equal parts. About one-third of the energy becomes useful work; rather more than one-third escapes into the atmosphere in the exhaust gases : and rather less than one-third gives its heat to the cylinders and pistons. As its name implies, an air-cooled engine is one in which the cylinders are cooled directly by air. While the engin.e can and does part with a small amount of its heat by radiation, the bulk of the heat is carried away by air moving more or less rapidly over the surface of the cylinders and cylinder head, which are provided with fins in order to increase the surface in contact with the air. As the loss of heat is nearly proportional to the velocity of the cooling air the faster the aeroplane goes the better will a given engine be cooled; consequently, on a high-speed aeroplane it would be permissible to reduce the area of the cylinders in contact with the air; that is to say, we might save weight by reducing the size and number of fins or, alternatively, the velocity of air over the cylinders need not be equal to the speed of the aeroplane.