The temperature of the combustion in a gasoline engine may be over 3,000 degrees Fahrenheit. Evidently it would melt the cylinder walls were not special provisions taken to keep them cool. The usual method is to surround the cylinders with water which absorbs the heat, and then cool the water by passing it through a radiator. Air driven by a fan through the radiator carries off the heat. Of course this represents just so much wasted energy and lowers the efficiency of the motor, but it is the most convenient way of getting rid of the intense heat of combustion. Smaller motors, such as are used on motorcycles, have their cylinders cooled by direct action of air on the outer surface of the cylinder. The heat radiated depends upon the surface exposed and the velocity of the air current. The radiating surface of the cylinders is increased by forming them with external ribs or flanges, so that the air that flows over them, while the machine is in motion, carries off enough heat to keep the temperature of the cylinders within safe limits. In some systems, particularly in the case of stationary engines, to insure a good circulation, the air is positively driven against the flanges by means of a fan. There are serious disadvantages in using water to cool an engine. Besides the bother of keeping the water system supplied with water there is the danger of clogging the radiator with lime deposits, and in extremely cold weather the water is liable to freeze and burst the water jackets or the radiator; for this reason, air cooling is recommended by some for automobile engines. In certain air-cooled automobile engines the cylinders are air-jacketed; that is, they are surrounded with casings through which the air is sucked by a powerful fan driven by the engine shaft, so that the air is bound to flow if the engine is turning. The cylinders are ribbed, so that they present a large radiating surface. [Illustration: Copyright, Kadel & Herbert GIANT CAPRONI HYDROAEROPLANE WHICH WAS WRECKED AT LAGO MAGGIORE Seating capacity 100 persons; power, eight 450-h.p. Liberty motors] [Illustration: A BOAT OF STONE--THE CONCRETE STEAMSHIP “FAITH”] [Illustration: THE “VIRGINIAN,” THE LARGEST LOCOMOTIVE IN THE WORLD Length over all, 97 ft.; weight of engine, 684,000 lbs.; of tender, 214,300 lbs.; tractive power, simple, 176,600 lbs.; compound, 147,200] Air-cooled engines have hotter cylinders than water-cooled engines and hence in cylinders of the same size less fuel is drawn into the air-cooled cylinder. But the fuel is used more efficiently because less of its heat energy is wasted; so that air-cooled engines show a slight fuel economy over water-cooled engines. On the other hand, when the air-cooled engine is overloaded or is run at very high speeds it is liable to become overheated and may ignite the incoming charge of gas prematurely. THE SELF IGNITING DIESEL ENGINE