Water when converted into vapor or steam absorbs a great quantity of heat, and of course the steam carries the heat of evaporization with it when it escapes to the atmosphere. As the hopper is open to the air, the temperature of the cylinder cannot exceed 212° F. (temperature of boiling water) as long as there is sufficient water left to cover the cylinder. The hoppers contain sufficient water for runs of several hours’ duration, and as the water boils away or evaporates, it may be replenished by simply pouring more water in the top of the hopper. Hopper cooling is used principally for small portable engines where the weight of a water tank or other cooling device would be objectionable and also where there is danger of freezing the pipes and connections of other systems. The loss of water by evaporization is from .3 to .6 of a gallon per horsepower hour; that is, for a 5 hp. engine the loss would be from 1.5 to 3 gals. for every hour that the engine was operated under full load. The cylinder and the water jacket are cast in one integral piece, with no joints of any kind in either the combustion chamber or in the water jacket. [Illustration: Fig. 124. Air Cooled “Grey Eagle” Aeronautical Motor. Note the Depth of Cooling Ribs. ] A system of cooling by which the heat of the walls is radiated to the air directly without the medium of water is often used on small high speed engines, and is known as “=AIR COOLING=.” This type of cylinder is surrounded with radiating ribs or spires which increases the radiating surface of the cylinder to the extent that the required amount of heat is lost to allow of economical lubrication. This system is desirable where the weight of radiators and water would be a drawback, where it would be inconvenient to obtain water, or where there would be trouble from freezing. An air cooled motor generally is provided with a fan that increases the efficiency of the radiating surface by changing the air between the ribs. With aeronautical motors such as the Gnome, and Gray Eagle, shown by Fig. 124, the circulation of the air due to the propeller and the rush of the aeroplane is sufficient to thoroughly cool the machine. As a rule, the air cooled motor is made more efficient in fuel consumption than the water cooled type because of the high temperature of the cylinder walls. In fact all engines are air cooled eventually, whether the heat is radiated at a high temperature by the fires, or at a lower temperature through the circulating water and radiator.