Another factor governing the output of an engine with a given size cylinder is the amount of air required to burn the fuel. The quantity of air necessary for the combustion of the fuel determines the amount of fuel that can be drawn into a given cylinder volume, and as we are dependent upon the fuel for the expansion it is evident that with two fuels of the same calorific value, the one requiring the least air will develop the most power. Since the air required to burn hydrogen gas is only one fourth of that required to burn the same amount of methane it is clear that more hydrogen can be burned in the cylinder than methane. This great increase in output due to the hydrogen charge is however, considerably offset by the greater calorific value of the methane. Should the air be in excess of that required for complete combustion, or should a great quantity of incombustible gas, such as nitrogen be present in the mixture, the fuel will be completely burned, but the speed of burning will be reduced owing to the dilution. As the air is increased beyond the proper proportions the explosions become weaker and weaker as the gas becomes leaner until the engine stops entirely. Because of the fact that it is impossible in practice to so thoroughly mix the gas and air that each particle of gas is in contact with a particle of air, the volume of air used for the combustion is much greater than that theoretically required. A =SLIGHT= excess of air, making a lean mixture, increases the efficiency of combustion although it reduces the temperature and pressure attained in the cylinder. This is due to the fact that while the temperature of the mixture is lower than with the theoretical mixture the temperature of the burning gas itself is much higher. A mixture that is too lean to burn at ordinary temperatures will respond readily to the ignition spark if the temperature or pressure is raised. (4) Compression. In the practical gas engine the gas is not ignited at the beginning of the suction stroke by which it is drawn into the cylinder, but is compressed in the front end of the cylinder by the return stroke of the piston, and then ignited. The process of compression adds greatly to the power output of a given sized cylinder and increases the efficiency of the fuel and expansion. In order to understand the relation that the compression bears to the expansion let us refer to Fig. 2 in which C is the working cylinder, P the piston and G the crank. While the piston is moving towards the crank in the direction of the arrow A it draws the mixture, indicated by the marks x x x x x, into the cylinder, the quantity being proportional to the position of the piston. In this particular case let us assume that the area of the piston is 50 square inches and that the entire stroke (B) of the piston is 12 inches. To prevent confusion due to considerations of heat loss we will further assume that the cylinder is constructed of non-conducting material.