Most essential of all, for the man who would become an airplane mechanic, is a thorough knowledge of gasolene-engines. This should include not only a knowledge of such fundamentals as the theory of the internal-combustion engine, carburetion, compression, ignition, and explosion, but also a keen insight into the whims of the human, and terribly inhuman, thing--the gasolene-motor. Nothing can be sweeter when it is sweet, and nothing more devilish when it is cranky, than an airplane engine. There are certain technical details which distinguish an airplane motor from an automobile motor, but a man who knows automobile engines can master the airplane motor in short order. Generally speaking, the airplane motor differs from the automobile motor in shape. The Liberty type of engine is V-shaped, with both sets of cylinders driving toward a common center, the crankshaft. Most airplane motors have special carbureters, and their oiling systems are extremely finely adjusted to take up any friction at their high speed. They will be found to be lighter in weight, with pistons, piston heads and other parts made of aluminium. They are, as a rule, more carefully made than most automobile motors, with especial attention to the fitting of all working parts. One advantage which an airplane mechanic has is standardization, which has reached a high point with Liberty, Hispano-Suiza, and Curtiss engines. Once a mechanic has learned his type he has learned practically every engine of that type. For a long time to come the 18,000 Liberty engines which this country had at the time the armistice was signed will be carrying commercial airplanes across broad stretches of the United States. If it had not been for the pressure of the war this engine might have been developed slowly, as the automobile engines were, with changes from year to year. The Liberty engine has reached a high standard of efficiency, and is likely to be the standard airplane engine in this country for several years to come. An airplane mechanic who knows his Liberty engine will be able to look after most of the airplanes with which he will come into contact.