Early in the war most of the army, navy, and private aviation schools of the United States adopted the penguin system of learning to fly. That method, invented by the French, consisted of using as a training-machine an aeroplane that had so small a wing spread or so weak a motor that it merely hopped five or six feet off the ground when the motor was wide open. The small wing spread caused it to zigzag along the ground like a drunken man. For those reasons, perhaps, it was named after the penguin, which does not remain long on the ground or in the air and which has an irregular gait. The first step in learning to fly consists in studying the structure of the aeroplane and of the aeronautical engine, and aerodynamics, or the science of the forces that aid or hinder the flight of heavier-than-air machines. During the last half-dozen years many of the manufacturers of aircraft maintained schools in order to encourage men to learn the art of flying, and have given their pupils the chance to study at first hand the designing, the building, and the assembling of aeroplanes and hydroplanes. That has given the pupils a thorough knowledge of every detail of the aircraft—an invaluable asset to an aviator who has been compelled to make a forced landing far from a repair-shop. In the “ground” schools conducted by the United States Government for instructing aviation officers at the various institutions, like Cornell, Massachusetts Institute of Technology, and Princeton, a great deal of time was devoted to assembling aeroplanes. Most of the manufacturers of aircraft in this country do not make the motors used to propel their aeroplanes. The aeronautical motor is one of the most difficult machines to build successfully. A motor that runs as smoothly as a watch on the ground may hesitate and sputter at an altitude of a thousand feet, and at three thousand feet may stop altogether. Engineers say that that is because the change in temperature and in atmospheric pressure causes a difference in carburization. All these things the prospective flier had to learn as well as the reasons for the same. Contrary to the general notion, the construction of the aeronautical motor differs radically from that of the automobile engine. In point of weight the difference is marked. Seldom is any stipulation made that limits the weight of the automobile motor in proportion to the amount of horse-power; a few pounds more or less is not an important consideration in a pleasure-car or a motor-truck. But in an aeroplane every ounce of superfluous weight must be eliminated from the engine, which must nevertheless be strong enough to withstand the most violent strain.