Although Aviation is a new field of human endeavor, its appli cation to the art of warfare is already becoming a specialty. Only recently has it been appreciated, that military requirements have a most vital and important in?uence on many features of aeroplanes not only in the art of using them in military operations, but in their fundamental design and construction. It is planned, therefore, to give particular attention here to the military aeroplane, as we find it today emerged from a crude state of invention and development into a more or less finished product, which, in the greatest war of history, has gloriously demonstrated its strategical and tactical importance. It is no longer necessary to speculate on the uses of aeroplanes in warfare. What has actually been accomplished in directing artillery fire, in reconnaissance, in dispatch-carrying, and in offensive work has opened a new phase of warfare, as significant as it is surprising. The technique of the use of aeroplanes in strategy and tactics, is decidedly a subject for the military expert, but the general design and construction of aeroplanes to accomplish certain definite purposes, and their operation and maintenance in the field, are subjects that may properly be considered here. In addition to expert ability in their operation, it is found that a sound and practical knowledge of the design and construction of aeroplanes is exceedingly helpful to the military aviator. A full consideration, therefore, is given to elementary theory and practice applied in aviation, and the information used is primarily designed to be of definite service, in the field, where many unforeseen difficulties constantly arise. Before taking up the determination of its elements, it is noces sary, clearly, to distinguish the aeroplane from other craft designed to navigate the air.