The subject of this Paper is one that presented itself in a piece of work of a practical nature. The development of aeroplane flight naturally suggested the investigation of the motion of a body in a resisting medium. In general this problem was too difficult for solution. One therefore looked for legitimate means of simplifying it. One simple type of motion allied somewhat to aeroplane motion was that of a plane lamina moving in air under the earth’s attraction. Even here difficulties arose in the solution of the resulting equations of motion; yet further simplification was not quite permissible, for care had to be taken that the simplified problem did bear some relation to the actual problem presented by nature.