The Rolling Carriage was constructed for the purpose of determining the pressure of the air on a plane moving normal to its direction of advance. ^[*] Whatever be the importance of this subject to aerodynamics or engineering, we are here interested in it only in its direct bearing on the aerodromic problem, and carry these observations only as far as this special object demands. Before this instrument was constructed, a few results had already been obtained with the Resultant Pressure Recorder (chapter IV), but additional observations were desired with an instrument that would be susceptible of greater precision. The statement has frequently been made that the law that the pressure is proportional to the square of the velocity fails for low velocities as well as for very high ones. As it appears to me that this conclusion was probably based on imperfect instrumental conditions due to the relatively excessive influence of the friction of the apparatus at low velocities, particular pains were taken in the present experiments to get as frictionless an action as possible. Plates IX and X contain drawings in elevation and plan of the apparatus devised for this purpose.

A metal carriage 8 1/2 inches long is suspended on a set of delicately constructed brass wheels 5 inches in. diameter, which roll on planed ways. Friction wheels bearing against the sides and bottom of the planed ways serve as guides to keep the carriage on its track. Cushions of rubber at each end...