AERODYNAMICS may be defined broadly as the science of motion of air or an afiriform fluid. Commonly, air alone is implied in the word. This is especially true when the name is used by engineers. With them it is the analogue of hydraulics, which is the science of motion of water. Both sciences treat of not only the motion of their peculiar media, but also of its effect on objects or machinery connected with the fluids.
An important function of theoretical afirodynamics is to determine the velocity and stress of a fltaid at every point of the mediuln when it flows past an obstacle, the physical properties and Coliditions of the fluid being assmned or given. From the point-velocity the stream-lines may be mapped; from the point-stress about an object the resultant pressure and friction may be found by summing over its surface. Equivalent results may be obtained if the object is assumed to move against the fluid, since only the relative motion is of consequence. This is regarded as self-evident.
It will be convenient to treat of afirodynamics together with the general science of fluid dynamics.
Theoretical fluid dynamics, being a difficult subject, is, for convenience, commonly divided into two branches, one treating of frictionless or perfect fluids, the other treating of viscous or imperfect fluids. The frictionless fluid has no existence in nature, but is hypothecated by mathematicians in order to facilitate the investigation of important laws and principles that may be approximately true of viscous or natural fluids. We may first r~otice some of the principles and conclusions in this easier branch, then pass to the treatment of viscous media.
DYNAMICS OF FRICTIONLESS FLUIDS.
The motion of a fluid may be ro.tational or irrotational, and each of these kinds of flow may be steady or unsteady. The motion of the medium is irrotational if every part of it moves 25r '" Measurement of Air Velocity and Pressure," Physical Review, December, 3t9o3.