Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/122

§ 78 infinity in either direction. It would be more proper to discover by trial some combination of sources and sinks that would give an easy termination to the form than to effect this by an arbitrary mutilation, for the true stream lines round the modified form could then be plotted. Beyond this there is no advantage in the one course over the other; the criterion in either case is the eye of the designer. In the hydrodynamic theory of an inviscid fluid, every conceivable body is of stream-line form, and the conditions that obtain in practice do not exist; it is therefore useless to attempt to rationalise the ichthyoid or stream-line form by existing analytical theory.

The foregoing example illustrates the graphic method as applied to effecting the combination of motion in two dimensions; certain cases of motion in three dimensions may be solved by