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 
Fig. 44. useless to attempt to rationalise the ichthyoid or stream-line form
by existing analytical theory.[1]
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
- ↑ In the paper from which quotations are given it would appear that Prof. Rankine believed there to be some particular virtue in the forms derived from the special case of the simple source and sink system, that the stream lines of such a system constitute in fact natural water lines. In actuality ichthyoid or stream-line form is governed by conditions not yet amenable to rigid treatment, and the design of a stream-line form to work in a real fluid with a minimum of resistance is largely a matter of art. The underlying principles have been discussed in the previous chapters.
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