are distributed along an axis (or axis plane for two-dimensional
motion in a three-dimensional space), and it is an established
proposition[1] that any solid whatever, in motion in a fluid, may
be imitated by an appropriate distribution of sources and sinks
situated on its surface, and it follows that within certain limitations
as to abruptness of contour, an equivalent exists for every
stream line solid of revolution in point sources and sinks
distributed along an axis, and for every cylinder of stream line
section in line sources and sinks located on an axial plane.
The distribution of sources and sinks that will produce any
particular form is only known in a few special cases, such as
those of the elliptical cylinder and ellipsoid, in which the number 
Fig. 43. is infinite. Any finite distribution can be investigated by the
graphic method by repeated compounding of system on system;
a comprehensive way of investigating cases of infinite distribution
is at present lacking. It may be noted that in all cases the
investigation commences with the source and sink system, the
form of the corresponding solid being obtained as a resultant;
the reverse process can only be effected by recognising the solid
as belonging to some particular system, and consequently only
certain solutions are possible.
It is evident that if we take any pair of Rankine's “oogenous neoids” and trim fore and aft to form water lines (Fig. 43), we can regard the process as equivalent to a number of sources in the region a a a, and sinks in the region b b b, in order to generate and absorb the stream flux that otherwise runs to
- ↑ Lamb, “Hydrodynamics,” pp. 56, 57 (3rd ed.).
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