§ 65. When a stream of fluid flows over a solid surface, or conversely
when a solid moves in still fluid, a resistance to the motion is
generated, commonly termed fluid friction. It is due to the viscosity
of the fluid, but generally the laws of fluid friction are very
different from those of simple viscous resistance. It would appear
that at all speeds, except the slowest, rotating eddies are formed by
the roughness of the solid surface, or by abrupt changes of velocity
distributed throughout the fluid; and the energy expended in producing
these eddying motions is gradually lost in overcoming the viscosity of
the fluid in regions more or less distant from that where they are
first produced.