When the form and movements of the model are made to approximate in form to
those of a whiting or a mackerel the conditions are somewhat different, and it is
more difficult to plot the flow with accuracy. It can, however, be seen that particles
situated near the surface of the posterior regions of the body are accelerated backwards
towards the trailing surface of the caudal fin (Fig. 3). This movement is
particularly noticeable in the case of particles lying in the vicinity of the leading
surface of the body, for when such particles reach the peduncle of the tail they pass
rapidly over the dorsal or ventral surface of the body and are drawn in at the trailing
surface of the fin (Fig. 3). It seems clear that the latter surface acts as a centre
of low pressure which induces a backward acceleration of all the water lying in the
vicinity of the whole of the posterior part of the body.