[Footnote 1: The integral of tangential component velocity all round any
closed curve, passing once through the aperture, is defined as the
"cyclic-constant" or the "circulation" ("Vortex Motion," § 60 (a), _Trans_.
R.S.E., April 29, 1867). It has the same value for all closed curves
passing just once through the aperture, and it remains constant through all
time, whether the solid body be in motion or at rest.]

It might be imagined that the action at a distance thus provided for by
fluid motion could serve as a foundation for a theory of the equilibrium,
and the vibrations, of elastic solids, and the transmission of waves like
those of light through an extended quasi-elastic solid medium. But
unfortunately for this idea the equilibrium is essentially unstable, both
in the case of magnets and, notwithstanding the fact that the forces are
oppositely directed, in the hydro-kinetic analogue also, when the several
movable bodies (two or any greater number) are so placed relatively as to
be in equilibrium. If, however, we connect the perforated bodies with
circulation through them in the hydro-kinetic system, by jointed rigid
connecting links, we may arrange for configurations of stable equilibrium.
Thus, without fly-wheels, but with fluid circulations through apertures, we
may make a model spring balance or a model luminiferous ether, either
without or with the rotational quality corresponding to that of the true
luminiferous ether in the magnetic fluid--in short, do all by the
perforated solids with circulations through them that we saw we could do by
means of linked gyrostats. But something that we cannot do by linked
gyrostats we can do by the perforated bodies with fluid circulation: we can
make a model gas. The mutual action at a distance, repulsive or attractive
according to the mutual aspect of the two bodies when passing within
collisional distance[1] of one another, suffices to produce the change of
direction of motion in collision, which essentially constitutes the