ional circulation through each, be placed at rest near one another.
The resultant fluid motion due to the two circulations, will give rise to
fluid pressure on the two bodies, which, if unbalanced, will cause them to
move. The force systems--force-and-torques, or pairs of forces--required to
prevent them from moving will be mutual and opposite, and will be the same
as, but opposite in direction to, the mutual force systems required to hold
at rest two electromagnets fulfilling the following specification: The two
electro magnets are to be of the same shape and size as the two bodies, and
to be placed in the same relative positions, and to consist of infinitely
thin layers of electric currents in the surfaces of solids possessing
extreme diamagnetic quality--in other words, infinitely small permeability.
The distribution of electric current on each body may be any whatever which
fulfills the condition that the total current across any closed line drawn
on the surface once through the aperture is equal to 1/4 [pi] of the
circulation[1] through the aperture in the hydro-kinetic analogue.

[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," Sec. 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, eithe