the two bodies have repelled
each other. An American student of the problem, Mr. T.J.J. See, has
shown that the same action has served to give to the double stars the
exceeding eccentricity of their orbits.

Although these recent studies of tidal action in the celestial sphere
are of the utmost importance to the theory of the universe, for they
may lead to changes in the nebular hypotheses, they are as yet too
incomplete and are, moreover, too mathematical to be presented in an
elementary treatise such as this.

* * * * *

We now turn to another class of waves which are of even more
importance than those of the tides--to the undulations which are
produced by the action of the wind on the surface of the water. While
the tide waves are limited to the open ocean, and to the seas and bays
which afford them free entrance, wind waves are produced everywhere
where water is subjected to the friction of air which flows over it.
While tidal waves come upon the shores but twice each day, the wind
waves of ordinary size which roll in from the ocean deliver their
blows at intervals of from three to ten seconds. Although the tidal
waves sometimes, by a packing-up process, attain the height of fifty
feet, their average altitude where they come in contact with the shore
probably does not much exceed four feet; usually they come in gently.
It is likely that in a general way the ocean surges which beat against
the coast are of greater altitude.

Wind waves are produced and perform their work in a manner which we
shall now describe. When the air blows over any resisting surface, it
tends, in a way which we can hardly afford here to describe, to
produce motions. If the particle is free to move under the impulse
which it communicates, it bears it along; if it is linked together in
the manner of large masses, which the wind can not transport, it tends
to set it in motion in an alternating way. The sounds of our musical
instruments which act by wind are due to these alternating vibrations,
such as all air currents tend to produce. An AEolian harp illustrates
the action which we are considering. Moving over matter which has the
qualities that we denote by the term fluid, the swayings which the air
produces are of a peculiar sort, though they much resemble those of
the fiddle string. The surface of the liquid rises and falls in what
we term waves, the size of which is determined by the measure of
f