make epochs, and to prevail over that through whose agency they first
obtained strength.

Thus, everywhere, through all realms, do the opposite principles of Rest
and Motion depend upon and reciprocally empower each other. In every
act, mechanical, mental, social, must both take part and consent
together; and upon the perfection of this consent depends the quality
of the action. Every progress is conditioned on a permanence; every
permanence _lives_ but in and through progress. Where all, and with
equal and simultaneous impulse, strives to move, nothing can move, but
chaos is come; where all refuses to move, and therefore stagnates, decay
supervenes, which is motion, though a motion downward.

Having made this general statement, we proceed to say that there are two
chief ways in which these universal opposites enter into reciprocation.
The first and more obvious is the method of alternation, or of rest
_from_ motion; the other, that of continuous equality, which may be
called a rest _in_ motion. These two methods, however, are not mutually
exclusive, but may at once occupy the same ground, and apply to the same
objects,--as oxygen and nitrogen severally fill the same space, to the
full capacity of each, as though the other were absent.

Instances of the alternation, either total or approximative, of these
principles are many and familiar. They may be seen in the systole and
diastole of the heart; in the alternate activity and passivity of the
lungs; in the feet of the pedestrian, one pausing while the other
proceeds; in the waving wings of birds; in the undulation of the sea; in
the creation and propagation of sound, and the propagation, at least,
of light; in the alternate acceleration and retardation of the earth's
motion in its orbit, and in the waving of its poles. In all vibrations