tually
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
and undulations there is a going and returning, between which must exist
minute periods of repose; but in many instances the return is simply a
relaxation or a subsidence, and belongs, therefore, to the department of
rest. Discourse itself, it will be observed, has its pauses, seasons of
repose thickly interspersed in the action of speech; and besides these
has its accented and unaccented syllables, emphatic and unemphatic
words,--illustrating thus in itself the law which it here affirms.
History is full of the same thing; the tides of faith and feeling now
ascend and now subside, through all the ages, in the soul of humanity;
each new affirmation prepares the way for new doubt, each honest doubt
in the end furthers and enlarges belief; the pendulum of destiny swings
to and fro forever, and earth's minutest life and heaven's remotest star
swing with it, rising but to fall, and falling that they may rise again.
So does rhythm go to the very bottom of the world: the heart of Nature
pulses, and the echoing shore and all music and the throbbing heart and
swaying destinies of man but follow and proclaim the law of her inward
life.

The universality and mutual relationship of these primal principles
have, perhaps, been sufficiently set forth; and this may be the place to
emphasize the second chief point,--that the perfection of this mutuality
measures the degree of excellence in all objects and actions. It
will everywhere appear, that, the more regular and symmetrical their
relationship, the more beautiful and acceptable are its results. For
example, sounds proceeding from vibrations wherein the strokes and
pauses are in invariable relation are such sounds as we denomina