An introduction to mathematics, by A. N. Whitehead.

PERIODICITY IN NATURE 171
push "in tune" with the oscillations of the
stone, so as always to secure a favourable
moment for a push. If the pushes are out
of tune, some increase the oscillations, but
others check them. But when they are in
tune, after a time all the pushes are favourable. The word "resonance" comes from considerations of sound: but the phenomenon
extends far beyond the region of sound. The
laws of absorption and emission of light depend on it, the "tuning" of receivers for wireless telegraphy, the comparative importance
of the influences of planets on each other's
motion, the danger to a suspension bridge as
troops march over it in step, and the excessive
vibration of some ships under the rhythmical
beat of their machinery at certain speeds.
This coincidence of periodicities may produce
steady phenomena when there is a constant association of the two periodic events, or it may
produce violent and sudden outbursts when
the association is fortuitous and temporary.
Again, the characteristic and constant
periods of vibration mentioned above are
the underlying causes of what appear to
us as steady excitements of our senses. We
work for hours in a steady light, or we listen
to a steady unvarying sound. But, if modern
science be correct, this steadiness has no
counterpart in nature. The steady light is
due to the impact on the eye of a countless