[Illustration: FIG. 90.--Waves formed by a pebble.]
A quiet pool and a pebble will help to make it clear to us. If we
throw a pebble into a quiet pool (Fig. 90), waves or ripples form and
spread out in all directions, gradually dying out as they become more
and more distant from the pebble. It is a strange fact that while we
see the ripple moving farther and farther away, the particles of water
are themselves not moving outward and away, but are merely bobbing up
and down, or are vibrating. If you wish to be sure of this, throw the
pebble near a spot where a chip lies quiet on the smooth pond. After
the waves form, the chip rides up and down with the water, but does
not move outward; if the water itself were moving outward, it would
carry the chip with it, but the water has no forward motion, and hence
the chip assumes the only motion possessed by the water, that is, an
up-and-down motion. Perhaps a more simple illustration is the
appearance of a wheat field or a lawn on a windy day; the wind sweeps
over the grass, producing in the grass a wave like the water waves of
the ocean, but the blades of grass do not move from their accustomed
place in the ground, held fast as they are by their roots.
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