oving 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.

If a pebble is thrown into a quiet pool, it creates ripples or waves
which spread outward in all directions, but which soon die out,
leaving the pool again placid and undisturbed. If now we could quickly
withdraw the pebble from the pool, the water would again be disturbed
and waves would form. If the pebble were attached to a string so that
it could be dropped into the water and withdrawn at regular intervals,
the waves would never have a chance to disappear, because there would
always be a regularly timed definite disturbance of the water. Learned
men tell us that all hot bodies and all luminous bodies are composed
of tiny particles, called molecules, which move unceasingly back and
forth with great speed. In Section 95 we saw that the molecules of all
substances move unceasingly; their speed, however, is not so great,
nor are their motions so regularly timed as are those of the
heat-giving and the light-giving particles. As the particles of the
hot and luminous bodies vibrate with great speed and force they
violently disturb the medium around them, and produce a series of
waves similar to those produced in the water by the pebble. If,
however, a pebble is thrown into the water very gently, the
disturbance is slight, sometimes too slight to throw the water into
waves; in the same way objects whose molecules are in a state of
gentle motion do not produce light.

The particles of heat-giving and light-giving bodies are in a state of
rapid vibration, and thereby disturb the surrounding medium, which
transmits or conveys the disturbance to the earth or to other objects
by a train of waves. When these waves reach their destination, the
sens