139. How Light and Heat Travel from the Sun to Us. Astronomers tell us that the sun--the chief source of heat and light--is 93,000,000 miles away from us; that is, so far distant that the fastest express train would require about 176 years to reach the sun. How do heat and light travel through this vast abyss of space? [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. 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 If the pebble were attached to a string so that it could be dropped into the water and withdrawn at regular intervals, the waves would never have a chance to disappear, because there would always be a regularly timed definite disturbance of the water.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.