CHANGE OF SOLAR RAYS INTO LIGHT, HEAT, AND OTHER FORMS OF ENERGY AS THEY ARE ABSORBED BY OUR ATMOSPHERE OR AS THEY ENCOUNTER THE EARTH—TEMPERATURES OF WATER, EARTH, AND AIR—HOW SANITARY HOMES MAY BE CHEAPLY CONSTRUCTED BELOW GROUND, COOL IN SUMMER AND WARM IN WINTER =Difference between Climate and Weather.= One may speak of the weather of to-day or of some time that is past, but not of the climate of to-day, or of any day, month, or year that is gone: for the climate of a place is determined by a study of its weather records for a long period of years. Climate changes so slowly that we speak of the movement as a mutation rather than as a change. The time that has elapsed since the discovery of the barometer and the thermometer—about two and a half centuries—is so short as to show little if any change in climate, while the weather changes from day to day. =The Sun Our Only Source of Appreciable Heat.= Each one of the stars visible to the eye and many of the millions that are not visible, are suns accompanied by planets. Their conditions are similar to those of our sun, except that most of them are larger than our sun, some a million times larger. But their distance is so great that they exercise little or no influence in the heating of the earth. Light travels at about the rate of 186,400 miles per second, and yet these stars are so distant that if the nearest one had been created at the time of the signing of the Declaration of Independence we still would be in ignorance of its existence, for its first rays of light would not reach us for many years yet to come; and light from some of the remote suns that we call stars requires thousands of years to come. It is apparent therefore that we depend exclusively upon our own luminary for the heat that warms our atmosphere and gives life to the surface of the earth. [Illustration: FIG. 21.—Equinoxes, March 21 and September 22. Axis perpendicular to Sun’s rays. Day and night everywhere equal.]