The uninformed will now learn that Australia lies in the southern hemisphere, while we are in the northern, and that there they live in midst of summer, while we are buried in snow. Nor will he now be surprised when he reads, that it snowed in Australia in the month of August, and that his friend or relative there reposed by the fireside, and read the letter from home by the light of the lamp, at the same hour that we here were taking an afternoon walk in the summer shade. The heat of summer, however, does not altogether depend upon the length of the day; nor does the cold of winter upon its shortness; but principally on this, that during summer-time the sun at noon stands directly over head; that therefore his vertical rays are enabled to pierce the soil with intense heat; while in winter-time the sun at noon stands nearer to the horizon; his rays fall on the earth obliquely, therefore heating the soil with but feeble power. We shall presently see that this position of the sun exercises great influence upon the weather. CHAPTER III. THE CURRENTS OF AIR AND THE WEATHER. In order to fully understand the conditions of the atmosphere, one must carefully notice the following: Though the sun produces summer and winter, and although his beams call forth heat, and the absence of heat causes intense cold on the surface of the globe, yet the sun alone does not make what we call "Weather." If the sun's influence alone were prevalent, there would be no change at all during our seasons; once cold or warm, it would invariably continue to be so, according to the time of the year. The sun, however, produces certain movements in the air; currents of air or winds pour from cold countries into warm ones, and _vice versa_ from warm ones into cold ones. It is this that makes our sky be cloudy or clear; that produces rain and sunshine, snow and hail, refreshing coolness in summer and warmth sometimes in midwinter, as also chilly nights in summer and thaw in winter. In other words, it is more properly the motion of the air, the wind, that produces what we call _weather_; that is, that changeableness from heat to cold, from dryness to moisture, all of which may be comprised in one name, weather. But whence does the wind arise? It is caused by the influence of the sun's heat upon the air. The whole earth is enveloped with a misty cover called "air." This air has the peculiar quality of expanding when it becomes heated. If you put a bladder that is filled with air and tied up, into the pipe of a heated stove, the air inside will expand so much as to burst the bladder with a loud report. The warm expanded air is lighter than the cold air, and always ascends in the atmosphere.