No bird warbles its sweet music in the air, no insect breathes, save by his power, and all animals love to bask in the genial glow of his light and heat. He rolls the scorching air of the tropics to frozen lands, and wafts the ships across the seas. He forces the heated waters of the equator to the poles, tempering all the earth. He lifts the water from the sea to sprinkle all the land and cap the distant mountains with eternal snow. Now, let us examine a little more minutely how this influence is exerted upon the _air_, which is the subject we are especially interested in at present. Does it commence at the top, and heat it, layer by layer, until it reaches the bottom? Not at all; but it passes through the whole forty-five miles of air, heating it very little, if any, and falls upon the solid substances at the earth's surface, heating them, which, in turn, heat the air by its individual particles coming into immediate contact with those solid hotter substances. We will endeavor to illustrate this in a crude way. [Illustration: Fig. 7.] Here we have a tin tube, _a_, fifteen feet long and ten inches in diameter, open at both ends; two feet from one end we introduce this ascending pipe, _b_, the upper end of which is merely inserted in a small flue, extending to the top of the building. The height of this flue is sufficient to make a current of air pass through this tube, as you will see by holding this smoking taper at the far end. We will now place a large heated ball, _c_, at this end, and outside of that we will place this reflector, _d_, pressing it quite close to the end of the tube, so that no air can enter here. The rays of heat from this ball, or from any other warm body, are thrown like rays of light, in every direction equally; there would, therefore, be some of the rays thrown through this tube to the other end without any reflector, but the proportion that would reach the other end would, of course, be small. We therefore collect those going the other way, and change their course, and then send them straight through the tube to the far end. We will place another reflector, _e_, at the far end, to receive and concentrate those rays, in the focus of which we will place a candle, F, with a little phosphorus on it, to show you that the rays of heat are passing through. There you see the candle is lighted, thus proving that there is a strong current of radiant heat coming from the hot ball, through the tube to this end. And you see by this smoke that there is a current of air passing the other way.