A great portion of the heat-rays which are emitted by the sun are absorbed in their passage through the atmosphere which surrounds our globe. It is calculated that about one-third of the heat-rays which fall on it never reach the earth, which fact adds another to the many beneficent purposes fulfilled by our gaseous envelope, screening us from the otherwise scorching heat. It is curious to trace the varied fates of the calorific rays which strike on the surface of the earth. Some at once on falling are reflected, and, passing back through the atmosphere, are lost amid the immensity of space; others are absorbed or imbibed by different bodies, and, after a time, are radiated from them; but the greater part of the beams which reach the earth during the summer are absorbed by it, and conveyed downward to a considerable distance, by conduction from particle to particle. Heat also spreads laterally from the regions of the equator toward the poles, thereby moderating the intense cold of the arctic and antarctic circles, and in winter, when the forest-trees are covered with snow, their deeply-penetrating roots are warmed by the heat, which, as in a vast store-house, has been laid up in the earth, to preserve life during the dreary winter. The rays which fall on the tropical seas descend to the depth of about three hundred feet. The sun's attraction for the earth, being also stronger at that quarter of the world, the heated waters are drawn upward, the colder waters from the poles rush in, and thus a great heated current is produced, flowing from the equator northward and southward, which tends to equalize the temperature of the earth. The sailor also knows how to avail himself of this phenomenon. When out at sea, despite his most skillful steering, he is in constant danger of shipwreck, if he fails to estimate truly the force and direction of those currents which are dragging him insensibly out of the true course. His compass does not help him here, neither does any log yet known give a perfectly authentic result. But he knows that this great gulf-stream has a stated path and time, and, by testing from hour to hour the temperature of the water through which he is proceeding, he knows at what point he is meeting this current, and reckons accordingly.