We have already said that heat was the producer of the winds, which are so essential to the preservation of the purity of the atmosphere. In order to understand their action, we shall consider the stupendous phenomenon of the trade-winds, which is similar to that of the current we have described. The rays of the sun falling vertically on the regions between the tropics, the air there becomes much heated. It is the property of air to expand when heated, and, when expanded, it is necessarily lighter than the cooler air around it. Consequently it rises. As it rises, the cooler air at once takes its place. Rushing from the temperate and polar regions to supply the want, the warm air which has risen flows toward the poles, and descends there, loses its heat, and again travels to the tropics. Thus a grand circulation is continually maintained in the atmosphere. These aerial currents, being affected by the revolution of the earth, do not move due north and south, as they otherwise would. Hence, while they equalize the temperature of the atmosphere, they also preserve its purity; for the pure oxygen evolved by the luxuriant vegetation of the equatorial regions is wafted by the winds to support life in the teeming population of the temperate zones, while the air from the poles bears carbonic acid gas on its wings to furnish food for the rich and gorgeous plants of the tropics. Thus the splendid water-lily of the Amazon, the stately palm-tree of Africa, and the great banyan of India, depend for nourishment on the breath of men and animals in lands thousands of miles distant from them, and, in return, they supply their benefactors with vivifying oxygen. Little less important, and still more beautiful, is the phenomenon of dew, which is produced by the power of radiating heat, possessed in different degrees by all bodies. The powers both of absorbing and of radiating heat, in great measure, depend on the color of bodies--the darker the color, the greater the power; so that each lovely flower bears within its petals a delicate thermometer, which determines the amount of heat each shall receive, and which is always the amount essential to their well-being. The queenly rose, the brilliant carnation, the fair lily, and the many-colored anemone, all basking in the same bright sunshine, enjoy different degrees of warmth, and when night descends, and the heat absorbed by day is radiated back, and bodies become cooler than the surrounding air, the vapor contained in the atmosphere is deposited in the form of dew. Those bodies which radiate most quickly receive the most copious supply of the refreshing fluid. This radiating power depends on the condition of the surface, as well as upon color, so that we may often see the grass garden bathed in dew, while the gravel walks which run through it are perfectly dry, and, again, the smooth, shining, juicy leaves of the laurel are quite dry, while the rose-tree beneath it is saturated with moisture.