It is not, however, wholly by the latitude of the region in which the evaporation takes place that the quantity of rain which falls from the atmosphere is determined; for the condition on which the falling back, in rain, of the water which has been taken up by evaporation mainly depends, is the cooling of the atmospheric stratum which contains it; and this effect is produced in very various ways, and many different causes operate to modify it. Sometimes the stratum is cooled by being wafted over ranges of mountains; sometimes by encountering and becoming mingled with cooler currents of air; and sometimes, again, by being driven in winds toward a higher, and, consequently, cooler latitude. If, on the other hand, air moves from cold mountains toward warm and sunny plains, or from higher latitudes to lower, or if, among the various currents into which it falls, it becomes mixed with air warmer than itself, its capacity for containing vapor in solution is increased, and, consequently, instead of releasing its hold upon the waters which it has already in possession, it becomes thirsty for more. It moves over a country, under these circumstances, as a warm and drying wind. Under a reverse of circumstances it would have formed drifting mists, or, perhaps, even copious showers of rain. It will be evident, from these considerations, that the frequency of the showers, and the quantity of the rain which will fall, in the various regions respectively which the surface of the earth presents, must depend on the combined influence of many causes, such as the warmth of the climate, the proximity and the direction of mountains and of seas, the character of the prevailing winds, and the reflecting qualities of the soil. These and other similar causes, it is found, do, in fact, produce a vast difference in the quantity of rain which falls in different regions. In the northern part of South America, where the land is bordered on every hand by vast tropical seas, which load the hot and thirsty air with vapor, and where the mighty Cordillera of the Andes rears its icy summits to chill and precipitate the vapors again, a quantity of rain amounting to more than ten feet in perpendicular height falls in a year. At St. Petersburg, on the other hand, the quantity thus falling in a year is but little more than one foot. The immense deluge which pours down from the clouds in South America would, if the water were to remain where it fell, wholly submerge and inundate the country. As it is, in flowing off through the valleys to the sea, the united torrents form the greatest river on the globe--the Amazon; and the vegetation, stimulated by the heat, and nourished by the abundant and incessant supplies of moisture, becomes so rank, and loads the earth with such an entangled and matted mass of trunks, and stems, and twining wreaths and vines, that man is almost excluded from the scene. The boundless forests become a vast and almost