Page:Physical Geography of the Sea and its Meteorology.djvu/144

 118 rain in the north temperate zone is half as much again as that of the south temperate. How is it, then, that this vapour gets, as stated, from the southern into the northern hemisphere, and comes with such regularity that our rivers never go dry and our springs fail not? It is because of these air-crossings—these beautiful operations, and the exquisite compensation of this grand machine, the atmosphere. It is exquisitely and wonderfully counterpoised. Late in the autumn of the north, throughout its winter, and in early spring, the sun is pouring his rays with the greatest intensity down upon the seas of the southern hemisphere, and this wonderful engine which we are contemplating is pumping up the water there (§ 208) with the greatest activity, and sending it over here for our rivers. The heat which this heavy evaporation absorbs becomes latent, and, with the moisture, is carried through the upper regions of the atmosphere until it reaches our climates. Here the vapour is formed into clouds, condensed, and precipitated. The heat which held this water in the state of vapour is set free, it becomes sensible heat, and it is that [(4), § 288] which contributes so much to temper our winter climate. It clouds up in winter, turns warm, and we say we are going to have fallen weather. That is because the process of condensation has already commenced, though no rain or snow may have fallen: thus we feel this southern heat, that has been collected from the rays of the sun by the sea, been bottled away by the winds in the clouds of a southern summer, and set free in the process of condensation in our northern winter. If Plate I. fairly represent the course of the winds, the south-east trade-winds would enter the northern hemisphere, and, as an upper current, bear into it all their moisture, except that which is precipitated in the region of equatorial calms, and in the crossing of high mountain ranges, such as the Cordilleras of South America.

291. More rain in the northern than in the southern hemisphere.—The South Seas, then (§ 290), should supply mainly the water for this engine, while the northern hemisphere condenses it; we should, therefore, have more rain in the northern hemisphere. The rivers tell us that we have—the rain-gauge also. The yearly average of rain in the north temperate zone is, according to Johnston, thirty-seven inches. He gives but twenty-six in the south temperate. The observations of mariners are also corroborative of the same. Log-books, containing altogether the