Page:Physical Geography of the Sea and its Meteorology.djvu/38

12 year come round, and the surplus temperature, thus withdrawn and stored away at the time it might have proved superfluous or inconvenient, is rendered back so soon as it is required; thus the cold of night and the rigour of winter are modified by the heat given out at the point of condensation by dew, rain, hail, and snow.

35. Effects upon the earth.—"The earth is a bad conductor of heat; the rays of the sun, which enter its surface and raise the temperature to 100° or 150°, scarcely penetrate a foot into the ground; a few feet down, the warmth of the ground is nearly the same night and day. The moisture which is there preserved free from the influence of currents of air is never raised into vapour; so soon as the upper stratum of earth becomes thoroughly dried, capillary action, by means of which all excess of water was withdrawn, ceases; so that, even under the heats of the tropics, the soil two feet down will be found, on the approach of the rains, sufficiently moist for the nourishment of plants. The splendid flowers and vigorous foliage which burst forth in May, when the parched soil would lead us to look for nothing but sterility, need in no way surprise us; fountains of water, boundless in extent and limited in depth only by the thickness of the soil which contains them, have been set aside and sealed up for their use, beyond the reach of those thirsty winds or burning rays which are suffered to carry off only the water which is superfluous, and would be pernicious. They remove it to other lands, where its agency is required, or treasure it up, as the material of clouds and dew, in the crystal vault of the firmament, the source, when the fitting season comes round again, of those deluges of rain which provide for the wants of the year. Such are some of the examples which may be supplied of general laws operating over nearly the whole surface of the terraqueous globe. Among the local provisions ancillary to these are the monsoons of India, and the land and sea breezes prevalent throughout the tropical coasts.

36. The tides.—"We have not noticed the tides, which, obedient to the sun and moon, daily convey two vast masses of water round the globe, and which twice a month, rising to an unusual height, visit elevations which otherwise are dry. During one half of the year the highest tides visit us by day, the other half by night; and at Bombay, at spring tide, the depths of the two differ by two or three feet from each other. The tides