Page:Physical Geography of the Sea and its Meteorology.djvu/379

Rh changes from day to day are very great. He would observe that the zones of winds and calms have their tropics or stationary nodes, about which they linger near three months at a time; and that they pass from one of their tropics to the other in a little less than another three months. Thus he would observe the whole system of belts to go north from the latter part of May till some time in August. Then they would stop and remain nearly stationary till winter, in December; when again they would commence to move rapidly over the ocean, and down towards the south, until the last of February or the first of March; then again they would become stationary, and remain about this, their southern tropic, till May again. Having completed his physical examination of the equatorial calms and winds, if the supposed observer should now turn his telescope towards the poles of our earth, he would observe a zone of calms bordering the north-east trade-winds on the north (§ 210), and another bordering the south-east trade-winds on the south (§213). These calm zones also would be observed to vibrate up and down with the trade-wind zones, partaking (§ 296) of their motions, and following the declination of the sun. On the polar side of each of these two calm zones there would be a broad band extending up into the polar regions, the prevailing wands within which are the opposites of the trade-winds, viz., south-west in the northern and north-west in the southern hemisphere. The equatorial edge of these calm belts is near the tropics, and their average breadth is 10° or 12°. On one side of these belts (§ 210) the wind blows perpetually towards the equator; on the other, its prevailing direction is towards the poles. They are called (§ 210) the "horse latitudes" by seamen.

655. Rainy seasons of the tropical calm belts.—Along the polar borders of these two calm belts (§ 296) we have another region of precipitation, though generally the rains here are not so constant as they are in the equatorial calms. The precipitation near the tropical calms is nevertheless sufficient to mark the seasons; for whenever these calm zones, as they go from north to south with the sun, leave a given parallel, the rainy season of that parallel, if it be in winter, is said to commence. Hence we may explain the rainy season in Chili at the south, and in California at the north.

656. Their position.—We can now understand why the calm belts of Cancer and Capricorn occupy a medial position between