Page:Physical Geography of the Sea and its Meteorology.djvu/107

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81
THE ATMOSPHERE.

observation as to direction of the wind, reason would teach us to look for the prevailing winds on each side of this calm belt to be from it.

212. The equatorial calm belt.—Following our imaginary particle of air, however, from the north across this calm belt of Cancer, we now perceive it moving on the surface of the earth as the north-east trade-wind; and as such it continues till it arrives near the equator, where it meets a like hypothetical particle, which, starting from the south at the same time the other started from the north pole, has blown as the south-east trade-wind. Here, at this equatorial place of meeting, there is another conflict of winds and another calm region, for a north-east and south-east wind cannot blow in the same place and at the same time. The two particles have been put in motion by the same power; they meet with equal force; and, therefore, at their place of meeting, they are arrested in their course. Here, therefore, there is a calm belt, as well as at Capricorn and Cancer. Warmed now by the heat of the sun, and of vapour in the process of condensation, and pressed on each side by the whole force of the north-east and south-east trades, these two hypothetical particles, taken as the type of the whole, cease to move onward and ascend. This operation is the reverse of that which took place at the meeting (§ 210) near the belt between the parallels of 30°-35°.

213. The calm belt of Capricorn.—This imaginary particle then, having ascended to the upper regions of the atmosphere again, travels there counter to the south-east trades, until it meets, near the calm belt of Capricorn, another particle from the south pole; here there is a descent as before. (§ 210); it then (§ 211) flows on towards the south pole as a surface wind from the north-west.

214. The polar calms and the return current.—Entering the polar regions obliquely, it is pressed upon by similar particles flowing in oblique currents across every meridian; and here again is a calm place or node; for, as our imaginary particle approaches the parallels near the polar calms more and more obliquely, it, with all the rest, is whirled about the pole in a continued circular gale; finally, reaching the vortex of the calm place, it is carried upward to the regions above, whence it commences again its flow to the north as an upper current, as far as the calm belt of Capricorn; here it encounters (§ 213) its fellow from the