Page:Physical Geography of the Sea and its Meteorology.djvu/106

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but, were the explanation to rest here, a north-east trade-wind extending from the polo to the equator would satisfy it; and were this so, we should have, on the surface, no winds but the north-east trade-winds on this side, and none but south-east trade-winds on the other side, of the equator.

209. From the Pole to 30°-35°.—Let us return now to our northern particle (§ 207), and follow it in a round from the north pole across the equator to the south pole, and back again. Setting off from the polar regions, this particle of air, for some reason which does not appear, hitherto, to have been very satisfactorily explained by philosophers, instead of travelling (§ 208) on the surface all the way from the pole to the equator, travels in the upper regions of the atmosphere until it gets near the belt between 30°-35°. Here it meets, also in the clouds, the hypothetical particle that is coming from the south, and going north to take its place.

210. The "horse latitudes." About this belt of 30°-35° north, then, these two particles press against each other with the whole amount of their motive power, and produce a calm and an accumulation of atmosphere: this accumulation is sufficient to balance the pressure of the two currents from the north and south. From under this bank of calms, which seamen call the "horse latitudes," two surface currents of wind are ejected or drawn out; one towards the equator, as the north-east trades, the other towards the pole, as the south-west "passage-winds," or counter-trades. These winds come out at the lower surface of the calm region, and consequently the place of the air borne away in this manner must be supplied, we may infer, by downward currents from the superincumbent air of the calm region. Like the case of a vessel of water which has two streams from opposite directions running in at the top, and two of equal capacity discharging in opposite directions at the bottom, the motion of the water would be downward;—so is the motion of the air in this calm zone.

211. The barometer there.—The barometer, in this calm region, stands higher than it does either to the north or to the south of it; and this is another proof as to the accumulation of the atmosphere here, and pressure from its downward motion. And because the pressure under this calm belt is greater than it is on either side of it, the tendency of the air will be to flow out on either side; therefore, supposing we were untaught by