THE ATMOSPHERE. 77 



philosophers, instead of travelHng (§ 208) on the surface all the 

 way from the pole to the equator, travels in the upper regions of 

 the atmosphere mitil it gets near the helt 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. About this belt of 30^-35° north, then, these two particles 

 Thr>" horse lati- prcss agaiust each other with the whole amount of 

 ludjs." tjjgjj. motive power, and produce a calm and an 

 accumulation of atmosphere : this accumulation is sufficient to 

 balance the pressm'e of the two cm-rents from the north and south. 

 From under this bank of calms, which seamen call the " horse 

 latitudes," two sm'face cm-rents 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 

 consequentl}- the place of the air borne away in this manner must 

 be supplied, we may infer, by downward cmTents from the superin- 

 cumbent 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 dowTiward ; — so is 

 the motion of the air in this calm zone. 



211. The barometer, in this calm region, stands higher than it 

 Th? barometer there, docs either to thc uortli 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 ; there- 

 fore, supposing we were untaught by 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. Following our imaginary particle of air, however, from the 

 The equatorial calm uorth across this calm belt of Cancer, we now per- 

 ^''^^- ceive it mo^ang 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 i\\Q 

 north pole, has blown as the south-east trade-mnd. 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 



