76 THE PHYSICAL GEOGKAI^HY OF THE SEA. 



off from the polar regions, this particle of air, for some reason 

 which does not appear to have been very satisfactorily explained 

 by philosophers, instead of traveling (§ 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 parallel of 80°-85°. 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 parallel of 30°-35° north, then, these two par- 

 The '' horse lati- ticlcs prcss agaiust cach 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 winds from the north and south. 

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

 tudes," two surface currents of wind are ejected or drawn out; 

 one toward the equator, as the northeast trades, the other to- 

 ward the pole, as the southwest " 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 direc- 

 tions 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, in this calm region, stands higher than it 

 The barometer there, does either to the north or to the south of it; and 

 this is another proof as to the accumulation of the atmosphere 

 liere, 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; 

 iliercforc, supposing we were untaught by observation, reason 

 would teach us to look for the prevailing winds on each side of 

 this calm belt to he from it. 



212. Following our imaginary particle of air, however, from 

 The equatorial calm ^hc uorth across this calfn belt of Cancer, we now 

 ^^^^' feel it moving on the surface of the earth as the 

 northeast 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 



