THE ATMOSPHERE. 77 



130. About tliis parallel of 30° 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 

 winds from the north and south. 



131. From under this bank of calms, which seamen call the 

 " horse latitudes" (I have called them the calms of Cancer), two 

 surface currents of wind are ejected ; one tow^ard the equator, as 

 the northeast trades, the other toward the pole, as the southwest 

 passage-winds. 



132. These w4nds 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, w^e may infer, by downward currents 

 from the superincumbent air of the calm region. Like the case 

 of a vessel of water wdiich has two streams from opposite direc- 

 tions running in at the to^^, and two of equal capacity dis- 

 charging 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. 



133. The barometer, in this calm region, is said to stand high- 

 er than it does either to the north or to the south of it ; and this 

 is another proof as to the banking up here of the atmosphere, and 

 pressui-e from its downward motion. We can understand why 

 there should be an uprising of the air which the two systems of 

 trade-winds pour into the equatorial calms. But wdien this air 

 commences to flow toward the poles as an upper current, we can 

 not understand why it should not continue gradually to descend 

 and turn back (§ 144) all the way from the equator to the poles, 

 nor as far as investigation has gone, has any explanation been 

 suggested for the calm belts of the tropics ; nor can w- e tell why 

 the upper currents should meet at one j^arallel in preference to 

 another. But the fact of a meeting and a preference is certain. 



134. Following our imaginary particle of air, however, from the 

 north across this calm 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, wdiich, starting from the south at the same 



