72 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Such is the process which is actually going on in nature ; and if 

 we take the motions of these two particles as the type of the mo- 

 tion of all, we shall have an illustration of the great currents in 

 the air, the equator being near one of the nodes, and there being 

 two systems of currents, an upper and an under, between it and 

 each pole. 



Halley, in his theory of the trade-winds, pointed out the key to 

 the explanation so far, of the atmospherical circulation ; but, were 

 the explanation to rest here, a northeast trade-wind extending 

 from the pole to the equator would satisfy it ; and were this so, 

 we should have, on the surface, no w^inds but the northeast trade- 

 winds on this side, and none but southeast trade-winds on the 

 other side, of the equator. 



99. Let us return now to our northern particle (Plate I., p. 70), 

 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 re- 

 gions, this particle of air, for some reason w^hich does not appear 

 to have been very satisfactorily explained by philosophers, instead 

 of traveling (§ 98) 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 30°. Here it meets, also in the clouds, 

 the hypothetical particle that is coming from the south, and going 

 north to take its place. 



100. About this 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 tw^o 

 winds from the north and south. 



101. 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 toward the equator, as 

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

 passage-winds. 



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 



