76 THE PHYSICAL GEOGRAPHY OF THE SEA. 



the diurnal rotation of the earth, and when the finger reaches the 

 equator, stop. It will now be seen that the place on the globe 

 under the finger is to the southward and westward of the place 

 from which the finger started ; in other words, the track of the 

 fino-er over the surface of the globe, like the track of the particle 

 of air upon the earth, has been from the northward and eastward. 



126. On the other hand, we can perceive how a like particle of 

 atmosphere that starts from the equator, to take the place of the 

 other at the pole, would, as it travels north, in consequence of its 

 vis inertice, be going toward the east faster than the earth. It 

 would therefore appear to be blowing from the southwest, and 

 going toward the northeast, and exactly in the opposite direction 

 to the other. Writing south for north, the same takes place be- 

 tween the south pole and the equator. 



127. 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 motion of all, we shall have an illustration of the great cur- 

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

 being at least two systems of currents, an upper and an u.nder, be- 

 tween it and each pole. 



128. 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 ex- 

 tending from the pole 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 southeast trade-winds 

 on the other side, of the equator. 



129. Let "us return now to our northern particle (Plate I., p. 

 75), and follow it in a round from the north pole across the equa- 

 tor to the south pole, and back again. Setting off from the polar 

 regions, this particle of air, for some reason which does not appear 

 to have been very satisfactorily explained by philosophers, in- 

 stead of traveling (§ 128) on the surface all the way from the pole 

 to the equator, travels in the upper regions of the atmosphere un- 

 til 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. 



