§ 208, 209. THE ATMOSPHERE. 75 



it did not partake of the diurnal motion of the earth, would, in 

 consequence of its vis inertice, find, as it travels south, the earth 

 slipping from under it, as it were, and thus it would appear to be 

 coming from the northeast and going toward the southwest; in 

 other words, it would be a northeast wind. The better to ex- 

 plain, let us take a common terrestrial globe for the illustration. 

 Bring the island of Madeira, or any other place about the same 

 parallel, under the brazen meridian ; put a finger of the left hand 

 on the place ; then, moving the finger down along the meridian 

 to the south, to represent the particle of air, turn the globe on its 

 axis from west to east, to represent 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 start- 

 ed ; in other words, the track of the finger over the surface of 

 the globe, like the track of the particle of air upon the earth, has 

 heen from the northward and eastward. 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 ineriice, be going toward the 

 east faster than the earth. It would therefore appear to be blow- 

 ing /ro?72 the southwest, and going toward the northeast, and ex- 

 actly in the opposite direction to the other. Writing south for 

 north, the same takes place between the south pole and the equator. 



208. Such is the process which is actually going on in nature ; 

 Two grand systems ^^^ if WO take the motious of these two particles as 

 of currents. ^-^^ ^^p^ q£ ^]^q motlou of all, wc shall havc an illus- 

 tration of the great currents in the air (§ 204), the equator being near 

 one of the nodes, and there being at least two systems of current."^, 

 an upper and an under, between it and each pole. Halley, in bis 

 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 winds but the northeast trade-winds on this side, and 

 none but southeast trade- winds on the other side, of the equator. 



209. Let us return now to our northern particle (§ 207), and 

 From the Pole to foUow it in a rouud from the north pole across the 

 35°-3o°. equator to the south pole, and back ngain. Setting 



