76 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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

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

 coming from the north-east and going towards the south-west ; in 

 other w^ords, it would be a north-east wind. The better to explain, 

 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, tm^n the globe on its axis 

 from west to east, to represent the dim^nal 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 finger over the surface of the globe, like the 

 track of the particle of air upon the earth, has been from the north- 

 ward 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, and in 

 consequence of its vis inertise, be going towards the east faster than 

 the earth. It would therefore appear to be blowing from the south- 

 west, and going towards the north-east and exactly 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 ^^d if WO take the motious of these two particles as 

 of currents. ^]^g ^^^ q£ ^^le motiou of all, WO shall have 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 

 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 north-east trade-wind extending from 

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

 should have, on the sm-face, no winds but the north-east trade-winds 

 on this side, and none but south-east trade-winds on the other side, 

 of the equator. 



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

 From the Pole to follow it in a rouud froui the north pole across the 

 30*^-05-. equator to the south pole, and back again. Setting ofl:" 

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

 not appear, hitherto, to have been very satisfactorily explained hj 



