THE ATMOSPHERE. 79 



opposite direction as they move from the equator towards the poles. 

 This tiiniing is caused b3^ the rotation of the earth on its axis. 



207. Effect of diurnal rotation on the course of the trade-icinds. — 

 The earth, we know, moves from west to east. Now if wo 

 imagine a particle of atmosphere at the north pole, where it is at 

 rest, to be put in motion in a straight line towards the equator, 

 we can easily see how this particle of air, coming from the very 

 axis of diurnal rotation, where it did not partake of tlie diurnal 

 motion of the earth would, in consequence of its vis inertioe, find, 

 as it travels south, the earth slipping 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 words, it would be a 

 north-east wind. The better to explain, let us take a common 

 teiTestrial 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 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/rom 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, and in consequence of its vis inertice, be going towards the 

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

 blowing /rom 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. Two grand systems of currents. — 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 currents in the air (§ 204), the 

 equator being near one of the nodes, and there being at least two 

 sj^stems 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 ; 



