of the A tmosplieres of the Earth and the Sim, 185 



ing tlie earth's rotation. A west wind moves round the 

 earth's axis more rapidly than the earth, and tends, by its 

 friction to accelerate the earth's rotation. An east wind, 

 for the opposite reason, tends to retard it ; and the two sets 

 of forces exactly neutralise each other. The friction of a 

 wind is approximately as the square of its velocity, and the 

 unbalanced effect of any wind on the earth's rotation = the 

 the east or west component of its force X the area it covers 

 X the radius of the parallel of latitude. The last factor 

 gives leverage. 



Were the whole equatorial region occupied by the trade- 

 winds, and the whole of both circumpolar regions by the 

 counter-trades, and were the east and west components of 

 the force everywhere the same, the dividing lines, in order 

 to produce the above-mentioned compensation, would be at 

 20"^ 19' 20" north and south nearly,* but they actually are at 

 about 28°, showing that, in order to produce the compensa- 

 tion, the force of the west winds must be greater than that of 

 the east ones. The greater force of the west winds is a neces- 

 sary consequence of the law of the conservation of areas, in 

 virtue of which, if friction were absent, the air at any lati- 

 tude would be moving round the earth's axis with an absolute 

 velocity inversely as the radius of the circle of latitude. The 

 excess or deficiency of the absolute velocity of a mass of air, 

 as compared with the earth's velocity of rotation at the same 

 latitude, is the velocity, west or east, of the wind. A mass 

 of air in moving towards the pole will consequently gain 

 absolute velocity, and increase in relative velocity as a west 

 wind ; in moving towards the equator, on the contrary, it 

 will lose absolute velocity, and increase in relative velocity 

 as an east wind. But the utmost increase towards the 

 equator will be finite ; towards the pole, on the contrary, 

 friction apart, it would be infinite ; the velocity at the pole 

 would be infinite, in consequence of the radius of the circle 

 of latitude there being nothing, which is physically inter- 

 preted by saying, that in the absence of friction no air would 

 reach the pole — being kept away by centrifugal force. 

 Friction prevents the centrifugal force of these aerial 



«- My friend, Mr Harlin, Fellow of St Peter's, Cambridge, has calculated 

 this for me. It is identical with the parallel that bisects the solid hemisphere. 



