346 PHYSIOGRAPHY. [chap. 



rises from the heated equatorial belt ? This air, on reaching 

 the higher regions of the atmosphere, flows over the cur- 

 rents which are sweeping across the surface below, and 

 thus produces currents which drift towards the north in the' 

 northern, and towards the south in the southern hemisphere. 

 But these upper currents are blowing from regions of high 

 velocity, to regions of lower velocity, of rotation ; they 

 therefore move more rapidly than the earth immediately 

 beneath them, and, as it were, outrun the earth in its rotation. 

 Hence they are deflected from a simple north-and-south 

 course, but in an opposite direction to that of the trades ; so 

 that, in the northern hemisphere, they blow from the S.W., 

 and, in the southern hemisphere, from the N.W. Such up- 

 per currents, moving directly counter to the surface winds, 

 may be recognised by their effects on high clouds. In the 

 higher regions of the atmosphere, they become chilled ; and, 

 at about the thirty-fifth parallel of latitude, they are suffi- 

 ciently dense to descend to the surface. Part of this air then 

 returns as an undercurrent to the equatorial belt, where it 

 becomes heated afresh, and once more rises, thus completing 

 the circulation in this part of the atmosphere ; while another 

 part of the descending air continues its course as a south- 

 west wind, in the northern hemisphere ; and, as a north-west 

 wind, in the southern hemisphere ; but these winds are not 

 so constant as the trades. The prevalent S.W. winds of 

 this country may have, in part, such an origin. These winds 

 are the chief rain-bearers to the British Isles ; and hence the 

 rotation of the earth is not without its effect upon the water- 

 supply of the Thames basin. 



The diurnal rotation of the earth sufficiently explains a 

 good many of the apparent movements of the heavenly 

 bodies. Thus, every day, the sun appears to rise towards the 



I 



