20 



Mr. R. M. Deeley on the 



create differences of pressure on its two sides is greater in 

 high than in low latitudes. 



In such a case as that above described the friction of the 

 wind against the earth's surface tends to reduce the velocity 

 of the lower portion. When this occurs the lower portion, 

 a!V a"b" (fig. 2), tends to flow towards the area of low 

 pressure, whilst the upper portion tends to flow towards the 

 region of higher pressure. This effect of surface friction was 

 pointed out by James Thomson ; and he considered, that it 

 accounted for the prevalent surface winds of middle latitudes, 

 especially in the Southern Hemisphere, blowing towards the 

 poles. But as such a lower wind moves in opposition to the 

 surface temperature gradient, he assumed that it was merely 

 a thin current, the main circulation resulting from the tem- 

 perature gradient consisting of two considerable upper 

 currents. 



The mean annual temperatures along two meridians are 

 The curve A is along 15" 3 W. long.,. 



shown in fig*. 5 



Fig. 5. 



80° 

 70° 

 60° 

 50° 

 40° 

 30° 

 2 0° 

 10° 

 0° 



Latitude 



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S. 80° 60° 40° 20° 0° 20° 40° 60° 80° N. 



Mean Annual Temperature Gradients. 



and, with the exception of that portion which passes over 

 the extreme westerly side of Africa, is wholly over water ; 

 whilst the curve B passes across Siberia, Central Asia, and 

 India, and then southwards over the Indian Ocean. Both 

 these curves show very considerable temperature gradients 

 from the equatorial towards the polar regions. 



The mean temperatures on Northern Asia are much lower 

 than those over the North Atlantic. India, however, is 

 warmer than the corresponding latitudes of the Atlantic. 

 An inspection of a chart showing the mean annual isotherms 

 of the world will show that this feature is a general one. 

 To recapitulate. In low latitudes the mean annual tempe- 

 rature over the land is greater than that over the sea ; but 



