the Distribution of Heat over the Globe. 103 



mean annual temperature, that the reverse is actually the case; 

 for the ocean in tropical regions has a mean annual temperature 

 below the normal, and the land a mean annual temperature 

 above the normal. Both in the Pacific and in the Atlantic the 

 mean temperature sinks to 2°*3 below the normal, while on the 

 land it rises 4°*6 above the normal. This can be accounted for 

 upon no other principle than that the ocean in tropical regions 

 is kept at a lower temperature in consequence of the enormous 

 amount of heat that is being constantly carried away from those 

 regions into temperate and polar regions, and of cold that is 

 being constantly carried from temperate and polar regions to the 

 tropical regions by means of ocean- currents. 



Were there no ocean-currents, it would follow, according to 

 theory, that the southern hemisphere should be warmer than the 

 northern, because the proportion of sea to land is greater on that 

 hemisphere than on the northern ; but we find that the reverse 

 is the case. 



Low Temperature of the Southern Hemisphere owing to Ocean- 

 currents. — The lower mean temperature of the southern hemi- 

 sphere is due to the amount of heat transferred over from that 

 hemisphere to the northern by ocean-currents. 



The great ocean-currents of the globe take their rise in three 

 immense streams from the Southern Ocean, which, on reaching 

 the tropical regions, become deflected in a westerly direction and 

 flow along the southern side of the equator for thousands of 

 miles. Perhaps more than one-half of this mass of moving water 

 returns into the Southern Ocean without ever crossing the equa- 

 tor; but the quantity which crosses over to the northern hemi- 

 sphere is enormous. Since there is then a constant flow of water 

 from the southern hemisphere to the northern in the form of 

 surface-currents, it must be compensated by undercurrents of 

 equal magnitude from the northern hemisphere to the southern. 

 The currents, however, which cross the equator are far higher in 

 temperature than their compensating undercurrents; conse- 

 quently there is a constant transference of heat from the south- 

 ern hemisphere to the northern. Any currents taking their rise 

 in the northern hemisphere and flowing across into the southern 

 are comparatively trifling, and the amount of heat transferred by 

 them is also trifling. There are one or two currents of consider- 

 able size, such as the Brazilian branch of the great equatorial 

 current of the Atlantic, and a part of the South Equatorial Drift- 

 current of the Pacilic, which cross the equator from north to 

 south ; but these cannot be regarded as northern currents ; they 

 are simply southern currents deflected back after crossing over 

 to the northern hemisphere. The heat which these currents 

 possess is chiefly obtained on the southern hemisphere before 



v 



