by the Southern than by the Northern Hemisphere. 227 



transparency for heat-rays, especially those from the sun, be- 

 comes heated to a considerable depth rapidly. The quantity of 

 heat stored up in the ground is comparatively small ; the quan- 

 tity stored up in the ocean is great. (4) The aqueous vapour of 

 the air acts as a screen to prevent the loss by radiation from 

 water, while it allows radiation from the ground to pass more 

 readily into space. (5) The air is heated more rapidly by con- 

 tact with the hot surface of the ground than it is by contact 

 with the surface of the ocean. Consequently the heat which 

 is carried up into the higher regions of the atmosphere and 

 thrown off into stellar space chiefly comes from the land. 



But it may be asked, If the southern hemisphere absorbs far 

 more heat than the northern, why, then, is its mean tempera- 

 ture so much below that of the northern ? The lower tempera- 

 ture of the southern hemisphere is evidently due, not to the 

 loss of heat by radiation as supposed by Adhemar and others, 

 but to a cause which has been completely overlooked, viz. to the 

 normous amount of heat transferred from that hemisphere to 

 the northern by means of 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. A considerable portion of these currents 

 returns into the Southern Ocean without ever crossing the 

 equator, but the greater portion of them crosses over to the 

 northern hemisphere. 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 cur- 

 rents of considerable 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 Pacific, 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 he- 

 misphere before crossing over to the northern ; and although 

 the northern hemisphere may not gain any temperature by 



