170 Mr. J. Croll on the Physical Cause of Ocean- currents. 



ranee of land in the northern hemisphere And to affirm, 



as Mr. Croll does, that the lower mean temperature of the south- 

 ern hemisphere is due to the amount of heat transferred over 

 from that hemisphere to the northern by ocean-currents, is to 

 repudiate all that has been established by the researches of me- 

 teorologists, as to the relative effects of land and sea/'' &c. (§ 101). 

 But what has all this to do with my estimate of the quantity 

 of heat conveyed by the Gulf-stream ? Supposing it to be true 

 that I "altogether ignore the influence on the distribution of 

 heat over the globe which is exerted by the great relative pre- 

 ponderance of land in the northern hemisphere," and supposing 

 it to be true that the lower mean temperature of the southern 

 hemisphere is not due, as I have concluded, to the amount of 

 heat transferred over from that hemisphere to the northern by 

 ocean -currents, this cannot in any way affect the value of my 

 figures regarding the amount of heat conveyed by the Gulf- 

 stream. It is not true, however, that I ignore and repudiate 

 all that has been established as to the effect of land and sea in 

 producing the difference of mean temperature between the two 

 hemispheres. So far from this being the case, I have devoted 

 an entire paper (Phil. Mag. Sept. 1869) to an examination of 

 the arguments which have been advanced to explain the lower 

 mean temperature of the southern hemisphere. 1 have given my 

 reasons for concluding that an enormous amount of heat is trans- 

 ferred from the southern hemisphere to the northern by means 

 of currents. These reasons may or may not be satisfactory ; but 

 nevertheless they are reasons, not assumptions. It would be 

 needless as well as out of place to repeat these arguments ; but I 

 may be permitted simply to refer to one of them, viz. my reason 

 for concluding that a great portion of the heat possessed by the 

 Gulf-stream is derived from the southern hemisphere. If all 

 that heat came from the northern hemisphere, it could only 

 come from that portion of the Atlantic, Caribbean Sea, and Gulf 

 of Mexico which lies to the north of the equator. The entire 

 area of these seas, extending to the tropic of Cancer, is about 

 7,700,000 square miles. Were the heat which passes through 

 the Straits of Florida derived exclusively from this area, the fol- 

 lowing Table would then represent the relative quantity per unit 

 surface possessed by the Atlantic in the three zones, assuming 

 that one half of the heat of the Gulf-stream passes into the arctic 

 regions, and the other half remains to warm the temperate 

 regions* : — 



From the Equator to the Tropic of Cancer . . 773 



From the Tropic of Cancer to the Arctic Circle 848 



From the Arctic Circle to the North Pole . . 610 



* See Phil. Mag. for October 1870, p. 258. 



