APPENDIX. 395 



which reaches it in the opposite part of its orbit, in the propor- 

 tion of 93^ to 90^, or about as 1000 to 936. Midsummer of the 

 southern hemisphere is the season when the earth is nearest to 

 the sun ; the winter of the southern and the summer of the 

 northern hemisphere occur when the earth is farthest from the 

 source of heat. The conclusion seems inevitable — the southern 

 hemisphere must have hotter summers and colder winters than 

 our hemisphere, where the heat of summer is tempered by the 

 greater distance, and the cold of winter mitigated by the com- 

 parative nearness, of the sun. 



The next point to be considered is the effect of ocean-currents, 

 and especially of the Gulf-stream, in modifying the climatal 

 conditions of some parts of the earth. Following in the track 

 of the late Captain Maury and Principal Forbes, Mr. Croll has 

 especially insisted on the importance of the great current which, 

 issuing from the Gulf of Mexico, and flowing northward between 

 Florida and the Bahamas, extends across the Atlantic towards 

 the western shores of Europe. He calculates that by this 

 current alone an amount of heat equal to that received on the 

 entire surface of the earth in a zone thirty-two miles in breadth 

 on each side of the equator is carried from the tropics to the 

 cooler regions of the northern hemisphere. Mr. Croll has, I 

 think, victoriously replied to several of the objections opposed 

 to this portion of his argument. His estimate of the volume of 

 water transferred by the Gulf-stream from the tropics to the 

 northern part of the Atlantic, which he reckons at the annual 

 amount of about 166,000 cubic miles, is, I think, in no degree 

 exaggerated ; and I also think that he is warranted in estimating 

 the mean initial temperature at about 65° Fahr. I am, however, 

 persuaded that in assuming 40° Fahr. as the temperature to 

 which, on an average, this vast body of water is reduced before 

 it returns to the equatorial zone, Mr. Croll has gone beyond the 

 probable limit. A large part of the stream is diverted eastward 

 about the latitude of the Azores, and is never cooled much 

 below 55° Fahr. before the waters enter the return current on 

 the eastern side of the Atlantic basin ; and I believe that, if we 

 allow the water of the Gulf-stream to undergo an average loss 

 of temperature of 20° Fahr., we shall be more likely to exaggerate 

 than to underrate the amount of cooling. 



In insisting on the importance of the Gulf-stream in modify- 



