and its Physical Relations to the Glacial Epoch. 127 



difying causes which would preveut the possibility of the sum- 

 mer temperature ever falling so low as 27° or even 36°; for sur- 

 rounded by a warm sea our summers could at this time no more 

 have been cold than during the glacial epoch they could have 

 been warm when the land was covered with cold ice. 



I feel satisfied that it will yet turn out, when the subject has 

 been better investigated, that the influence of ocean-currents in 

 modifying the climate of the polar regions of the globe has not 

 been duly estimated. Were there no ocean-currents, and the 

 polar regions to depend alone upon the direct heat of the sun, 

 the temperature of those regions would be enormously below 

 what it actually is. The difference of temperature between the 

 equatorial and polar regions of the globe is far less than it would 

 otherwise be, did the temperature of each zone depend alone 

 upon the quantity of heat received directly from the sun. A 

 very considerable amount of the temperature of- polar regions is 

 due to the heat absorbed by the ocean in equatorial regions, and 

 conveyed there by ocean- currents. This process tends to lower 

 the temperature of the equatorial regions and raise the tempera- 

 ture of the polar, and thus reduce the difference between the two. 



The truth of what has just been stated will be obvious if we 

 simply reflect on the quantity of heat transferred from the equa- 

 torial to the northern regions by one stream alone, namely the 

 Gulf-stream. I have not been able to find any trustworthy 

 estimate of the actual quantity of water conveyed by the Gulf- 

 stream. From a rough estimate made after an examination of the 

 Charts of the United States Coast Survey, I believe that the total 

 quantity of water transferred is at least equal to a stream fifty 

 miles broad and 1000 feet deep, flowing at the rate of four miles 

 an hour; and the mean temperature of the entire mass of 

 moving water is not under 65° at the moment of leaving the 

 Gulf. Captain Maury considers the Gulf-stream equal to a 

 stream 32 miles broad and 1200 feet deep, flowing at the rate of 

 five knots an hour*. This is a somewhat higher estimate. 

 Now the density of air to that of water is as 1 to 770, and its 

 specific heat to that of water is as 1 to 4*2. Consequently 

 the same amount of heat that would raise 1 cubic foot of water 

 1°, would raise 770 cubic feet of air 4°*2, or 3234 cubic feet 

 1°. The quantity of heat conveyed by the Gulf-stream is there- 

 fore equal to that which would be conveyed by a current of air 

 3234 times the volume of the Gulf-stream and at the same 

 temperature and moving with the same velocity. In order to 

 convey an equal amount of heat from the tropics by means of 

 an aerial current, it would be necessary to have a current about 

 1 \ mile deep and at the temperature of 65° blowing at the rate of 

 * Physical Geography of the Sea, § 24. 



