84 Mr. J. Croll on Ocean-currents in relation to 



of the whole zone, is ^-^- of that received at the equator; con- 

 sequently the quantity of heat conveyed by the Gulf-stream in 

 one year is equal to the heat which falls on an average on 

 6,873,800 square miles of the arctic regions. The frigid zone or 

 arctic regions contain 8,130,000 square miles. There is actually, 

 therefore, nearly as much heat transferred from tropical regions 

 by the Gulf-stream as is received from the sun by the entire 

 arctic regions, the quantity conveyed by the stream to that 

 received from the sun by those regions being as 15 to 18. 



But we have been assuming in our calculations that the per- 

 centage of heat absorbed by the atmosphere is no greater in polar 

 regions than it is at the equator, which is not the case. If we 

 make due allowance for the extra amount absorbed in polar 

 regions in consequence of the obliqueness of the sun's rays, the 

 total quantity of heat conveyed by the Gulf-stream will probably 

 nearly equal the amount received from the sun by the entire 

 arctic regions. 



If we compare the quantity of heat conveyed by the Gulf- 

 stream with that conveyed by means of aerial currents, the result 

 is equally startling. 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 ; con- 

 sequently 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 

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

 3234 times the volume of the Gulf-stream, at the same tem- 

 perature and moving with the same velocity. Taking, as before, 

 the width of the stream at 50 miles, and its depth at 10C0 feet, 

 and its velocity at 4 miles an hour, it follows that, 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 J 

 mile deep, and at the temperature of 65°, blowing at the rate of 

 four miles an hour from every part of the equator over the north- 

 ern hemisphere towards the pole. If its velocity were equal to 

 that of a good sailing-breeze, which Sir John Herschel states to 

 be about twenty-one miles an hour, the current would require to 

 be above 1200 feet deep. A greater quantity of heat is probably 

 conveyed by the Gulf-stream alone from the tropical to the tem- 

 perate and arctic regions than by all the aerial currents which 

 flow from the equator. 



We are apt, on the other hand, to overestimate the amount 

 of the heat conveyed from tropical regions to us by means of 

 aerial currents. The only currents which flow from the equa- 

 torial regions are the upper currents, or anti-trades as they are 

 called. But it is not possible that much heat can be conveyed 

 directly by them. The upper currents of the trade-winds, even 



