Mr. J. Croll on the Physical Cause of Ocean-currents. 435 



the equator; (2) that it cannot be the reflux of the Gulf-stream, 

 because its sectional area is 900 times as great as that of the 

 Gulf-stream ; (3) that the immense mass of water is, according 

 to my views, set in motion by the Gulf-stream. 



I shall consider these in their order. (1) That this immense 

 mass of cold water came originally from the polar regions I, of 

 course, admit, but that the whole is in a state of motion I cer- 

 tainly do not admit. There is no warrant whatever for any such 

 assumption. According to Dr. Carpenter himself, the heating- 

 power of the sun does not extend to any great depth below the 

 surface ; consequently there is nothing whatever to heat this mass 

 but the heat coming through the earth's crust. But the amount 

 of heat derived from this source is so trifling, that an under- 

 current from the Arctic regions far less in volume than that of 

 the Gulf-stream would be quite sufficient to keep the mass at 

 an ice-cold temperature. Taking the area of the North Atlantic 

 between the equator and the tropic of Cancer, including also 

 the Caribbean Sea and the Gulf of Mexico, to be 7,700,000 

 square miles, and the rate at which internal heat passes through 

 the earth's surface to be that assigned by Sir William Thomson, 

 we find that the total quantity of heat derived from the earth's 

 crust by the above area is equal to about 88 x 10 15 foot-pounds 

 per day. But this amount is equal to only gl ? that conveyed 

 by the Gulf-stream, on the supposition that each pound of water 

 carries 19,300 foot-pounds of heat. Consequently an under- 

 current from the polar regions of not more than -^ the volume 

 of the Gulf-stream would suffice to keep the entire mass of water 

 of that area within 1° of what it would be were there no heat 

 derived from the crust of the earth ; that is to say, were the 

 water conveyed by the under-current at 32°, internal heat would 

 not maintain the mass of the ocean in the above area at more 

 than 33°. The entire area of the North Atlantic from the 

 equator to the arctic circle is somewhere about 16,000,000 square 

 miles. An under-current of less than ^ that of the Gulf- 

 stream coming from the Arctic regions would therefore suffice to 

 keep the entire North Atlantic basin filled with ice-cold water. 

 In short, whatever theory we adopt regarding oceanic circulation, 

 it follows equally as a necessary consequence that the entire mass 

 of the ocean below the stratum heated by the sun's rays must 

 consist of cold water. For if cold water be continually coming 

 from the polar regions either in the form of under-currents, or 

 in the form of a general under-flow as Dr. Carpenter supposes, 

 the entire under portion of the ocean must ultimately become 

 occupied by cold water ; for there is no source from which this 

 influx of water can derive heat, save from the earth's crust. But 

 the amount thus derived is so trifling as to produce no sensible 



2F2 



