288 Mr. J. Croll on the Wind Theory 



as much a necessary result of the wind theory as of the gravi- 

 tation theory, and that there is no relation whatever between 

 the wind theory and warm water in the'depths of the sea. 



It is supposed that the return under- currents from the polar 

 regions are by far too insignificant to be able to maintain at a 

 polar temperature the great depths of the ocean. 



Let us examine this objection. It is freely admitted, nay 

 even strenuously maintained by the advocates of the gravitation 

 theory themselves, that the heating-power of the sun does not 

 extend to any great depth below the surface of the ocean ; con- 

 sequently there is nothing whatever to heat this mass of water 

 underneath except 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 of no great mag- 

 nitude would be sufficient to keep the mass at an ice-cold tem- 

 perature. 



On a former occasion* I showed that, taking the rate at 

 which internal heat passes through the earth's surface to be 

 that assigned by Sir William Thomson, the total amount re- 

 ceived per annum by the North Atlantic, between the equator 

 and tropic of Cancer, including the Caribbean Sea, is equal to 

 only ^ ¥ of that conveyed by the Gulf-stream, on the supposi- 

 tion that each pound of water carries 19,300 foot-pounds of 

 heat, — and that consequently an under-current from the polar 

 regions of not more than ¥ j 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 no heat derived from the 

 crust of the earth, and an under-current of less than -'y that 

 of the Gulf-stream coming from the polar regions would keep 

 the entire North Atlantic from the equator to the arctic circle 

 filled with ice-cold water. A polar under-current half the size of 

 the Gulf-stream would be sufficient to keep the entire water of 

 the globe (below the stratum heated by the sun's rays) at an 

 ice-cold temperature. Internal heat would not be sufficient 

 under such circumstances to maintain the mass 1° Fahr. above 

 the temperature it possessed when it left the polar regions. 



In short, whatever theory we adopt regarding oceanic circu- 

 lation, 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 con- 

 tinually coming from the polar regions, either in the form of 

 under-currents, or in the form of a general underflow as Dr. 

 Carpenter supposes, the entire under portion of the ocean must 

 ultimately become occupied by cold water; for there is no 



* Philosophical Magazine, June 1874 ; ' Nature, 5 vol. x. p. 52. 



