Mr. J. Croll on the Physical Cause of Ocean-currents, 111 



rature between them, and consequently the difference of density, 

 would be diminishing, and in course of time would disappear 

 altogether. As has already been shown, it is a necessary con- 

 sequence that the water flowing from equatorial to polar regions 

 must be compensated by an equal amount flowing from polar to 

 equatorial regions. Now, if the water flowing from polar to 

 equatorial regions were not being heated as rapidly as the water 

 flowing from equatorial to polar regions is being cooled, the 

 equatorial seas would gradually become colder and colder until 

 no sensible difference of temperature existed between them and 

 the polar oceans. In fact, equality of the two rates is necessary 

 to the very existence of such a general circulation as that advo- 

 cated by Dr. Carpenter. If he admits that the general inter- 

 change of equatorial and polar water advocated by him is caused 

 by the difference of density between the water at the equator 

 and the poles, resulting from difference of temperature, then he 

 must admit also that this difference of density is just as much 

 due to the heating of the equatorial water by the sun as it is to 

 the cooling of the polar water by radiation and other means — or, 

 in other words, that it is as much due to equatorial heat as to 

 polar cold. And if so, it cannot be true that polar cold rather 

 than equatorial heat is the " primum mobile" of this circulation; 

 and far less can it be true that the heating of the equatorial 

 water by the sun is of so little importance that it may be " prac- 

 tically disregarded. " 



Supposed influence of Heat derived from the Earth's Crust. — 

 There is, according to Dr. Carpenter, another agent concerned 

 in the production of the general oceanic circulation, viz. the 

 heat derived by the bottom of the ocean from the crust of the 

 earth (see §§ 20, 34*; also Brit. Assoc. Report for 1872, p. 49, 

 and other places). We have no reason to believe that the quan- 

 tity of internal heat coming through the earth's crust is greater 

 in one part of the globe than in another; nor have we any 

 grounds for concluding that the bottom of intertropical seas re- 

 ceives more heat from the earth's crust than the bottom of those 

 in polar regions. But if the polar seas receive as much heat 

 from this source as the seas within the tropics, then the differ- 

 ence of density between the two cannot possibly be due to heat 

 received from the earth's crust; and this being so, it is mecha- 

 nically impossible that internal heat can be a cause in the pro- 

 duction of the general oceanic circulation. 



Circulation without Difference of Level. — There is another part 

 of the theory which appears to me irreconcilable with mecha- 

 nics. It is maintained that this general circulation takes place 

 without any difference of level between the equator and the poles. 

 Referring to the case of the two cylinders W and C, which re* 



