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



polar cold, rather than equatorial heat, is the primum mobile of 

 this circulation"*. 



The influence of the sun in heating the waters of the intertro- 

 pical seas is, in Dr. Carpenter's manner of viewing the problem, 

 of no great importance. The efficient cause of motion he con- 

 siders resides in cold rather than in heat. In fact he even goes 

 the length of maintaining that, as a power in the production of 

 the general interchange of equatorial and polar water, the effect 

 of polar cold is so much superior to that of intertropical heat, 

 that the influence of the latter may he practically disregarded. 



" Suppose two basins of ocean-water," he says, " connected 

 by a strait to be placed under such different climatic conditions 

 that the surface of one is exposed to the heating influence of 

 tropical sunshine, whilst the surface of the other is subjected to 

 the extreme cold of the sunless polar winter. The effect of the 

 surface-heat upon the water of the tropical basin will be for the 

 most part limited (as I shall presently show) to its uppermost 

 stratum, and may here be practically disregarded f. 



Dr. Carpenter's idea regarding the efficiency of cold in pro- 

 ducing motion seems to me to be not only opposed to the gene- 

 rally received views on the subject, but wholly irreconcilable 

 with the ordinary principles of mechanics. In fact there are 

 so many points on which Dr. Carpenter's theory of a " General 

 Vertical Oceanic Circulation M differs from the generally received 

 views on the subject of circulation by means of difference of 

 specific gravity, that I have thought it advisable to enter some- 

 what minutely into the consideration of the mechanics of that 

 theory, the more so as he has so repeatedly asserted that eminent 

 physicists agree with what he has advanced on the subject. 



According to the generally received theory, the circulation is 

 due to the difference of density between the sea in equatorial and 

 polar regions. The real efficient cause is gravity ; but gravity 

 cannot act when there is no difference of specific gravity. If 

 the sea were of equal density from the poles to the equator, 

 gravity could exercise no influence in the production of circula- 

 tion ; and the influence which it does possess is in proportion to 

 the difference of density. But the difference of density between 

 equatorial and polar waters is in turn due not absolutely either 

 to polar cold or to tropical heat, but to both — or, in other words, 

 to the difference of temperature between the polar and equatorial 

 seas. This difference, in the very nature of things, must be as 

 much the result of equatorial heat as of polar cold. If the sea 

 in equatorial regions were not being heated by the sun as rapidly 

 as the sea in polar regions is being cooled, the difference of tempe- 



* Proc. Roy. Geog. Soc. January 9, 18/1. t Ibid. 



