Mr. J. Croll on the Physical Cause of Ocean-currents. 261 



would merely destroy the uniformity of the slope without aiding 

 in the least degree the general motion of the water. 



I trust, from what has been already stated, it is obvious that 

 the " primum mobile " of Dr. Carpenter has in reality no exist- 

 ence, and that the energy derived from the full slope, whatever that 

 slope may be, comprehends all that can possibly be obtained from 

 gravity. This being the case, there is, according to Dr. Carpen- 

 ter's own admission, not sufficient power to produce the circulation 

 which he assumes. 



If this full slope, as has been proved by direct experi- 

 ment *, be not sufficient to produce even sensible movement of 

 the water from the equator to the pole, a thing which Dr. Car- 

 penter seems to admit f, how much less able is it in addition to 

 this to overcome the resistance to the motion of the water in 

 the horizontal undercurrent and in the vertical descending and 

 ascending currents. 



The slope, according to Dr. Carpenter's data, ought to be less 

 than 18 feet. 



But is there in reality a slope of 18 feet ? Is the equatorial 

 column 18 feet higher than the polar ? My calculations of the 

 differences in the height of the two columns, as has been stated 

 on a former occasion, were made on the assumption that " the 

 temperature of the ocean at the equator decreases at a uniform 

 rate from the surface downwards, which is far from being the 

 case. The rate of decrease is most rapid at the surface, and 

 decreases as we descend. The principal part of the decrease of 

 temperature takes place within no very great depth from the 

 surface ; consequently the greater part of the excess of tempe- 

 rature at the equator over that at the poles affects the sea to no 

 great depth "J. It therefore follows that the actual slope must 

 be under 18 feet. I am glad to find that Dr. Carpenter agrees 

 with me, that the principal part of the excess of temperature at 

 the equator is at the surface, and does not extend to any great 

 depth. 



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

 smface-heat upon the waters of the tropical basin will be for the 

 most part limited (as I shall presently show, § 40) to its upper- 

 most stratum, and may here be practically disregarded. But the 

 effect of surface-cold upon the water of the polar basin will be 



* Phil. Mag. Oct. 1870, p. 254. 



f See his footnote to § 25. 



X Phil. Mag. for October 1870, p. 249. 



