Mr. J. Croll on the Physical Cause of Ocean- currents. 113 



ence of the circulation. The water being assumed to be the 

 same in both columns and differing only as regards tempera- 

 ture, and the equatorial column possessing more heat than the 

 polar, and being therefore less dense than the latter, it follows, 

 in order that the two columns may be in static equilibrium, that 

 the surface of the equatorial column must stand at a higher 

 level than that of the polar. This produces the slope W C from 

 the equator to the pole. The extent of the slope will of course 

 depend upon the extent of the difference of their temperatures. 

 But, as was shown on a former occasion (Phil. Mag. for Oct. 

 1871), it is impossible that static equilibrium can ever be fully 

 obtained, because the slope occasioned by the elevation of the 

 equatorial column above the polar produces what we may be 

 allowed to call a molecular disturbance of equilibrium. The 

 surface of the ocean, or the molecules of water lying on the 

 slope, are not in a position of equilibrium, but tend, in virtue 

 of gravity, to roll down the slope in the direction of the polar 

 column C. It will be observed that the more we gain of static 

 equilibrium of the entire ocean the greater is the slope, and 

 consequently the greater is the disturbance of molecular equi- 

 librium ; and, vice versa, the more molecular equilibrium is 

 restored by the reduction of the slope, the greater is the disturb- 

 ance of static equilibrium. It is therefore absolutely impossible 

 that both conditions of equilibrium can be fulfilled at the same time 

 so long as a difference of temperature exists between the two 

 columns. And this conclusion holds true even though we should 

 assume water to be a perfect fluid absolutely devoid of viscosity. 

 It follows, therefore, that a general oceanic circulation without a 

 difference of level is a mechanical impossibility. 



In a case of actual circulation due to difference of gravity, 

 there is always a constant disturbance of both static and mole- 

 cular equilibrium. Column C is always higher and column W 

 always lower than it ought to be were the two in equilibrium ; 

 but they never can be at the same leveL 



It is quite conceivable, of course, that the two conditions of 

 equilibrium may be fulfilled alternately. We can conceive 

 column C remaining stationary till the water flowing from 

 column W has restored the level. And after the level is re- 

 stored we can conceive the polar column C sinking and the 

 equatorial column W rising till the two perfectly balance each 

 other. Such a mode of circulation, consisting of an alternate 

 surface-flow and vertical descent and ascent of the columns, 

 though conceivable, is in reality impossible in nature ; for there 

 are no means by which the polar column C could be supported 

 from sinking till the level had been restored. But Dr. Carpen- 

 ter does not assume that the general oceanic circulation takes 

 Phil. Mag. S. 4. Vol. 47. No. 310. Feb. 1874. \ 



