Mr. J. Croll on the Physical Cause of Ocean-currents. 247 



also this power, whatever may be the way in which it acts, is 

 far more effective in producing motion than the slope ; for he 

 regards it as the primum mobile of the whole affair, whereas the 

 tendency of the water to run down the slope is regarded as a 

 secondary matter. 



It is perfectly true that I have not in my paper on the sub- 

 ject taken into account any such agency as this supposed by 

 Dr. Carpenter, for the simple reason that I know of no such 

 agency. With the exception of an exceedingly trifling force, 

 which I did not deem worthy of being taken into account, and 

 to which I shall have occasion shortly to refer, I know of no 

 possible agency arising from difference of specific gravity tend- 

 ing to produce circulation other than the force impelling the water 

 down the slope. 



The Work performed by Gravity. — But in order clearly to 

 understand this point, it will be better to treat the matter ac- 

 cording to the third method, and consider not the mere force of 

 gravity impelling the waters, but the amount of work which 

 gravitation is capable of performing. 



Assuming, then, the correctness of my estimate, that the height 

 of the surface of the ocean at the equator above that at the poles 

 is 18 feet, a pound of water in flowing down the slope from the 

 equator to either of the poles will perform 18 foot-pounds of 

 work. Or, more properly speaking, in the descent of a pound of 

 water down this slope from the equator to the pole gravitation 

 performs 18 foot-pounds of work. Now it is evident that 

 when this pound of water has reached the pole, it is at the bot- 

 tom of the slope, and consequently cannot descend further. 

 Therefore gravity cannot perform any more work upon it ; for 

 gravity cannot perform work unless the thing acted upon de- 

 scend — that is, moves under the force exerted. But the water will 

 not move under the influence of gravity unless it move down- 

 ward ; for it is only in this direction that gravity acts on the 

 water. " But," says Dr. Carpenter, " the effect of surface-cold 

 upon the water of the polar basin will be to reduce the tempe- 

 rature of its whole mass below the freezing-point of fresh water, 

 the surface stratum sinking as it is cooled in virtue of its dimi- 

 nished bulk and increased density, and being replaced by water 

 not yet cooled to the same degree." (§ 22.) The cooling of the 

 whole mass of polar water by cold and the heating of the water 

 at the equator by the sun's rays make, of course, as we have just 

 seen, the polar column of water to be denser than the equatorial 

 one, and consequently, in order that the two may balance each 

 other, the polar column is shorter than the equatorial by 18 

 feet ; and the slope of 18 feet to which we allude is thus formed. 

 It is perfectly true that the water which leaves the equator warm 



