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



Gravity then exerts more force in pulling the polar column down 

 than it does in preventing the equatorial column from rising ; 

 and the consequence is that the polar column begins to descend 

 and the equatorial column to rise. But as the polar column 

 continues to descend and the equatorial to rise, the power of 

 gravity to produce motion in the polar column diminishes, and 

 the power of gravity to prevent motion in the equatorial column 

 increases ; and when P ; descends to P and E' rises to E, the 

 power of gravity to prevent motion in the equatorial column is 

 exactly equal to the power of gravity to produce motion in the 

 polar column, and consequently motion ceases. It therefore 

 follows that the entire amount of work performed by the descent 

 of P' P is spent in raising E' E against gravity. 



It follows also that inequalities in the sea-bottom cannot in 

 any way aid the circulation ; for although the cold undercurrent 

 should in its progress come to a deep trough filled with water 

 less dense than itself, it would no doubt sink to the bottom of 

 the hollow ; but before it could get out again as much work 

 would have to be performed against gravity as was performed 

 by gravity in sinking it. But whilst inequalities in the bed of 

 the ocean would not aid the current, they would nevertheless 

 very considerably retard it by the obstructions which they would 

 offer to the motion of the water. 



We have been assuming that the weight of P' P is equal to 

 that of E E' ; but the mass P' P must be greater than E E', be- 

 cause P' P has not only to raise E E', but it has to impel the 

 undercurrent — to push the water along the sea-bottom from the 

 pole to the equator. So we must have a mass of water, in addi- 

 tion to P' P, placed on the polar column to enable it to produce 

 the undercurrent in addition to the raising of the equatorial 

 column. 



It follows also that the amount of work which can be per- 

 formed by gravity depends entirely on the difference of tempe- 

 rature between the equatorial and the polar waters, and is 

 wholly independent of the way in which the temperature may 

 decrease from the equator to the poles. Suppose, in agreement 

 with Dr. Carpenter's idea ('Nature' for July 6, 1871), that the 

 equatorial heat and polar cold should be confined to limited 

 areas, and that through the intermediate space no great differ- 

 ence of temperatnre should prevail. Such an arrangement as 

 this would not increase the amount of work which gravity could 

 perform ; it would simply make the slope steeper at the two 

 extremes and flatter in the intervening space. It would no 

 doubt aid the surface-flow of the water near the equator and 

 the poles, but it would retard in a corresponding degree the 

 flow of the water in the intermediate regions. In short, it 



