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



unless he can show that gravity performs work in the way of 

 impelling the water in addition to what I have pointed out, it 

 cannot be said that I have overlooked the influence of any force. 



That neither force nor energy can be derived from the mere 

 descent of the polar column P is easily proved. The reason 

 why the column P descends is because, in consequence of the 

 mass of water P' P resting on it, its weight is in excess of the 

 equatorial column E Q. But the force with which the column 

 descends is equal, not to the weight of the column, but to the 

 weight of the mass P' P ; consequently as much work would be 

 performed by gravity in the descent of the mass P ; P (the two 

 feet of water) alone as in the descent of the entire column P' 0, 

 10,000 feet in height. Suppose a ton weight is placed in each 

 scale of a balance : the two scales balance each other. Piace a 

 pound weight in one of the scales along with the ton weight and 

 the scale will descend. But it descends, not with the pressure 

 of a ton and a pound, but with the pressure of the pound weight 

 only. In the descent of the scale, say, 1 foot, gravity can per- 

 form only 1 foot-pound of work. In like manner, in the descent 

 of the polar column, the only work available is the work of the 

 mass P' P laid on the top of the column. But it must be ob- 

 served that in the descent of the column from P' to P, a distance 

 of 2 feet, each pound of water of the mass P' P does not perform 

 2 foot-pounds of work; for the moment that a molecule of 

 water reaches P, it then ceases to perform further work. The 

 molecules at the surface P' descend 2 feet before reaching P; 

 the molecules midway between P' and P descend only 1 foot 

 before reaching P, and the molecules at the bottom of the mass 

 are already at P, and therefore cannot perform any work. The 

 mean distance through which the entire mass performs work is 

 therefore 1 foot. One foot-pound per pound of water repre- 

 sents in this case the amount of work derived from the vertical 

 movement. 



That such is the case is further evident from the following 

 considerations. Before the polar column begins to descend, it is 

 heavier than the equatorial by the weight of two feet of water ; 

 but when the column has descended one foot, the polar column 

 is heavier than the equatorial by the weight of only one foot of 

 water; and, as the column continues to descend, the force with 

 which it descends continues to diminish, and when it has sunk 

 to P the force is zero. Consequently the mean pressure or 

 weight with which the two feet of water P'P descended was 

 equal to that of a layer of one foot of water ; in other words, 

 each pound of water, taking the mass as a whole, descended 

 with the pressure or weight of half a pound. But a half pound 

 descending two feet performs one foot-pound; so that whether 



