Mr. J. Croll on the Physical Cause of Ocean-currents, 257 



has descended 1 foot the equatorial column will have risen 1 

 foot. The pressure of the descending polar column will now 

 be reduced to 2 feet of water. And when the polar column has 

 descended another foot, P will have reached P, and E' will have 

 reached E ; the two columns will then be in equilibrium. It 

 therefore follows that the mean pressure with which the polar 

 column descended the 2 feet was equal to the pressure of 2 feet 

 of water. Consequently the mean amount of work performed 

 by the descent of the mass was equal to 2 foot-pounds per pound 

 of water; this, added to the 16 foot-pounds derived from the 

 slope, gives a total of 18 foot-pounds. 



In whatever way we view the question, we are led to the con- 

 clusion that if 18 feet represent the amount of slope between the 

 equatorial and polar columns when the two are in equilibrium, 

 then 18 foot-pounds is the total amount of work that gravity 

 can perform upon a pound of water in overcoming the resistance 

 to motion in its passage from the equator to the pole down the 

 slope, and then in its vertical descent to the bottom of the ocean. 



But it will be replied, not only does the 2 feet of water P' P 

 descend, but the entire column P 0, 10,000 feet in length, de- 

 scends also. What, then, it will be asked, becomes of the force 

 which gravity exerts in the descent of this column ? We shall 

 shortly see that this force is entirely applied to the overcoming 

 of the resistance offered by gravity to the motion of the water 

 in other parts of the circuit ; so that not a single foot-pound 

 of this force goes to overcome cohesion, friction, and other 

 resistances; it is all spent in counteracting the efforts which 

 gravity exerts to stop the current in another part of the circuit. 

 This vertical descent is therefore, not as Dr. Carpenter concludes, 

 some power which I had omitted to take into account in my 

 former determinations. 



I shall now consider the next part of the movement, viz. the 

 under or return current from the bottom of the polar column 

 to the bottom of the equatorial. What produces this current ? 

 It is needless to say that it cannot be caused directly by gra- 

 vity. Gravitation cannot directly draw any body horizontally 

 along the earth's surface. The water that forms this current 

 is pressed out laterally by the weight of the polar column, and 

 flows, or rather is pushed, towards the equator to supply the 

 vacancy caused by the ascent of the equatorial column. There 

 is a constant flow of water from the equator to the poles along 

 the surface, and this draining of the water from the equator 

 is supplied by the under or return current from the poles. 

 But the only power which can impel the water from the bottom 

 of the polar column to the bottom of the equatorial column 

 is the pressure of the polar column. But whence does the polar 



Phil. Mag. S. 4. Vol. 42. No. 280. Oct. 187L S 



