Mr. J. Croll on the Physical Cause of Ocean-currents. 251 



But let the point at issue be here clearly and distinctly un- 

 derstood. In my last paper I showed that the difference of tem- 

 perature between the sea in equatorial and polar regions makes a 

 difference of level of 18 feet, and that the force of gravity derived 

 from a slope so small is insufficient to produce the circulation 

 which Dr. Carpenter supposes. Dr. Carpenter admits this, but 

 maintains that there is a vertical descent of the water at the 

 poles, and that the force derived from this vertical descent is the 

 primum mobile of the circulation, — in short, that though the force 

 derived from the mere slope is insufficient, nevertheless when we 

 take into account the force of the vertical descent of the water, a 

 thing which he supposes I have overlooked, we have a cause per- 

 fectly sufficient to produce the necessary circulation. This is 

 the general conclusion arrived at by Dr. Carpenter. Now let it 

 be observed that I admit, when water circulates from difference 

 of specific gravity, this vertical movement is just as real a part 

 of the process as the flow down the slope ; but the point which 

 I maintain is that there is no additional power derived from this 

 vertical movement over and above what is derived from the full 

 slope — or, in other words, that this primum mobile, which be 

 says I have overlooked, has in reality no existence. Now, if I 

 manage to establish this point, I trust it will be obvious to the 

 reader that Dr. Carpenter's theory is untenable. 



In Dr. Carpenter's way of looking at the problem, as in mine, 

 gravity can perform only 18 foot-pounds of work per pound 

 of water. In his way of treating the problem, the amount 

 of work performed by the pound of water in first flowing- 

 down the slope and then in descending vertically, when added 

 together, amounts to exactly 18 foot-pounds. The reason is 

 obvious ; for whatever work is gained by vertical descent is 

 just so much deducted from the work of descending the slope. 

 If the pound of water on reaching the pole descends verti- 

 cally 2 feet, it must have descended only 16 feet in coming 

 down the slope. Suppose water has flowed down upon the polar 

 seas so as to restore the level by, say, 2 feet ; the polar column 

 is now too heavy by the extent of 2 feet of water, and the slope 

 is therefore reduced from 18 feet to 16 feet. Suppose that 

 while this condition of things remains, a pound of water leaves 

 the equator and flows down the slope to the pole. In perform- 

 ing this journey it descends only 16 feet, and consequently only 

 16 foot-pounds of work is performed. The pressure of the 2 

 feet of water on the top of the polar column is supposed now to 

 begin to act ; water is pressed out laterally from beneath ; the 

 column descends 2 feet, and equilibrium between the equatorial 

 and polar columns is restored. The pound of water in this pro- 

 cess of course descends vertically 2 feet, and consequently 2 



