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



entire column may be said to be in a state of constant descent, 

 like that which exists in the water of a tall jar when an opening 

 is made at its bottom, and the water which flows away through 

 it is replaced by an equivalent supply poured into the top of 

 the jar." (§ 23.) 



But if this be his theory, as it evidently is, then the 18 foot- 

 pounds (the amount of work performed by the descent of the 

 water down the slope) comprehends all the work that gravitation 

 can perform on a pound of water in making a complete circuit 

 from the equator to the pole and from the pole back to the 

 equator. 



This, I trust, will be evident from the following considera- 

 tions. When a pound of water has flowed down from the equa- 

 tor to the pole, it has descended 18 feet, and is then at the foot 

 of the slope. Gravity has therefore no more power to pull it 

 down to a lower level. It will not sink through the polar water ; 

 for it is not denser than the water beneath on which it rests. 

 But Dr. Carpenter will reply, Although it will not sink through 

 the polar water, it has nevertheless made the polar column 

 heavier than the equatorial, and this excess of pressure forces a 

 pound of water out from beneath and allows the column to 

 descend. Suppose a quantity of water to flow down from the 

 equator so as to raise the level of the polar water by, say, 2 

 feet. The polar column is now heavier than the equatorial by 

 the weight of this 2 feet of water. The pressure of the 2 feet 

 of water on the polar column will force a quantity of water 

 laterally from the bottom and cause the entire column to de- 

 scend till the level of equilibrium is restored. In other words, 

 the polar column will sink 2 feet. Now in the sinking of 

 this column work is performed by gravity. A certain amount 

 of work is performed by gravity in causing the water to fall 

 down the slope from the equator to the pole, and, in addition 

 to this, a certain amount is performed by gravity in the vertical 

 descent of the column. 



I freely admit this to be sound reasoning, and admit that so 

 much is due to the slope and so much to the vertical descent of 

 the water. In my original way of looking at the problem, I re- 

 garded the entire force as due to the slope. In Dr. Carpenter's 

 way of looking at it there is also the slope, but there is, in ad- 

 dition to it, the vertical descent of the water. 



But here we come to the most important point, viz. is there 

 the full slope of 18 feet and an additional vertical movement ? 

 Dr. Carpenter seems to conclude that there is, and that this ver- 

 tical force is something in addition to the force which I derive 

 from the slope. And here, I venture to think, is the radical 

 error into which he has fallen in regard to the whole matter. 



