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



foot-pounds of work is performed. But this 2 foot-pounds, 

 added to the 16 foot-pounds, gives simply 18 foot-pounds. 

 Therefore, so far as the amount of work is concerned, it is the 

 same whether the pound of water is supposed to descend through 

 the full slope of 18 feet, or to descend first through a slope of 

 16 feet and then vertically through the remaining 2 feet. 



Perhaps the following diagram will help to make the point 

 still clearer. 



Kg. 1. 



Let P (fig. 1) be the surface of the ocean at the pole, and E 

 the surface at the equator ; P a column of water at the pole, 

 and E Q a column at the equator. The two columns are of equal 

 weight and balance each other; but as the polar water is colder, 

 and consequently denser than the equatorial, the polar column 

 is shorter than the equatorial, the difference in the length of the 

 two columns being 18 feet. The surface of the ocean at the 

 equator E is 18 feet higher than the surface of the ocean at the 

 pole P ; there is therefore a slope of 18 feet from E to P. The 

 molecules of water at E tend to flow down this slope towards P. 

 The amount of work performed by gravity in the descent of a 

 pound of water down this slope from E to P is therefore 18 

 foot-pounds. This represents the state of things in the way in 

 which I have viewed the problem. Dr. Carpenter does not ob- 

 ject to all this. It is an essential part of his theory that there is 

 a slope from the equator to the pole. The amount of this slope 

 is, in his way of looking at the question, as in mine, proportional 

 to the difference of density between the polar column and the 

 equatorial ; and as he has not called in question my estimate of 

 the difference of density of the two columns, I presume that he 

 admits that the slope is about 18 feet. And of course he will 

 admit that, in the descent of a pound of water down this slope, 

 18 foot-pounds of work is performed by gravity. But he repre- 

 sents the operation as occurring thus : — As the equatorial column 

 is higher than the polar, when the two are in equilibrium water 

 tends to flow down this slope from the equator to the pole as a 

 surface- current. He assumes this flow to continue till the level 



