Mr. J. Croll on the Physical Cause of Ocean-currents. 249 



18 + 1 = 19 foot-pounds as the total amount that could be per- 

 formed by gravitation on the pound of water from the time that 

 it left the equator till it returned to where it started. The 

 amount of work performed in descending the slope from the 

 equator to the pole and in sinking to a depth of 10,000 feet or 

 so through the polar water assumed to be warmer than the sur- 

 face-water, comprehends the total amount of work that gravita- 

 tion can possibly perform ; so that the amount of force gained 

 by such a supposition over and above that derived from the slope 

 is trifling. 



But it would appear that this, after all, is not what Dr. Car- 

 penter means, but something entirely different. What he means 

 seems to be this ; when a quantity of water, say a layer one foot 

 thick, flows down from the equator to the pole, the polar column 

 becomes then heavier than the equatorial by the weight of this 

 additional layer. A layer of water equal in quantity is therefore 

 pressed away from the bottom of the column and flows off in the 

 direction of the equator as an undercurrent, the polar column at 

 the same time sinking down one foot until equilibrium of the 

 polar and equatorial columns is restored. Another foot of water 

 now flows down upon the polar column and another foot of water 

 is displaced from below, causing, of course, the column to de- 

 scend an additional foot. The same process being continually 

 repeated, a constant downward motion of the polar column is 

 the result. Or perhaps, to express the matter more accurately, 

 owing to the constant flow of water from the equatorial regions 

 down the slope, the weight of the polar column is kept always 

 in excess of that of the equatorial ; therefore the polar column 

 in the effort to restore equilibrium is kept in a constant state 

 of descent. Hence he terms it a " vertical " circulation. The 

 following will show Dr. Carpenter's theory in his own words. 



" The action of cold on the surface-water of each polar area will 

 be exerted as follows : — 



" (a) In diminishing the height of the polar column as com- 

 pared with that of the equatorial, so that a lowering of its 

 level is produced, which can only be made good by a surface- 

 flow from the latter towards the former. 



" (b) In producing an excess in the downward pressure of the 

 column when this inflow has restored its level, in virtue of 

 the increase of specific gravity it has gained by its reduction in 

 volume ; whereby a portion of its heavy bottom-water is displaced 

 laterally, causing a further reduction of level, which draws in a 

 further supply of the warmer and lighter water flowing towards 

 its surface. 



" (c) In imparting a downward movement to each new surface- 

 stratum as its temperature undergoes reduction; so that the 



