Mr. J. Croll on the Physical Cause of Ocean-currents. 121 



before, P C represents the polar column and E W the equatorial 

 column. 



Fig. 2. 



It will be observed that the water in that wedge-shaped por- 

 tion W C W forming the incline cannot be in a state of static 

 equilibrium. A molecule of water at O, for example, will be 

 pressed more in the direction of C than in the direction of W, 

 and the amount of this excess of pressure towards C will depend 

 upon the height of W above the line C W. It is evident that 

 the pressure tending to move the molecule at towards C will 

 be far greater than the direct pull of gravity tending to draw a 

 molecule at 0' lying on the surface of the incline towards C. 

 The experiments of M. Dubuat prove that the direct force of 

 gravity will not move the molecule at 0' — that is, cause it to roll 

 down the incline W C ; but they do not prove that it may not 

 yield to pressure from above, or that the pressure of the column 

 W W will not move the molecule at 0. The pressure is caused 

 by gravity, and cannot, of course, enable gravity to perform more 

 work than what is derived from the energy of gravity ; it will 

 enable gravity, however, to overcome resistance, which it could 

 not do by direct action. But whether the pressure resulting 

 from the greater height of the water at the equator due to its 

 higher temperature be actually sufficient to produce displacement 

 of the water is a question which I am wholly unable to answer. 



If we suppose 9 feet to be the height of the equatorial surface 

 above the polar required to make the two columns balance each 

 other, the actual difference of level between the two columns will 

 certainly not be more than one half that amount, because, if a 

 circulation exist, the weight of the polar column must always be 

 in excess of that of the equatorial. But this excess can only be 

 obtained at the expense of the surface-slope, as was shown at 

 length in my last paper. The surface- slope probably will not 

 exceed more than 4 feet or 4J feet. Suppose the ocean to be of 

 equal density from the poles to the equator, and that by some 

 means or other the surface of the ocean at the equator is raised, 

 say, 4 feet above that of the poles, then there can be little 



