Mr. J. Croll on the Physical Cause of Ocean-currents. 265 



whether or not 9 foot-pounds is sufficient to carry a pound of 

 water from the equator to the pole as a surface-current, and 

 back from the pole to the equator as an undercurrent. 



Dr. Carpenter must either show that the total amount of work 

 that can be performed by gravity on a pound of water in its 

 entire circuit from the equator to the pole and back to where it 

 started exceeds 9 foot-pounds, or else that the mean force of the 

 resistances to the motion of the pound of water is under xoVo" 

 of a grain. Unless he can do this, he is not warranted to con- 

 clude that the General Oceanic Circulation which he advocates 

 is even a physical possibility, far less a physical necessity. 



But this force of ^^ of a grain, infinitesimal as it is, holds 

 true only in regard to the water at the surface. The power of 

 gravity to produce motion decreases rapidly as we proceed down- 

 wards ; at a short distance below the surface it may be practi- 

 cally disregarded. 



In determining whether a certain slope be sufficient to produce 

 motion of the water, there is a distinction of the utmost import- 

 ance which must be borne in mind; viz. we must consider 

 whether the slope be caused by difference of specific gravity or by 

 some other agency, such, for example, as the wind heaping up 

 the water. Suppose the ocean to be the same in density from the 

 equator to the poles, and that by some means or other the water 

 should be so heaped up or raised at the equator as to produce a 

 difference of level of 18 feet between the equator and the poles. 

 It is more than probable that the force of gravity would in this 

 case be sufficient to cause such a motion of the water as would 

 restore the level of the ocean ; for the ocean would not be in a 

 state of equilibrium when the water stood 18 feet higher at the 

 equator than at the poles, because the equatorial column would 

 exceed the polar by the weight of this 18 feet of water; conse- 

 quently the entire weight of this mass would be employed in 

 restoring the level. But when the rise of 18 feet at the equator 

 is the result of specific gravity, the sea is in perfect equilibrium 

 and no pressure whatever is exerted ; the only thing in such a 

 case that could restore the level or produce motion is that almost 

 infinitesimal tendency that the molecules at the surface have to 

 roll down the slope. 



I cannot but think that, on this account, Dr. Coldinghas very 

 much overestimated the power of gravity in the case of the Gulf- 

 stream. He states that, in order to impel the waters of the 

 Gulf-stream through the Straits of Florida, a slope of 9 feet from 

 the Gulf of Mexico to St. Augustine is a necessity. But differ- 

 ence of specific gravity affords only 6 feet of slope ; the addi- 

 tional three feet he assumes to be derived from the heaping up 

 of the water in the Gulf of Mexico under the influence of the 



