Mr. J. CrolJ on the Physical Cause of Ocean-currents. 119 



It is no doubt a somewhat difficult problem to determine ac- 

 curately the total amount of force exercised by gravity on the 

 ocean ; but for our present purpose this is not necessary. All 

 that we require at present is a very rough estimate indeed. And 

 this can be attained by very simple considerations. Suppose we 

 assume the mean depth of the sea to be, say, three miles. The 

 mean depth may yet be found to be somewhat less than this, or 

 it may be found to be somewhat greater; a slight mistake, 

 however, in regard to the mass of the ocean will not materially 

 affect our conclusions. Taking the depth at 3 miles, the force 

 or direct pull of gravity on the entire waters of the ocean tend- 

 ing to the production of the general circulation will not amount 

 to more than iat000 ' 000t000 that of gravity, or only about ^ that 

 of the attraction of the moon in the production of the tides. 

 Let it be observed that I am referring to the force or pull of 

 gravity, and not to hydrostatic pressure. 



The moon, by raising the waters of the ocean, will produce a 

 slope of 2 feet in a quadrant ; and because the raised water 

 sinks and the level is restored, Mr. Ferrel concludes that a similar 

 slope of 2 feet produced by difference of temperature will there- 

 fore be sufficient to produce motion and restore level. But it is 

 overlooked that the restoration of level in the case of the tides is 

 as truly the work of the moon as the disturbance of that level 

 is. For the water raised by the attraction of the moon at one 

 time is again, six hours afterwards, pulled down by the moon 

 when the earth has turned round a quadrant. 



No doubt the earth's gravity alone would in course of time 

 restore the level ; but this does not follow as a logical conse- 

 quence from Mr. FerreFs premises. If we suppose a slope to 

 be produced in the ocean by the moon and the moon's attrac- 

 tion withdrawn so as to allow the water to sink to its original 

 level, the raised side will be the heaviest and the depressed side 

 the lightest; consequently the raised side will tend to sink and 

 the depressed side will tend to rise, in order that the ocean may 

 regain its static equilibrium. But when a difference of level is 

 produced by difference of temperature, the raised side is always 

 the lightest and the depressed side is always the heaviest ; con- 

 sequently the very effort which the ocean makes to maintain its 

 equilibrium tends to prevent the level being restored. The 

 moon produces the tides chiefly by means of a simple yielding 

 of the entire ocean considered as a mass ; whereas in the case of 

 a general oceanic circulation the level is restored by a flow of 

 water at or near the surface. Consequently the amount of 

 friction and molecular resistance to be overcome in the restora- 

 tion of level in the latter case is much greater than in tho 

 former. The moon, as the researches of Sir Williaui Thomson 



