Mr. J. Croll on the Physical Cause of Ocean-currents. 251 



But water is not a perfect fluid, and its molecules do offer con- 

 siderable resistance to motion. Water flowing down an incline, 

 however steep it may be, soon acquires a uniform motion. There 

 must therefore be a certain inclination below which no motion 

 can take place. Experiments were made by M. Dubuat, with 

 the view of determining this limit *. He found that when the 

 inclination was 1 in 500,000, the motion of the water was 

 barely perceptible ; and he came to the conclusion that when 

 the inclination is reduced to 1 in 1,000,000, all motion ceases. 

 But the inclination afforded by the difference of temperature 

 between the sea in equatorial and polar regions does not ex- 

 ceed the half of this, and consequently it can have absolutely 

 no effect whatever in producing currents, no, not even the 

 M trifling surface-drift" which Sir John Herschel is willing to 

 attribute to it. 



There is an error into which some writers appear to fall to 

 which I may here refer. Suppose that at the equator we have 

 to descend 10,000 feet before water equal in density to that at 

 the poles is reached. We have in this case a plain with a slope 

 of 10,000 feet in 6200 miles, forming the upper surface of the 

 water of maximum density. Now this slope exercises no influ- 

 ence in the way of producing a current, as some seem to sup- 

 pose ; for this is not a case of disturbed equilibrium, but the 

 reverse. This slope is the condition of static equilibrium 

 when there is a difference between the temperature of the water 

 at the equator and the poles. The only slope that has any 

 tendency to produce motion of the water is the slope formed by 

 the surface of the ocean in the equatorial regions being higher 

 than the surface at the poles ; but this is a slope of only 18 feet 

 in 6200 miles. 



Objections to Dr. Carpenter's theory of a general interchange 

 of equatorial and polar ivaters, 

 Lieut. Maury's theory of a general interchange of water 

 between the equator and the poles resulting from a difference 

 of specific gravity, caused by difference of temperature, has 

 lately been advocated by Dr. Carpenter f. He considers that 

 the great masses of warm water found by him and his col- 

 leagues in their late important dredging-expeditions in the 

 depths of the North Atlantic must be referred, not to the 

 Gulf-stream, but to a general movement of water from the 

 equatorial regions. " The inference seems inevitable," he says, 



* Dubuat's ' Hydraulique,' tome i. p. G4 (1816). See also British. As- 

 sociation Report for 1834, pp. 422, 451. 



t See Proceedings of the Royal Society for Dec. 1868, Nov. 1869. 

 Lecture delivered at the Royal Institution : i Nature/ vol, i. p. 490. 



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