552 



Dr. W. B. Carpenter on the 



[June 13, 



action of the southerly underflow upon the long rope hanging to the buoy, 

 yet the coincidence of this fact with the preceding, and of both with what 

 the doctrine in question would predicate, may be fairly regarded as evidence 

 of some value in its favour. 



21.1 now proceed to the second head of the discussion, — viz. the demon- 

 stration, which Mr. Croll considers himself to have given, that the difference 

 of Temperature between Polar and Equatorial water cannot possibly produce 

 the effect I attribute to it. He affirms (1) that the utmost difference 

 which can be supposed to exist between the levels of the Polar and the 

 Equatorial columns will not serve to produce a movement of the Equatorial 

 surface-water towards the Poles ; and (2) that the utmost difference that 

 can be supposed to exist between the Specific Gravities of the Equatorial 

 and Polar columns will not serve to produce a flow of bottom-water from 

 the Polar area towards the Equator. — Assuming the temperature of a 

 column of water at the Pole to be 32° from the surface downwards, and the 

 temperature of the Equatorial column to be 80° at the surface, gradually 

 falling to 32° at 10,000 feet, Mr. Croll calculates that the extreme 

 elevation of level of the Equatorial column above the Polar would be 18 

 feet. " The distance from the Equator to the Poles is about 6200 miles. 

 "The force impelling the water down this slope of 18 feet in 6200 miles 

 "would therefore be equal to about 1-1, 820,000th that of gravity. For 

 " example, the force impelling a cubic foot (64lbs.) of water at the surface of 

 " the ocean. would scarcely be equal to the weight of one fourth of a grain. 

 " But in reality it would not nearly equal this " *. As evidence of the in- 

 adequacy of this force to produce motion in Ocean-water, Mr. Croll relies 

 on the experiments of M. Dubuat; who "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 

 " exceed the half of this ; and consequently it can have absolutely no effect 

 " whatever in producing currents." 



22. Now the experiments of M. Dubuat had reference, not to the sloio 

 restoration of level produced by the motion of water on itself but to the 

 sensible movement of water flowing over solid surfaces and retarded by 

 its friction against them ; and I have the authority of Mr. Hawksley 

 (whose large experience in the construction of Waterworks enables him to 

 speak with high authority on this point) for the statement, that whilst the 

 latter source of retardation is one with which Hydraulic Engineers are well 

 acquainted, the friction with which the former is attended is so slight that 

 it is altogether disregarded in practice. According to Mr. Croll, if a trough 

 1,820,000 feet long were partly filled with water, and enough water were 

 then poured into it at one end to raise the surface at that end one foot, with 

 a uniform slope to the other end, that inclination would be permanent ; 

 * " On Ocean-currents," in Phil. Mag. Oct. 1870, p. 249. 



