Mr. J. Croll on the Physical Cause of Ocean-currents. 117 



down a slope of 1 in 1,820,000*, he says, "Now the experi- 

 ments of M. Dubuat had reference, not to the slow restoration 

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

 sensible movement of water flowing over solid surfaces and re- 

 tarded by its friction against them " (§ 22). Dr. Carpenter's 

 meaning, I presume, is that if the incline consist of any solid 

 substance, water will not flow down it; but if it be made of 

 water itself, water will flow down it. But in M. Dubuat' s ex- 

 periments it was only the molecules in actual contact with the 

 solid incline that could possibly be retarded by friction against 

 it. The molecules not in contact with the solid incline evidently 

 rested upon an incline of water, and were at perfect liberty to 

 roll down that incline if they chose ; but they did not do so ; 

 and consequently M. Dubuat's experiment proved that water 

 will not flow over itself on an incline of 1 in 1,000,000. 



A begging of the question at issue, — " It is to be remembered," 

 says Dr. Carpenter, " that, however small the original amount of 

 movement may be, a momentum tending to its continuance must 

 be generated from the instant of its commencement ; so that if the 

 initiating force be in constant action, there will be a progressive 

 acceleration of its rate, until the increase of resistance equalizes 

 the tendency to further acceleration. Now, if it be admitted that 

 the propagation of the disturbance of equilibrium from one 

 column to another is simply retarded, not prevented, by the vis- 

 cosity of the liquid, I cannot see how the conclusion can be re- 

 sisted, that the constantly maintained difference of gravity be- 

 tween the polar and equatorial columns really acts as a vis viva 

 in maintaining a circulation between them" (§ 35). 



If it be true, as Dr. Carpenter asserts, that in the case of the 

 general oceanic circulation advocated by him " viscosity " simply 

 retards motion, but does not prevent it, I certainly agree with 

 him "that the constantly maintained difference of gravity between 

 the polar and equatorial columns really acts as a vis viva in 

 maintaining a circulation between them." But to assert that it 

 merely retards, but does not prevent, motion, is simply begging 

 the question at issue. It is an established principle that if the 

 force resisting motion be greater than the force tending to pro- 

 duce it, then no motion can take place and no work can be per- 

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

 the molecular resistance of water to motion is greater than the 

 force derived from a slope of. 1 in 1 ,000,000 ; and therefore it 

 is simply begging the question at issue to assert that it is less. 

 The experiments of MM. Barlow, Rainey, and others to which 



* The slope, however, taking Dr. Carpenter's own data, amounts only 

 to little more than one half, viz. to 1 in 3,500,000. See Phil. Mag. for 

 October 1871, p. 263. 



