560 



Dr. W. B. Carpenter on the 



[June 13, 



come under the direct influence of insolation ; whereby its temperature will 

 be greatly augmented, and its level raised, so as to promote the return-flow. 

 Thus at the two extremities of the basin there will be two forces in constant 

 renewal, the opposing actions of which will as constantly tend to a disturb- 

 ance of equilibrium ; and to deny that a movement will thus be produced 

 and sustained, tending to the restoration of equilibrium through the entire 

 basin, is simply to assert that a constant force acting on particles that are 

 free to move does not move them. 



35. It is to be remembered that, however small the original amount of 

 movement may be, a momentum tending to its continuance must be gene- 

 rated 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 accelera- 

 tion. 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 " viscosity " of the liquid, I cannot see how the conclusion can be 

 resisted, that the constantly maintained difference of Gravity between the 

 Polar and Equatorial columns really acts as a vis viva in maintaining a Cir- 

 culation between them. A remarkable confirmation of my argument as 

 to the sufficiency of difference in Specific Gravity to keep up a Vertical 

 Circulation, is afforded by recent researches on the Dardanelles Under- 

 current. (See § 173.) 



36. Those who are accustomed to look only at the movements of 

 great volumes of fluid, and to discuss them en masse, are apt to ignore facts 

 perfectly well known to such as have had occasion to observe those minuter 

 changes which are constantly taking place within any collection of liquid 

 under the influence of slight alterations of Temperature. — The following is 

 a remarkable example of this kind, which fell within my own knowledge 

 many years ago, and made a strong impression upon me. Mr. West, a very 

 ingenious mechanician at Bristol, having heard of the success of Mr. Peter 

 Barlow's plan of constructing object-glasses for Telescopes of moderate 

 aperture, in which the double concave of flint-glass was replaced by a highly 

 refracting fluid (such as sulphuret of carbon, or oil of cassia), carried out 

 this plan on a large scale in the construction of an object-glass of eighteen 

 inches in diameter. By the mathematical aid of Mr. Barlow, and his 

 own great mechanical ability, this object-glass (exceeding in diameter that 

 of any refracting telescope which had been at that time constructed) was 

 completed with theoretical correctness ; but when it was brought into use, its 

 performance was found to be so seriously impaired by movements produced 

 in the fluid by the very slight disturbances in the equality of the Tempera- 

 ture of its different parts, occasioned by atmospheric currents, that in spite 

 of every precaution which could be taken for its protection, it was found to 

 be valueless for the purpose of Astronomical research. — Those, again, who 

 have been accustomed to the minute observation of Aquaria, have frequently 

 noticed currents, marked by the movement of minute suspended particles, 



