54 Mr. J. LeConte on the Apparent Attractions 



balance each other, so that the floating solid can have no motion 

 of translation *. In like manner, Dupre, in his admirable 

 researches on " Molecular Actions," investigates this anoma- 

 lous deduction from Laplace's theory, indicating the com- 

 pleteness of the latter, and showing that Poisson is correct in 

 the conclusion that the horizontal molecular forces exactly 

 balance one another |. 



Notwithstanding the a priori mechanical impossibility of an 

 isolated floating body taking on a motion of translation under 

 the conditions above designated, I made the following arrange- 

 ments with the view of experimentally testing the question. 

 A plate of well-cleaned glass (fig. 4), g g r , and a plate of 

 polished steel, s s', were cemented together so as to constitute 

 a single compound plate of these substances. This was floated 

 in a vertical position by securing masses of cork c and c' to 

 the two faces, and attaching a leaden sinker (w) of the proper 

 weight to the lower extremity of the plates. If such a com- 

 posite plate is plunged vertically into alcohol contained in a 

 large vessel, a concave meniscus will be formed by the ascend- 

 ing liquid on the glass face, while on the steel face no meniscus 

 will be formed, and the adjacent surface of the alcohol will be 

 horizontal. Now Dr. Young insisted that, according to 

 Laplace's theory, the horizontal molecular pressures on the 

 opposite faces being unequal, the composite plate should be 

 drawn in the direction of the centre of concavity of the meniscus 

 on the glass face, and thus cause the entire system to take on 

 a motion of translation toward the right. The arrangement 

 represented in fig. 4 was so sensitive that it was difficult to 

 avoid the influence of slight currents of air; nevertheless 

 there were no indications of the action of unbalanced forces on 

 the floating apparatus. Hence the idea that such a system 

 would take on a horizontal and perpetual motion under the 

 action of molecular forces is not only inconsistent with funda- 

 mental mechanical principles, but is contradicted by direct 

 experiment ; for no motion takes place in such a composite 

 vertical plate when partly immersed and floating in the liquidf . 



Moreover it seems to me that, according to any theory of 

 capillarity which is based upon the action of a tense elastic 

 superficial film, it is clear that the tensile reaction of the 



* Poisson, op. cit. ante, arts. 85 and 96, pp. 172 and 194. 



t Dupre, op. cit. ante, chap. ix. pp. 396-400. 



X It is scarcely necessary to add that the irregular and perplexing capil- 

 lary motions due to the difference in the surface-tensions of different 

 liquids or of the same liquid in different states are here excluded from 

 consideration. The curious motions of small isolated masses of camphor, 

 when floating on the surface of warm water, come under this category 

 These phenomena have been carefully studied by Tomlinson and others. 



