On the Descent of a Solid Body on an Inclined Plane. 99 



density in the single (partial) layers of the entire surface-layer is 

 therefore not constant at all points, and we cannot assign to 

 a 2 the meaning given above. The following appears to me to 

 represent most accurately the present state of knowledge on the 

 subject. 



Taking the radius of the sphere of action as equal for all sub- 

 stances, which makes the volume V of the inside fluid particles 



which work on the particles at the surface the same, - measures 



the force which the mass 1, uniformly distributed over the vo- 

 lume V, exercises on the surface-layer of the fluid. In other 



words, the half of Poisson's constant a 2 = —measures the attrac- 



(7 



tion which is exerted on a portion of the surface-layer of the 

 fluid, the base of which is unity, by a mass 1 inside the fluid, 

 and it may be called specific capillary attraction or specific cohesion. 



From the preceding Tables it follows that the specific cohe- 

 sion of the metals and many other substances in a molten condition, 

 at temperatures little above their melting -points , is nearly as the 

 numbers 1 , 2, 3, §c. 



The law expressed in the preceding statement as to the spe- 

 cific cohesion of the fluids becomes intelligible if we assume that 

 the molecular function is the same for all bodies, and that in 

 the surface -layer, the density of which is not the same in all its 

 parts, masses are enclosed which, in different substances, bear to 

 each other the proportions of the series of the natural numbers. 



Berlin, October 1868. 



XII. On the Descent of a Solid Body on an Inclined Plane when 

 subjected to alternations of Temperature. By Henry Moseley, 

 M.A., Canon of Bristol, F.R.S., Instil. Imp. Sc. Paris, Cor- 

 resp.* 



LET AB(fig.l) be an elementary plate of the solid, and conceive 

 it to be divided into an infinite number of equal elements 

 by planes perpendicular to its length. Let X be a point so taken 

 in it that, if it were divided in X. the thrust necessary to push 

 the part X A up the plane would equal that necessary to push 

 X B down it. Let the element at X be imagined to have its 

 temperature so raised as just to equal this thrust; and let the 

 temperatures of all the elements in X A, beginning from X, be 

 equally raised in succession. Each will thus be dilated more 

 than the one before it, because its dilatation will be opposed by 





Communicated by the Author, 



H2 



