1879.] Professor Cornu on the Elastieity of Solid Bodies. 191 



WEEKLY EVENING MEETING, 

 Friday, May 16, 1879. 



William Spottiswoode, Esq. M.A. D.C.L. LL.D. President E.S. 

 Vice-President, in tlie Chair. 



Professor A. Cornu, 



PROFESSOR AT THE rOLTTECHMIC SCHOOL, MEMBER OF INSTITUTE OF FRANCE. 



The Ojptical Study of the Elasticity cf Solid Bodies. 



Preliminary Remarks. 



All solid bodies utilized in scientific and industrial applications are 

 more or less elastic : and it is very important, in a practical as well 

 as a theoretical point of view, to be able to predict the deformations 

 due to given forces, or, conversely, to know the forces which corre- 

 spond to given deformations. 



Mathematical Calculation enables us to solve both problems for 

 every description of form and of force in all their details, provided 

 that it borrows from experience certain results obtained in very 

 simple cases. 



Homogeneous and Isotropic Bodies. 



An elastic bar urged by external traction forces extends itself 

 along its largest dimensions (longitudinal extension) : at the same 

 time, by the natural play of internal forces, its transversal dimensions 

 diminish (transversal contraction). 



[Illustration of this general fact with an indiarubber bar.] 



In order to calculate all the circumstances of deformation of an 

 elastic isotropic body, whatever may be its shape and acting forces, 

 it is sufficient to know the rate of longitudinal extension (modulus of 

 elasticity), and its ratio to the transversal contraction. 



Various opinions amongst the Physicists upon the value of this Batio. 



A considerable number of physicists (Cagniard-Latour, Wertheim, 

 Prof. Kirchhoff, Dr. Everett, &c.) have made a series of experiments 

 on various supposed isotropic bodies. 



The question, a very important one in a theoretical point of view, 

 is to know if this ratio is a variable one according to the nature of 



