PROCEEDINGS 



OF THE 



ROYAL SOCIETY OF EDINBURGH. 



VOL. II. 



1847-8. No. 32. 



Sixty-Sixth Session. 



Monday, 20(h March 1848. 



Dr CHRISTISON, V.P., in the Chair. 



The following Communications were read : — 



1. On an Instrument for measuring the extensibility of 



Elastic Solids. By Professor Forbes. 

 This instrument is almost a faithful reproduction of S'Graves- 

 ande's apparatus described in his " Physices Elementa Mathema- 

 tica," 1742 (but not in the previous editions). It is described or 

 alluded to by few modern writers, except Biot in his " Traite de 

 Physique." It consists of a strong wooden table or frame, with a vice 

 at each end, between which a wire or lamina may be stretched with a 

 determinate tension by means of a weight attached by a cord, pass- 

 ing over a pulley in the manner of the musical apparatus, called a 

 Monochord. After the tension is adjusted both vices are screwed 

 fast, the space included between them being exactly 50 inches. If 

 now, any deviation of the middle point of the wire included by the 

 vices be made (similar to the action of sounding a harp-string), the 

 force required to pull it a certain distance aside will depend, 1st, oa 

 the length of the wire; 2d, on its tension ; 3d, on its extensibility, 

 or the modulus of elasticity. 



S'Gravesande employed his apparatus to verify Hooke's law, 

 that the extension is as the extending force within the limits of per- 

 fect elasticity. But it does not seem to have occurred to him, nor 

 (singularly enough) to later experimenters, to deduce from the forces 

 required to produce given deviations, the specific extensibility, or 

 what Dr Young calls the Modulus of Elasticity of the body. 

 VOL. II. ^ 



