temporarily produced in Isotropic Bodies. 



85^ 



sum of the surfaces of the four pistons is 181-456 square xenti- 

 metres . 



The first numbers were obtained with the piece of flint No. 13, 

 and it was not until 1000 kilogrammes had been reached that 

 we made use of the larger piece of plate glass. The only difficulty- 

 consists in always having two plates, one fixed and the other 

 moveable, well cut and parallel so as to exhibit no sensible flex- 

 ure. The results have then all the exactitude that can be desired, 

 and they show how great are the errors which we commit when 

 we imagine that we can neglect all the frictions in our calcula- 

 tions of the useful efibct of the hydraulic press. 



B. Determination of the coefficient of elasticity. 

 We have 



E< 



d . La 



This determination w^ould not ofi^er any difficulty if the index 

 of extraordinary refraction could be found directly, or if it were a 

 known function of lo, the ordinary index ; but, in the actual state 

 of matters, this formula can only serve for approximate estimates 

 in cases where all other means fail, as, for example, in determining 

 the elasticity of precious stones. I have attempted to apply it 

 to the diamond, which presents a double interest, both as a 

 simple and dimorphous body, and as the hardest of all known 



