374 ON PHYSICAL DATA, &C. 



And because F(cri) and -F(.«-,) are each equal 

 to nothing, we shall have 



Since h is very small we may neglect the 

 second and all higher powers of h, and then we 

 shall have 



That is, the compressive force is equal to the 

 tensile force. This investigation explains why 

 the forces of extension and compression are 

 proportional to the distances compressed and 

 extended, which was established by Tredgold 

 and others. Professor Hodgkinson, F. R. S., 

 M.R.I. A., &c. was the first to show that when 

 the distances increased this law was not correct. 

 I shall here leave the subject till some future 

 opportunity. 



