1921] Elasticity 377 



WEEKLY EVENING MEETING, 

 Friday, May 27, 1921. 



Colonel E. H. Grove-Hills, C.M.G. D.Sc. F.R.S., 



Secretary and Vice-President, in the Chair. 



A. Mallock, F.R.S. 



Elasticity. 



Before speaking of the more special problems relating to elasticity, 

 on which I have been recently engaged, it may be as well to define 

 the meaning of the word " elasticity " in its scientific sense. 



In ordinary conversation the terms " elastic " and " elasticity " 

 have many different interpretations, but in physics it denotes simply a 

 property possessed by all matter, but in various degree — namely, that 

 of returning to its original form and dimensions after any cause 

 which produces the alterations has ceased to act. 



It has nothing to do with the hardness, softness, brittleness, or 

 flexibility of the material. These are terms which depend on the 

 limits of elasticity, and not on the elasticity itself. A billiard ball, 

 for instance, is not by any means soft, but it is exceedingly elastic, as 

 is shown by the absence of any permanent mark at the point where 

 it has received the impacts of other balls. 



The measure of elasticity is given by the force required to pro- 

 duce some known alteration of form, not exceeding in amount that 

 which will naturally disappear when the force is removed. 



Elasticity is of two fundamentally distinct kinds. All isotropic 

 matter (i.e. matter which has the same properties in all directions), 

 whether solid, liquid or gaseous, may be compressed bodily without 

 change of shape, and it may also be distorted without change of 

 volume. The forces which resist these alterations are known respec- 

 tively as volume elasticity, or compressibility, and rigidity. 



Any possible change of shape can be brought about by a suitable 

 combination of volume compression (or dilation) and distortion (or 

 " shear "as it is often called, from the identity of the action of dis- 

 tortion with the effect produced by a pair of shears). 



Liquids and gases oppose hardly any resistance to distortion ; in 

 fact the mathematical definition of a " perfect fluid " is a material 

 which offers no resistance whatever to distortion. Matter, however, 

 in whatever state it may exist, resists compression, and in most cases 

 this resistance is very large. Water, for instance, though its rigidity 

 is nearly negligible, is only compressed by one three-hundred- 



