46 



PRESIDENT S ADDRESS — SECTION A. 



Or we may employ a sort of inverse of the preceding 

 analogy. We may substitute for displacement momentum 

 per unit volume, and for force of restitution rate of spin. 

 Then elasticity must be replaced by the reciprocal of moment 

 of inertia ; and so in iron the particles must be thought of as 

 possessing much moment of inertia. This analogy has been 

 used by Dr. Lodge, as giving some idea of the way in which 



electromagnetic waves are propagated. He supposes the ^ 



of a medium to be its elasticity, and fx to be its density, so 



that the velocity of waves will be -j^=- We have in this 



analogy to suppose that there are lines and tubes of spin, and 

 that along a tube of spin the momentum of the spinning- 

 matter between two sections of the tube, a small and constant 

 distance apart, is uniform along the tube. 



In this table of analogues I have grouped the main facts 

 which we have been considering : — 



Table of Analogves. 



