242 POPULAR LECTURES AND ADDRESSES. 



a field unaffected by magnetic force. But now 

 let there be a different rotational velocity 

 imparted to 'the jointed square round the axis 

 of the two projecting hooked rods, such as to 

 give a resultant moment of momentum round 

 any given line through the centre of inertia of 

 the system, and let pairs of the hooked rods 

 in the model thus altered, which is no longer 

 a model of a mere spring-balance, be applied as 

 connections between millions of pairs of particles 

 as before with the lines of resultant moment 

 of momentum all similarly directed : we now 

 have a model elastic solid which will have the 

 property that the direction of vibration in waves 

 of rectilinear vibrations propagated through it 

 shall turn round the line of propagation of the 

 waves ; just as Faraday's observation proves to 

 be done by the line of vibration of light in a 

 dense medium between the poles of a powerful 

 magnet. The case of wave front perpendicular 

 to the lines of resultant moment of momentum 

 (that is to say, the direction of propagation 

 being parallel to these lines) corresponds, in our 



