TRANSACTIONS OF SECTION A. 



619 



the vibration by hand, the weight will hang down at rest, the pin drawn out to a 

 certain degree; and the distance drawn out will be simply proportional to the 

 weight hung on, as in an ordinary spring balance. 



Here, then, out of matter possessing rigidity, but absolutely devoid of elasticity, 

 we have made a perfect model of a spring in the form of a spring balance. Connect 

 millions of millions ot particles by pairs of rods such as these of this spring balance, 

 and we have a group of particles constituting an elastic solid ; exactly°fulnllin^ 

 the mathematical ideal worked out by Navier, Poisson, and Cauchy, and many 

 other mathematicians who, following their example, have endeavoured to found a 

 theory of the elasticity of solids on mutual attraction and repulsion between a 

 group of material particles. All that can possibly be done by this theory, with 

 its assumption of forces acting according to any assumed law of relation to distance, 

 is done by the gyrostatic system. But the gyrostatic system does, besides, what 



Fig. 2. 



the system of naturally acting material particles cannot do : it constitutes an 

 elastic solid which can have the Faraday magneto-optic rotation of the plane of 

 polarisation of light ; supposing the application of our solid to be a model of the 

 luminiferous ether for illustrating the undulatory theory of light. The gyrostatic 

 model spring balance is arranged to have zero moment of momentum as a whole, 

 and therefore to contribute nothing to the Faraday rotation ; with this arrange- 

 ment the model illustrates the luminiferous ether in 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. 



