240 POPULAR LECTURES AND ADDRESSES. 



The enclosing circle represents in each case 

 in section an enclosing spherical shell to prevent 

 the interior from being seen. In the inside of 

 one there are fly-wheels, in the inside of the 

 other a massless spring. The projecting hooked 

 rods seem as if they are connected by a spring 

 in each case. If we hang any one of the^ systems 

 up by the hook on one of its projecting rods, 

 and hang a weight to the hook of the other 

 projecting rod, the weight when first put on 

 will oscillate up and down, and will go on doing 

 so for ever if the system be absolutely unfrictional. 

 If we check 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 of 

 particles by pairs of rods such as these of this 

 spring-balance, and we have a group of particles 



