﻿262 



Dr. J. A. Ewing on the 



are to follow, if we study the action of a model 

 grouping a number of polarized " molecules " 



formed by 

 in one plane, 

 supporting them on fixed centres about which they are free 

 to turn. In the model before you the centres are uniformly 

 spaced in rectangular rows, and the " molecules " are + shaped 

 pieces of hardened steel, strongly magnetized along each of 

 the crossed axes, each having, therefore, two north poles and 

 two south poles. The third axis is omitted in the model, the 

 movement to be studied with the help of the model being- 

 movement in one plane. On placing these " molecules " on 

 their centres they readily take up the position already indi- 

 cated in fig. 3. Each one within the group has its four poles 

 in close proximity to four poles of contrary name, and is, 

 therefore, highly stable. If disturbed by being turned through 

 a small angle, and let go, it swings back, transmitting a wave 

 of vibration through the group, which is reflected from the 

 edges, and is finally damped out in the model by pivot friction 

 and air friction. We may assume some damping action (say 

 by the induction of eddy-currents) in the actual solid, of which 

 the model may be taken as a very crude representation. 



By turning two molecules carefully round together, each 

 through one right angle in opposite senses, we set up a dis- 

 senting pair, whose equilibrium has feeble stability. A slight 

 displacement, such as might be produced by the transmission 

 of a vibrational wave, breaks them up, and they swing back 

 to the normal configuration, giving out energy, which is 

 taken up by the rest and is ultimately dissipated. By making 

 the dissenting coterie consist of three or more we can give it 

 additional strength. 



4*4*4*4* 

 4*4-4-4- 



4*4*4-4*4*4*4+ 

 4-4-4-4-4-4-4- 



An example is shown in fig. 4, where the three molecules 

 marked a, b, and c are turned round in this way. 



Notice that the normal molecule d, adjoining a line of such 

 dissenters, is in a peculiar position. His neighbours present 



