662 



TRANSACTIONS OF SECTION G. 



their normal neiglibours, may be conjectured to differ from ibe normal parts of 

 the grain in respect of electrolytic quality, and to be more readily attached by an 

 etching reagent, Hence, perhaps, the conspicuous isolated geometrical pits that 

 appear on etching a polished surface of wrought iron. 



It will help in making clear these points, and others that are to follow, if we 

 study the action of a model formed by grouping a number of polarised 

 ' molecules ' 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 magnetised 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 tlie model by pivot friction and air friction. We may 



Fig. i. 



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 dissenting 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 con- 

 figuration, 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. 



An example is shown in fig. 4, where the three molecules marked a, h, 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 to him three N. poles and one S. pole. 

 He has the choice of conforming to the majority, or of throwing in his lot with the 

 dissenters ; and he has a third possible position of equilibrium (very feeble equi- 

 librium) which is reached when his two S. poles are turned until the one neighbour- 

 ing south pole faces just between them. I have laboured these points a little 

 because they seem important when we come to speak of the efiects of strain. 



Consider now the straining action, which we may imitate in the model by 

 sliding one part of the group past the other part. For this purpose the centres 

 are cemented to two glass plates which can slide parallel to one of the axes. 



