338 Lord Kelvin on the 



elongations perpendicular to the three pairs of parallel sides 

 produce electro-polarizations with electric axis in each case 



A 



Fig. 3. 



A 



/K 



A 



A 



A 



% 



perpendicular to the line of elongation, equal elongations in 

 the directions of the diagonals produce electric polarizations 

 equal to those, but having their axes along the lines of elon- 

 gation instead of perpendicular to thern. 



§ 12. Consider now two simple elongations in the direction 

 shown by the arrow-heads A in fig. 4. These two elongations 

 produce electro-polarizations with axes and signs indicated 

 by aa and bb respectively. The resultant of our present two 

 simple elongations is clearly a dilatation equal in all directions 

 in the plane of the diagram, compounded with a single simple 

 elongation in the line K'OK, bisecting the angle between them, 



