88 Proceedings of Royal Society of Edinburgh. [sess. 
the linear dilatations along and perpendicular to the magnetic field 
tend to he of opposite sign. Where the one is positive the other is 
negative. Each has a comparatively early maximum, and each 
changes sign in moderate fields. Their manners of variation differ, 
however, in detail ; and hence arises the undulatory variation which 
has just been described as characteristic of the volume changes of a 
particular iron tube under the influence of increasing magnetising 
forces. 
