Permanent Magnets. 



501 



remarks, " so that the cause of the gradual decline of power 

 has yet to be discovered." 



Regarding heat as a species of molecular agitation, it seems 

 reasonable to expect that this agitation should act in a manner 

 analogous to mechanical disturbances, in facilitating the decay 

 of permanent magnetism. In fact we know that, by exposing 

 permanent magnets to moderately high temperatures, we can 

 destroy their magnetism altogether. 



The following are the mean results of the sets of observa- 

 tions that have been made. The changes in the value of M 

 are shown in the figure. It will be seen that they are much 

 greater in summer than in winter. 



M 1 



12,000 1 

















11,000 1 















10,000 













1884 



n 1885 i 



N.B. The temperatures were always noted, and the tempe- 

 rature-coefficients of the magnets were determined by a careful 

 discussion. They are not large enough to produce any differ- 

 ence in the numbers which need be taken into account for 

 the present purpose. 



Phil. Mag. S. 5. Vol. 22. No. 139. Dec. 1886. 2 M 



