88 
Mr. Christie on the mutual action of 
in all the cases, been made with the same disc ; and certainly 
the agreement is as near as we could expect from observa- 
tions made with different discs. 
If the agreement of all these results with so simple a law 
is to be considered as fortuitous, the coincidence is certainly 
most singular ; but I am disposed to think that the law in 
nature is really this, that the quantities of magnetism destroyed 
by removing concentric portions from the interior of the disc, 
are proportional to the mass removed, but that they have not 
generally, the same ratio to the whole magnetism developed 
in the uncut disc, which the portion removed from the disc 
has to the whole disc ; that is, if d is the diameter of the 
uncut disc, and & represent the magnetism developed in it ; 
d l and d tl the diameters of portions removed, and u l , the 
magnetism developed in the disc when those portions are 
removed, the magnets revolving at the same distance from 
the axis in each case, then 
at — a, d* 
a — a,i d IX 
and - is not equal to , but JHUL* _ „ 
where q will be constant so long as the magnets revolve at 
the same distance from the axis, but will vary with that 
distance. In the present instance, the magnets revolving at 
the distance 3-2 inches from the axis, the values of q derived 
from the different observations are 1-48, i'45, 1 53, 154, 
giving a mean 1-50. Here then the magnetism destroyed by 
the removal of concentric portions from the interior of the 
disc, has a considerably greater ratio to the whole magnetism 
developed than the mass removed has to the whole. 
From the great diminution in the intensity of the induced 
