72 Mr. Christie on the mutual action of 
law, according to which the force acting during rotation 
varied at different distances, would be determined with great 
precision. In this expectation I was not disappointed ; and I 
now propose laying before the Society an account of these 
experiments, and of the results which I obtained from them. 
In making some preliminary trials with thin flat rings, I 
found the effects produced so very much less than with a disc 
of the same weight, that, previously to pursuing the enquiry 
which I had at first in view, I was induced to make some 
experiments, in order to ascertain the effects that would be 
produced by a simple solution of continuity, in a circular 
disc, by concentric sections, and likewise by successively 
removing concentric portions. These experiments clearly 
show, that the intensity of the magnetism developed during 
rotation, is not alone materially affected by a separation across, 
what may be termed, the path of the induced pole, as has 
been found to be the case by Mr. Babbage and Mr. Her- 
schel ; but that a separation, concentric with that path, by 
which the pole is undisturbed in its progress, is equally effi- 
cacious in diminishing the intensity of that pole ; and that in 
the magnetism of the whole, when all the parts are continu- 
ous, there is in all cases a great accumulation of intensity 
above the sum of the intensities of the separate parts. This 
is so important a feature in the phaenomena depending upon 
rotation, that these experiments may not be considered unin- 
teresting : I shall therefore give an account of them previously 
to entering upon the principal object of this communication. 
The nature of the apparatus which I employed on this and 
subsequent occasions, will be best understood by a reference 
to Plate X. A B is a very firm table, having a vertical axis 
