494 Mr. Babbage and Mr. Herschel's accoujit of the 
have recourse to some neutral system, such as that described 
in the foregoing pages, in its place, or it may be left unneu- 
tralized. It ought too to be so small, or so remote, as not to 
produce induced polarity in the shell, which would react on 
itself when the sphere is set in motion, and destroy the suc- 
cess of the experiment. 
37. The effect of a solution of continuity in the revolving 
bodies comes next to be considered. It is difficult ; but the 
difficulty is not a consequence of our principles of explana- 
tion, but of our ignorance of the very complicated laws 
which regulate the distribution and communication of mag- 
netism in bodies of irregular figure. So far hov/ever as the 
operation of the general principle can be traced, its results 
are consonant to observation. 
38. In the first place, it is obvious that where one or more 
slits are cut in a metallic plate, over which the pole of a 
magnet is revolving, that immediate and free communication 
between particle and particle, on which probably the rapid, 
and certainly the intense developement of magnetism depends, 
is destroyed. The induced pole (by which we mean now the 
whole of that space in which sensible magnetism is deve- 
loped, and which is, of course, a spot of sensible, and proba- 
bly considerable magnitude — of a figure more or less elon- 
gated according to the velocity of the motion) — instead of 
travelling regularly round, retaining a constant magnetism 
and force, will now be in a perpetual state of change. Instead 
of being carried uniformly across the slit, it will die away in 
intensity, and shrink into a point in dimension on the hinder 
neutralize the earth’s action on the compass needle, cannot be without some dis- 
turbing influence of this kind. 
