repetition of M. Arago’s experiments on magnetism, &c. 485 
netic action observed by us, lie among the best conductors of 
electricity.* Another feature, no less striking, is the extreme 
feebleness of this species of action compared w^ith that which 
takes place in cases of sensible attraction and polarity. This 
will appear more evidently, if we consider the mode of action 
which probably obtains in these experiments, and the me- 
chanism, if we may so express it, by which the effects of such 
almost infinitesimal forces are rendered perceptible in them. 
26. The rationale of these phsenomena, as well as of those 
observed by Mr. Barlow in the rotation of iron, which form 
only a particular case (though certainly the most prominent 
of any) of the class in question, seems to depend on a principle 
which, whether it has or has not been before entertained or 
distinctly stated in words, it may be as well, once for all, to 
assume here as r postulatmn, viz. that i?i the induction of mag- 
netism, time enters as an essential element, and that no finite 
degree of magnetic polarity can he communicated to, or taken from 
any body whatever susceptible of magnetism, in an instant, f 
* The meagre statements and imperfect reports which have hitherto reached us 
of M. Arago’s researches, had prepared us to expect a much more appreciable 
amount of magnetic force in non-metallic bodies than we have observed. Glass, 
wood, water, ice, and indeed every description of substance, have been included in 
the list of bodies capable of producing a notable deviation from the magnetic meri • 
dian in a suspended bar, by their rotation. This naturally renders us desirous of 
seeing that eminent philosopher’s own account of the means employed by him to 
render sensible such very minute forces, which must have been unusually delicate. 
This may perhaps be the proper place to mention, that the numerical estimates in 
this paper are merely intended to be received as gross approximations, valuable 
only in the absence of all other information of the kind. The metals used were 
those of commerce, no pains having been taken to free them from iron. Much 
refinement would have been thrown away on such materials. 
f It is now some years since one of the authors of this account (Mr. Babbage) 
