REPORT ON ELECTRICITY, MAGNETISM, AND HEAT. IS 
ory of electricity, treated as M. Poisson had treated it in 1811. 
Mr. Bonnycastle found that all Mr. Barlow’s results agreed with 
such a theory. 
But a peculiar circumstance in the experiments attracted Mr. 
Barlow’s notice, and made him imagine that the theory required 
modification. He found that the attraction of a solid iron sphere 
was the same as that of a hollow shell, even when the shell was 
thin ; and he was led to believe that the magnetic power of iron 
resides wholly in the surface. This result was confirmed, as to 
the facts, by Capt. Kater*. Mr. Barlow considered this result 
as inconsistent with the theory of Coulomb, in which the mag¬ 
netic fluids in every particle of the mass were supposed to be 
dislodged by the action of a neighbouring magnet. 
Yet a little attention shows us that this is in fact a conse¬ 
quence of the Coulomhian theory. I have already (p. 11) quoted 
the passage in Coulomb’s memoir on magnetism in which he as¬ 
serts that the distribution of the sensible magnetism will be the 
same as if the fluids were transferrible from one part of the body 
to the other. Now it is easily shown that on this supposition all 
the sensible magnetism is repelled to the surface, as all the sen¬ 
sible electricity is, according to the parallel theory of electricity, 
and as Coulomb had shown that it is in fact. It is true, that 
though the superficial disposition of magnetism followed from 
the theory, and was involved in the general proposition above 
quoted, I do not know that Coulomb anywhere expressly asserts 
the fact respecting magnetic bodies, or that he made any expe¬ 
riments to confirm it. Yet it may be observed, that in his se¬ 
cond memoir on electricity and magnetism f he proved that in 
a long needle the magnetic force may be conceived to be col 
lected very near each end, which is an indication of the same 
kind of effect of the theoretical properties of magnetism. 
It was probably the experimental labours of the English phi¬ 
losophers which led M. Poisson to perform the same office for 
the magnetic which he had executed so well for the electric the¬ 
ory ;—to trace the consequences of Coulomb’s hypotheses by 
the aid of powerful and general analytical methods. In Febru¬ 
ary 1824 a memoir of his upon this subject was read to the In¬ 
stitute, and published in 1826, in the memoirs for 1821 and 
L822, according to the strange method of publication of the 
French Academy. In this memoir he obtains expressions for 
the attractions and repulsions of a body magnetised by influence 
upon any point, and examines in particular the case in which 
the body is a sphere. M. Poisson gives the name of magnetic 
* See his memoir in the Phil. Trans. 1821. f Acad. Par. 1785, p. 578. 
