OF ALL THE PHENOMENA OF TERRESTRIAL MAGNETISM. 
101 
netism had resisted every prior attempt to reduce them to the power of ana- 
lysis. It will now be necessary to refer to those which relate to the reciprocal 
action of iron bodies and a magnetic needle on each other, which were still 
more uncertain, and apparently more anomalous, than the former. 
In 1819 I undertook this investigation, and, after an extensive series of ex- 
periments, I succeeded in reducing this reciprocal action likewise to very 
simple laws, derived empyrically from the results I had obtained, but which 
were obviously only approximative, although the approximation was very close : 
in the course of these experiments also a very remarkable fact was discovered ; 
namely, that all the magnetic power of an iron sphere resides on its surface. 
These first experiments, with the exception of the fact last mentioned, which I 
had not then discovered, were presented to the Royal Society in 1819. I after- 
wards extended my first researches, and published the whole in a small tract, 
entitled, “ An Essay on Magnetic Attraction.” 
The simplicity of these results, and particularly the circumstance of the 
magnetic power residing on the surface only of the sphere, led Mr. Charles 
Ronnycastle, (at present mathematical professor in the University of Virginia,) 
to undertake an investigation of the laws which an iron sphere ought to ex- 
hibit according to a certain hypothesis relative to induced magnetism ; and he 
succeeded in eliciting most of the formulae I had obtained empyrically from my 
experiments. In the second edition of my Essay, I also examined the subject 
analytically, making a slight alteration in the hypothesis employed by Pro- 
fessor Bonnycastle ; and by this means have been enabled to deduce all my 
experimental laws without exception, and to supply the small corrective part 
which was obviously wanted in the several formulae as they were first obtained. 
Since that time, M. Poisson has employed his powerful analysis to investigate 
the subject in all its generality; and I have had the satisfaction of seeing con- 
firmed by so distinguished a mathematician all my original propositions ; and 
an action considered till that time anomalous, reduced to laws nearly as 
general and certain as those which govern the planetary motions. 
The application of these deductions to the present inquiry is important. It 
follows from these laws, that if an iron sphere, such as we have supposed in a 
transient state of magnetic induction, be made to act upon a magnetic needle, 
isolated from the terrestrial magnetism, it will produce in that needle all the 
