Dr. Ritchie on a supposed new Law of Magnetic Action. 55 
eminent engineers, althongh they have not thrown the prin- 
ciple into the language of analysis, have nevertheless used it 
arithmetically ; and indeed I have never before heard any 
doubt expressed about it. 
During the Jast autumn I have been engaged in an exten- 
sive course of experiments on rail-roads in different parts of 
the kingdom, with a view to determine with greater precision 
than has been hitherto attained, the values of the different con- 
stant quantities which enter into their theory. The results of 
all these experiments are in the most perfect accordance with 
the principle you have called in question. 
I remain, dear Sir, yours very truly, 
Dron. Larpner. 
36, Cambridge Terrace, Edgeware Road, 
December 14, 1835. 
XII. Remarks ona supposed new Law of Magnetic Action. By 
the Rev. Witiiam Ritcuie, LL.D., F.R.S, Professor of 
Natural Philosophy in the Royal Institution of Great Britain 
and in the University of London.* 
N the last Number of the London and Edinburgh Philoso« 
phica! Magazine}, Mr. Fox has endeavoured to show 
that the mutual attraction of two magnets does not follow 
the law formerly adopted by all philosophers, viz. the law 
of the inverse square of the distance ; but the law of the simple 
inverse of the distance. This law he deduces from experi- 
ments on the attraction of the opposite ends or poles of mag- 
nets placed at very small distances from each other. Thus, 
for example, when the ends of the magnets are at the distance 
of 5555 of an inch, he found the effect to be only one half of 
what it was when they were in contact; when removed to the 
distance of =,55 of an inch the effect was one half of one half, 
or one fourth; when separated by a distance of ;1, of an inch, 
the force was only one half of one fourth, or one eighth, &c.; 
which numbers are to each other in the inverse ratio of the 
distances 
I admit the truth of the experiments, but differ from Mr. 
Fox in the conclusion he has drawn from them. To show 
that the deduction is unfounded, we must first describe what 
is meant by the pole of a magnet, and its position with regard 
to the extremity of the magnet. The pole of a magnet is 
the centre of parallel forces of all the attractions and repulsions 
of the elementary magnets of which it is composed. Now the 
position of this centre will obviously depend on the form of 
* Communicated by the Author, t Vol. vii. p. 439, 
