Inquiries concerning the Magnetism of the Earth. 1S5 
half axis of an artificial magnet to be used in the experiment. This 
magnet was then placed on the line, at different distances from 
the needle; and the magnitude of the angle, which the needle 
made with the magnetic meridian, was accurately observed for each 
distance.” As the effect of the terrestrial magnet is measured by 
a constant quantity, multiplied into the needle’s angle of devia^ 
tion ; the tangent of that angle will likewise express the effect of 
the artificial magnet, — when the distances are such that all the 
lines drawn from the end of that magnet to the needle may 
without error be reckoned parallel. On comparing the results 
of the experiment with those of the calculation, it appears, that 
the supposition of t being equal to 1 or to 3, entirely disagrees 
with observation ; and that, on the other hand, if we make t =: % 
or suppose that the magnetic force diminishes inversely as the 
square of the distance, the numbers found by experiment differ 
very little from those found by calculation. The value of r has 
less efiect on the result. The following Table exhibits the con- 
sequences obtained by assigning to r and t the only values which 
yield quantities in the smallest degree conformable to observa-? 
tion, 
Distance 
in half 
axes of 
the mag- 
net. 
Increase of the Force. 
By Ob- 
servation. 
By Calculation. 
t — 2 
r = 1 
t — 2 
r=2 
t=2 
r=:S 
11 
10 
9 
7 
5 
4 
1.000 
1.306 
1.834 
3.947 
11.015 
22.441 
1,000 
1,334 
1.835 
3.938 
11.072 
22.245 
1,000 
1.334 
1.836 
3.945 
11.119 
22.411 
1.000 
1.325 
1.836 
3.949 
11.154 
22.530 
From this coincidence between the hypothesis and the expe- 
riment, Mr Hansteen thinks himself entitled to lay it do\vn as a 
fact, that ^ = S ; in other words, that The attractive or repulsive 
force with which two magnetic particles affect each other ^ is al- 
ways directly as their intensities^ and inversely as the square of 
their mutual distance. Certainly the data from which this de- 
duction is drawn, do not seem to be much more unexceptionable 
