Inquiries cbncerning the Magnetism of the Earth. 1^1 
sary to find means of simplifying the experiments to such a de- 
gree, that, being disengaged from all foreign circumstances, and 
all mechanism of too complex a nature, they may present no^ 
thing but the mere effect of the law which we wish to discover. 
But this presupposes an acquaintance -v^ith those circumstances, 
and that mechanism, and a knowledge more or less accurate of 
the influence they may have in altering the issue of the pro- 
posed e:iperiment. The magnet presents all these obstructions. 
One of its poles attracts, while the other repels ; and the effect 
which results is always mixed. The number of poles may be 
augmented ; but no art has yet succeeded in reducing them to 
one. That, however, would be the method of discovering its 
individual effect, unmingled with the alteration proceeding from 
the directly opposite effect of the other pole. Besides, it is not 
at the pole alone that the magnet attracts : it attracts more or 
less at every point of its surface, with very unequal degrees of 
force. Hence, the mean direction and the compound force vary 
in a manner extremely complicated. To succeed in finding the 
elementary laws, we should have it in our power to make the 
experiment for each isolated point ; but experiment can shew us 
nothing more than the sum that results from the combined ac- 
tion of all the points. 
To these difficulties still more must be added. In order 
to determine by experiment the forces of the magnet, we have 
nothing to present to it but ferruginous matters^ or another 
magnet. Now, how short soever the experiment may be, the 
iron which we bring to be attracted by the magnet, begins 
itself to share in the magnetic force, and the effect which results 
begins of course to be more or less disturbed. The same thing 
happens, if, instead of iron, we employ another magnet. And 
farther, if the experiments require a considerable space of time, 
it is not certain that the magnet we use will preserve its force 
invariable till the process is concluded ; the magnetic force, and 
its mean direction also, being known to change from one moment 
to another.''’ 
Obscured by these uncertainties, the true law of magnetic at- 
traction was long unknown or mistaken. Newton, and bis com- 
mentators Le Jacquier and Le Sueur, imagined the force to be 
inversely as the cube of the distance. Muschenbroek’s experi- 
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