demanding his assent as a right, I must solicit a suspension of his judgment as a courtesy; and, after all, however firmly the hypothesis may support the phenomena piled upon it, we can deduce no more than a practical rule, grounded on a strong presumption. The license of arithmetic, however, furnishes instances that a rule may be usefully applied in practice, and for the particular purpose may be sufficiently authenticated by the result, before it has itself been duly demonstrated. It is enough, if only it hath been rendered fully intelligible.
In a system where every position proceeds from a scientific preconstruction, a power acting exclusively in length, would be magnetism by virtue of our own definition of the term. In like manner, a surface power would be electricity, as far as that system was concerned, whether it accorded or not with the facts ordinarily so called. But it is incumbent on us, who must treat the subject analytically, to show by experiment that magnetism does in fact act longitudinally, and electricity superficially; and that, consequently, the former is distinguished from, and yet contained in, the latter, as a straight line is distinguished from, yet contained in, a superficies.
First, that magnetism, in its conductors, seeks and follows length only, and by the length is itself conducted, has been proved by Brugmans, in his philosophical Essay on the Matter of Magnetism, where he relates that a magnet capable of supporting a body four times heavier than itself, and which acted as a magnetic needle at the distance of twenty inches, was so weakened by the interposition of three cast-iron plates of considerable thickness, as scarcely to move the magnetic needle from its place at
