On the laws of the deviation of magnetized needles towards iron

The author had pointed out, several years ago, the law of deviation of a magnetized needle, (either freely suspended or constrained to move in any particular plane,) from its natural position, by the influence of masses of iron in its vicinity. This law was founded on the hypothesis that the ironattracted both the poles of the needle: the position of which, resulting from this action, might be determined by that of an imaginary minute magnetic needle, freely suspended by its centre of gravity, reduced to the plane of revolution. The author had considered this law as fully established from its accordance with experiment; but Mr. Barlow, in a paper which was published in the last volume of the Philosophical Transactions, denies that such an accordance exists, and infers, from the results of some experiments which he made on horizontal needles, having their magnetism unequally distributed in their two branches, that the theory on which the preceding law is founded is fallacious. In opposition to the views of Mr. Barlow, the author contends that the phenomena observed are precisely those which must result from the theory he had himself adopted; and that they tend in no way to support the hypothesis of their being simply the effects of the magnetic power which the iron receives by induction from the earth. The author was also led to suspect the accuracy of another conclusion which had been drawn by Mr. Barlow, namely, that the length of the needle had no sensible influence upon the extent of its deviations. In order to determine this point, he began by ascertaining more scrupulously than had yet been done, the values of several of the elements of the calculation, such as the exact positions of the points where the intensity of the magnetism is the greatest, and also of the point of neutrality, or of the magnetic centre; and he next subjected to a more severe scrutiny a law which had been regarded as established by experiment; namely, that the tangent of the deviation is proportional to the rectangle of the cosine of the longitude, into the sine of the double latitude of the position of the centre of the needle, with relation to the mass of iron as referred to a hollow sphere.