I can only allude to Gauss's investigations respecting the Absolute Measure of the Earth's Magnetic Force. To determine the ratio of the magnetic force of the earth to that of a known magnet, Poisson proposed to observe the time of vibration of a second magnet. The method of Gauss, now universally adopted, consists in observing the position of equilibrium of the second magnet when deflected by the first. The manner in which the business of magnetic observation has been taken up by the governments of our time makes this by far the greatest scientific undertaking which the world has ever seen. The result will be that we shall obtain in a few years a knowledge of the magnetic constitution of the earth which otherwise it might have required centuries to accumulate. The secular magnetic changes must still require a long time to reduce to their laws of phenomena, except observation be anticipated or assisted by some happy discovery as to the causes of these changes. But besides the special gain to magnetic science by this great plan of joint action among the nations of the earth, there is thereby a beginning made in the recognition and execution of the duty of forwarding science in general by national exertions. For at most of the magnetic observations, meterological observations are also carried on; and such observations, being far more extensive, systematic, and permanent than those which have usually been made, can hardly fail to produce important additions to science. But at any rate they do for science that which nations can do, and individuals cannot; and they seek for scientific truths in a manner suitable to the respect now professed for science and to the progress which its methods have made. Nor are we to overlook the effect of such observations as means of training men in the pursuit of science. "There is amongst us," says one of the magnetic observers, "a growing recognition of the importance, both for science and for practical life, of forming exact observers of nature. Hitherto astronomy alone has afforded a very partial opportunity for the formation of fine observers, of which few could avail themselves. Experience has shown that magnetic observations may serve as excellent training schools in this respect.” Letter of W. Weber. Brit. Assoc. Rep. 1845, p. 17. Addition to Note (D). The function V, mentioned p. 73, is of constant occurrence in investigations respecting attractions. It is introduced by Laplace in his investigations respecting the attractions of spheroids, Méc. Cel. Livr. III. Art. 4. Mr. Green and Prof. Mac Cullagh have proposed to term this function the Potential of the system; but this term (though suggested, I suppose, by analogy with the substantive Exponential,) does not appear convenient in its form. On the other hand, the term Integral Proximity does not indicate that which gives the function its peculiar claim to distinction; namely, that its differentials express the power or attraction of the system. Perhaps Integral Potentiality, or Integral Attractivity, would be a term combining the recommendations of both the others. Percussæ gelido trepidant sub pectore fibræ, Paullatim per membra levat; terrâque repulsum est LUCAN. VI. 752. The form which lay before inert and dead, The fibres quiver and the sinews brace, With tempered strength which genial life supplies, As though the earth the supine corse repelled. CHAPTER I. DISCOVERY OF VOLTAIC ELECTRICITY. E have given the name of mechanico-chemi WE cal to the class of sciences now under our consideration; for these sciences are concerned with cases in which mechanical effects, that is, attractions and repulsions, are produced; while the conditions under which these effects occur, depend, as we shall hereafter see, on chemical relations. In that branch of these sciences which we have just treated of, Magnetism, the mechanical phenomena were obvious, but their connexion with chemical causes was by no means apparent, and, indeed, has not yet come under our notice. The subject to which we now proceed, Galvanism, belongs to the same group, but, at first sight, exhibits only the other, the chemical, portion of the features of the class; for the connexion of galvanic phenomena with chemical action was soon made out, but the mechanical effects which accompany them were not examined till the examination was required by a new train of discovery. It is to be observed, that I do not include in the class of mechanical effects the convulsive motions in the limbs of animals which are occasioned by galvanic action; for these movements are produced, not by attraction and repulsion, but by muscular irrita |