that may be conceived to be placed. This discordance of position is still more remarkable in the several parts of a neighbouring line running on the E. by N. rhumb, and proves clearly that several magnetic bodies exert their influence in producing the disturbance which is there visible. Were it necessary to confirm this by any other observation, it would be fully proved by its deviation at a distant point situated near the N.W. part of the circle, where its position is such as to be evidently produced by some magnet unconnected with those which cause the great irregularities crowded together in the two places immediately before described. If I understand rightly the account which Humboldt has given of the affection of the needle by the hill of serpentine which he describes, it would appear that the whole rock consisted of a single magnet acting with great regularity on the needle. But in this case it is plain that its deviations from the true meridian are the consequences of the actions of several magnetic bodies dispersed over the summit of the hill, from the intricacy of whose combined influences it will evidently be impossible to determine the precise position or extent of any of them. It is probable that the meridians of all these and similar magnets occurring in the basaltic rocks, or in other rocks, are coincident with the magnetic meridian, and that they acquire this virtue as masses of iron are often known to do by long continued rest in favourable positions, combined probably with circumstances of which we are as yet doomed to remain in ignorance, little acquainted as we are with the history and causes of this obscure power. But I need not dwell longer on this particular example, since in a paper on Glen Tilt, which will be found in the present volume, I have entered at some length into the general question.
Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/104
