Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/336

 surface of the earth. Geologists, hereafter perhaps more attentive to this subject, may probably add to this catalogue, or at least augment the number of examples, and add many more to the few localities which I have had opportunities of examining.

To surveyors who are more deeply interested in the fact and its consequences, I shall also look for more extensive and more accurate observations, since the possession of more delicate instruments, and the devotion of time and attention to this particular object, are required to determine the quantity and extent of these influences, as well as their practical effect in producing permanent local variations in the magnetic meridian.

I need scarcely say that an ordinary ships compass is insufficient to detect these variations, unless where they are considerable, as the instrument, either from the rudeness of its workmanship, or the intentional insensibility given it by the maker to render it steady in steering small ships or in navigating through a cross sea, is rarely alive to minute quantities of the disturbing force.

I have attempted on various occasions to discover the positions of the poles of the natural magnets which thus disturb the needle brought within the sphere of their influence. From the observation of Humboldt we are led to imagine that he conceived the rock or mountain which he describes, to have possessed but two poles, and that its effects on the needle could in consequence be easily assigned. This is not impossible, but in all the cases where I have been able to make observations, I have found that the disturbances must have been produced by a number of independent magnets, each rock or fragment being possessed of a meridian of its own, and the disturbances of the needle being in consequence of that, extremely irregular and uncertain. I have already entered into a detail of this fact where I first observed it in Sky, in the