Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/339

 sights is so useful and convenient for ordinary purposes, it will at least render it one degree more correct, although even in this way we shall not avoid those disturbances which arise from the more extensive influence of larger masses of magnetic matter. This caution is equally necessary to the geologist in the taking of geological bearings or angles, since the errors which may arise from its position on the ground are generally considerable, on account of the delicacy of the needle of a pocket compass, and may often be as great or greater than those which proceed from the short radius of the instrument itself, and the consequent difficulty of accurate observation.

In the use of the miner's compass the species of error which I have here noticed is always in danger of becoming serious, from the difficulty of removing the instrument to a sufficient distance from the disturbing forces. Yet even in the narrowest shaft or level an attention to this will enable the miner to take precautions, which, if they cannot entirely remove, will at least diminish in a great degree the probability of serious error.

In adjusting the position of a common theodolite it is evident that one of the sources of error will be sometimes removed, and always diminished, as it rarely happens that the local influences of small masses are felt at the distance at which that instrument is elevated from the ground. Yet I have shewn in the instance of Glamich, that the disturbing power was sensible at four feet from the surface. Such cases are however rare, although the ordinary quantity of the variation will be altered at distances infinitely greater, where the action of mountains or large tracts of magnetic matter is concerned.

In general I may remark that no confidence can be reposed in magnetic angles until the regular action of the needle has been ascertained