Dr. Mac CuLLOCH 6« /;&^ Geofogy of Glen Tilt. 331 



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 geo- 

 logical bearings or angles, since the errors which may ari-se from 

 its position en 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 de- 

 gree 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 Glamlch, 

 that the disturbing power was sensible at four feet from the sur- 

 face. 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 con- 

 cerned. 



In general I may remark that no confidence can be reposed in 

 magnetic angles until the regular action of the needle has been as- 



2t 2 



