On Metallic Mine rals. 419 



or at most three, which would be likely to move the needle 

 upon approaching it. One of these enters as a constituent 

 among many rocks, particularly those of a dark green 

 colour, or those of a black which are not limestone. It 

 also much more rarely forms solid beds of magnetic ore, 

 such as have been described in this essay. To the former 

 cause I attribute the local attraction near Kingston, in the 

 neighbourhood of which place many rocks occur, both 

 fixed and detached, which would be liable to attract the 

 needle. 



Whatever may be the cause, however, of attraction in 

 any particular case, all authorities agree in stating that 

 local attraction is very common in Canada ; and this fact 

 should be particularly borne in remembrance by those who 

 use the compass, either for surveying or for the more 

 general purpose of security in the woods. It shews that 

 too much confidence may be placed in this instiiimcnt, and 

 that an implicit reliance upon it for security, while travers- 

 ing the forest in tliis country, might j)rove the destruction 

 of the traveller. 



Besides a natural local attraction, or a local attraction 

 resulting from natural causes, instruments are sometimes 

 liable to a similar influence from artificial ones, as appears 

 from Mr. Amos Eaton's observations in Si Hi man's Journal 

 for March, 1827, ?• 14. With the aid of a microscope, he 

 detected vi-ry minute steel scales attached to the limbs of 

 the instrtinunt, and left there in the manufacture of it. 



The theuilolite and the sextant are the only instruments 

 which should be used by surveyors in this country, without 

 any reference to the compass of the former, but starting 

 from a true meridian. 



