:332 Dr. Mac Culloch on the Geology of Glen Tilt. 



certained by repeated observations on its steadiness at different ele- 

 vations above the surface, and at different points upon it. It is not 

 a sufficient trial that detached specimens of the rocks in the vicinity 

 •exert no action on it, since the variations will often be sensible from 

 the joint action of a considerable mass of polar matter, when small 

 pieces of the same substance produce no effect. The dipping needle 

 would give the most correct information respecting the existence of 

 these local causes of error, but it is an instrument unfortunately 

 too nice and expensive for ordinary purposes. In all cases where 

 «uch a disturbing force is suspected to be in existence, and where 

 accuracy is necessary, we should not be content until the ground has 

 been examined and the actual variation ascertained by the observa- 

 tion of the magnetic azimuth, wherever it is practicable to procure 

 such an observation. It would however tend still more to the re- 

 moval of all possible errors arising from this cause, if surveyors 

 "were to reject the use of the needle altogether, and depend solely on 

 the back angle ; since although every one who lays claim to accu- 

 xacy will correct the one observation by ^^the other, yet the temp- 

 tation arising from the facility of using the needle ^one, is perhaps 

 too great to be always resisted.* 



** After this paper had been prepared for the Society, the voyage of Captain Flinders 

 ■was published. It gives xne great pleasure to find so thorough a confirmation of my ob- 

 servations and suspicions on this subject, in the original remaiks of that indefatigable 

 and unfortunate navigator. I was also pleased to see that he had frequently 

 irfbserved the polarity of granite ; more frequently I doubt not than it has occurred 

 to mc, had he published the details of all his observations. There seems to me 

 -also some reason to think, from a remark on which however he does not lay much 

 stress, that he imagined that property to be marc conspicuous on the stunmits of hill* 

 tlian elsewhere. 



