322 Rev. Mr Scoresby on the Uniform Permeability of 



the deviation also one degree, whilst another specimen of the 

 same mineral, highly magnetic, diminished the deviation exactly 

 the same quantity. These differences, therefore, do not affect 

 the general conclusion, but merely indicate either an error of ob- 

 servation occasioned by the magnetic condition of the substances 

 themselves, or a modified influence on the needle, through a 

 change in the magnetic condition of the minerals induced by the 

 proximity of the magnet. 



The facts now briefly stated — indicating the uniform and uni- 

 versal permeability of solid substances by the magnetic influence, 

 suggested the idea that the measure of the magnetic influence, 

 as transmitted through solid substances, would afford a mean 

 of ascertaining the thickness of substances not otherwise deter- 

 minable. For as the deviation of the compass needle under the 

 action of a straight bar-magnet, at a given distance, and in a 

 given direction, affords a satisfactory indication of the intensity 

 of the action of the magnet, of course the same deviation pro- 

 duced by the same magnet, under similar circumstances as to 

 direction, &c. will give a satisfactory indication of equality of 

 distance. 



This, indeed, is a corollary necessarily resulting from the ex- 

 periments already described, for determining the uniform per- 

 meability of solid substances to the magnetic influence, and 

 therefore needs no proof. But still verification experiments 

 were necessary, in order to determine, for practical purposes, 

 these two points ; 1,^^, The degree of accuracy with which the 

 thickness of solid substances may be measured by the observa- 

 tion of deviations ; and, Mly^ The distance to which the magnetic 

 influence may be so employed. 



I. As TO THE DEGREE OF ACCURACY WITH WHICH THE THICKNESS 

 OF SOLID SUBSTANCES MAY BE DETERMINED BY THE METHOD 

 OP MAGNETIC DEVIATIONS. 



It may be necessary in the outset briefly to state, the dif- 

 ferent processes employed for ascertaining and comparing the 

 magnetic influence. 



