of all known Substances to the Magnetic Influence. 321- 



the deviation was precisely the same. In like manner when the 

 interposed substances consisted of walls composed of various 

 materials, or large blocks of stone of many tons in weight, the 

 results were equally conclusive that the solid substances, though 

 placed in the most likely mode to interrupt the attraction, if 

 that were possible, had no influence whatever. 



This fact being verified in a great variety of bodies not 

 ferruginous, it next became an object to ascertain the effect of 

 interposed iron. As iron in its metallic state, however, does not 

 occur under circumstances at all likely to interfere with the ap- 

 plication of the magnetic influence, designed to be made by this 

 paper, my observations, in the present instance, will be chiefly 

 confined to the influence of such ferruginous substances, on 

 transmitted magnetism, as may possibly be encountered in tun- 

 nelling or mining operations. It is only, indeed, on a very 

 small scale that I have had opportunity of making experiments 

 on iron ores — the specimens of these ores to which I could have 

 access in this place, being chiefly merely cabinet specimens. 

 They may serve to shew, however, that no difficulty whatever is 

 to be apprehended in practical operations from the occurrence of 

 iron ores ; but that the magnetic influence is as freely transmit- 

 ted through such substances as through those not capable of a 

 magnetic condition. For in a series of experiments with iron 

 ores in considerable variety, successively interposed between a 

 small magnet of 4^ inches in length, and a pocket compass, the 

 north pole of the magnet, and the centre of the compass being 

 8 inches apart, the deviation of the needle, which was 20° when 

 nothing was interposed, remained uniformly, and, as far as the 

 eye could discern, precisely the same. The ores made use of 

 were from 1 to 7 inches in thickness, and consisted of the fol- 

 lowing varieties : — Haematites, iron-pyrites, sulphuret of iron, 

 Teitenisen, arseniate of iron, argillaceous clay-ironstone, spath- 

 ose-iron, compact red iron-ore, micaceous specular- iron, kidney 

 iron-ore, and iron-sand. Besides these, three magnetic speci- 

 mens were subjected to trial, one of which, a piece of Frank- 

 linite, being interposed, produced an increase of deviation of one 

 degree ; another, a piece of magnetic iron-ore, three inches in 

 thickness, being placed in its most neutral position, increased 



JANUARY MARCH 1832. Y 



