296 Royal Society. 



ing whether all bodies are equally and uniformly permeable to the 

 magnetic influence. Out of a great number of substances not ferrugi- 

 nous, but of various qualities, thickness, and solidity, which were 

 subjected to trial, no instance occurred of their offering any per- 

 ceptible obstruction to the action of a magnet on a compass, when 

 interposed between them. No interruption to this action occurred 

 even when the intervening bodies were iron ores, of which several 

 were tried, excepting in one or two cases in which the ore was found 

 to be itself magnetic. Hence the author was led to conceive that 

 an accurate estimation of the magnetic influence transmitted through 

 solid substances, might afford an excellent mode of ascertaining the 

 thickness of such substances which might not be otherwise deter- 

 minable. In order to judge of the degree of accuracy with which 

 this might be accomplished, he instituted various sets of experi- 

 ments ; first placing the magnet in a line pointing to the centre of 

 the compass, and on a level with it, in the east and west magnetic 

 direction ; and secondly in positions more or less oblique to this di- 

 rection. He found reason to conclude from these trials, that the 

 degree of accuracy attainable by this method was such as to render 

 it highly advantageous in mining operations. Thus the thickness of 

 a mass of freestone rock on the Liverpool and Manchester rail- way, 

 three feet two inches in thickness, was determined by this method 

 to within theeightli of an inch of its actual measurement, exhibiting 

 an error of only one SS^th part of the whole. 



Many experiments were made to determine the effect which the 

 form, dimensions, quality, and number of magnets have on the ex- 

 tent of their directive influence on the compass. It was found tliat 

 little, if any augmentation of power results from increasing the thick- 

 ness of the magnet ; but that, with magnets of similar form, the di- 

 rective forces are nearly in the direct ratio of -their lengths. The 

 author gives the results of an extensive series of experiments on the 

 combined influence of several magnets, arranged, either in contact 

 or in juxta-position, in a great variety of ways. The contact of 

 dissimilar poles was in all cases productive of an increase, and that 

 of similar poles of a diminution of efficiency. 



In the second part of this paper the author enters into an investi- 

 gation of the law of the magnetic directive power with reference to 

 distance : in which he finds it convenient to estimate all distances 

 in multiples of the length of the magnet employed, or, more cor- 

 rectly, of the interval between its two poles. From the established 

 law of magnetic force, — namely, that it is in the inverse duplicate ratio 

 of the distance, — the author deduces formulae for estimating the di- 

 rective power of a magnet on a compass at different distances. The 

 combined action of four magnets, on a compass of Captain Kater's 

 construction, which was five inches in diameter, will afford a tole- 

 rably accurate measurement of the thickness of any solid intervening 

 substance, when about forty feet thick ; but even at the distance of 

 eighty-two feet the deviation produced by the magnet will be two 

 minutes of a degree, and therefore still very appreciable. But the 

 sensibility of the compass to the magnetic influence might be much 



further 



