112 Rev. Mr Scoresby an the Uniform Permeability of 



A few of the results, however, with ordinary compasses and 

 moderate-sized bars, may be useful, as illustrative of the facility 

 of employing the magnetic influence practically for the purpose 

 proposed. 



With a single straight bar-magnet, and tempered only at the 

 ends, and a pocket-compass of only 1 J inches diameter, distances 

 of four times the length of the bar may be determined to within 

 1-lOth of the whole, and shorter distances to a more considerable 

 degree of accuracy. 



With the same single bar, and a Kater's compass of 5 inches 

 in diameter, distances of about six times the length of the mag- 

 net may be determined within a very moderate limit of error, and 

 an extent of even ten lengths of the bar may be measured to, 

 perhaps, a tenth of the whole. 



Two similar magnets placed parrallel to each other, a few 

 inches asunder, will indicate, within the same limits of error, 

 distances of nearly a length and a half more remote with the 

 pocket-compass ; or with a Eater's compass, distances from two 

 to three lengths greater, when the deviations are very small. 



Four bar-magnets of a similar kind will produce, on the Ka- 

 ter's compass, about the same deviation at \Qjhcal lengths, as a 

 single bar produces at ten focal lengths — thus affording, with 

 three feet magnets, a tolerably correct measure of the thickness of 

 any solid intervening substance, of about 40 feet. But the same 

 magnets will produce a sensible effect on the compass (namely, 

 a sum of deviations amounting to 2') at the distance of no less 

 than 33 focal lengths, or 82 feet ! 



These results, however, which are taken from the observation 

 of the effects of bar-magnets on a compass acted upon by the 

 whole force of terrestrial magnetism, will be greatly modified, and 

 vastly extended, when the directive force of the earth on the 

 compass is partially neutralized, by the proper arrangement of 

 one or two small magnets in juxtaposition. For, extending the 

 influence, therefore, to the greatest possible distance with a given 

 apparatus, such an arrangement will of course be used. 



But I proceed to give the particulars of a few experiments 

 by which the extent of influence expected to be produced was 



