132 On the Permeability of known Substances. 



ther in steel tempered or soft, cloes not prevent the influence of 

 the magnet being transmitted to the compass ; but, on the con- 

 trary, I have most commonly found, that the deviations produ- 

 ced by the magnet acting through a mass of iron, are greater 

 than when nothing whatever is interposed. I have also found 

 that the influence is not interrupted, though the interposed me- 

 tal be a mass of hardened steel, nor even when trial was made 

 on a body of powerfully magnetic bars, two or three inches in 

 solid thickness. When the mass of iron is not otherwise magne- 

 tic than as to its magnetism of position derived from the earth, 

 then the action of a magnet passed through its neutral or equa- 

 torial plane, is, in all cases, (as far at least as can be inferred 

 from very many trials), more energetic than when no such sub- 

 stance is near it. This was so particularly the case when the 

 magnetic influence was passed through the two double cylin- 

 ders and furnace of a locomotive steam-engine (consisting of 

 ten successive plates of iron, forming a total of 5| inches in 

 thickness of metal), that it required the magnets to be a foot 

 nearer to the compass in free space, than when placed beyond 

 the engine, to produce equal deviations. For, whilst the dis- 

 tance of the magnets and compass, when the influence was trans- 

 mitted through the engine, was 7 feet 8 inches, the distance on 

 the open side, producing the same deviation, was 6 feet 7 J 

 inches*. 



Now this anomaly may be easily shewn to arise from the de- 

 velopment of magnetism in the mass of iron, through the proxi- 

 mity of the magnets, by which the direct or permeable influence 

 of the magnet is necessarily augmented ; but that the whole of 

 the effect which takes place is not due to this cause, — what is 

 generally called " induced magnetism," — ^but chiefly to the ac- 

 tual transmission of influence through the very substance of the 

 mass of iron, I have distinctly and experimentally determined. 



• Some working engineers, who were present when this experiment was 

 made, were exceedingly inquisitive about the effects produced, watching the 

 deviations of the compass with great astonishment. One of them perceiving 

 that the magnet, which was hid from his view by the engine, equally affected 

 the compass as when presented in the open space, addressed himself to his 

 comrade, and made this characteristic remark, — " Why, Tom," said he, " it 

 sees through't!" 



