272 Rev. Mr Scoresby on some of the Laws 



may be applied. For there seems to be a particular relation be- 

 tween the ductility of iron, and its capacity for magnetism — 

 the softest and most ductile iron having the greatest capacity 

 for the magnetic influence. Hence it is considered, that the 

 experimental determination of the capacity of different speci- 

 mens of iron by the method adopted in the last experiment, 

 might (as already suggested) afford a simple and decided test 

 and measure of their relative quality and value in commerce. 



For this purpose a bar-magnet of moderate dimensions (say 

 of 12 inches in length) and a small compass would be all the 

 apparatus requisite. The different specimens of iron, whose 

 proportionate quality as to ductility is to be determined, might 

 require to be forged into the shape and size of the magnet, or 

 at any rate into a similar shape and size with each other, and 

 then softened equally by being moderately heated and allowed 

 to cool slowly in the same place, or under similar circumstances. 

 The magnet being then placed in the east or west direction of 

 the compass, say at two lengths distance, will indicate, by the 

 deviation of the needle, its own power of attraction, which, being 

 observed, one of the specimens of iron is laid exactly over the 

 top of the magnet separated by two small blocks of wood, or 

 other substance of equal thickness, and then the deviation pro- 

 duced by both extremities of the iron-bar alternately observed. 

 (See Fig. 5.) The difference between the mean of these devia- 

 tions and that of the magnet alone, will serve as a measure of 

 the capacity of that particular specimen for induced magnetism. 

 Whilst the compass and magnet remain undisturbed, the rest 

 of the specimens can be brought successively to the test, being 

 always kept at equal distances from the magnet by the same in- 

 terposed substances, when the comparison of the measures of 

 their different capacities for induced magnetism will^ if the theory 

 be correct, afford a certain indication of their relative qualities. 

 With a view of assisting me in verifying this theory, my friend 

 Edward Roscoe, Esq. of Liverpool, kindly furnished me with 

 several characteristic specimens of the different qualities of iron 

 most commonly manufactured in Britain. Though the examina- 

 tion of these did not enable me to discriminate small differences, 

 yet, when the common and host qualities were compared, the 

 magnetic capacity of the latter proved to be nearly one-tenth 

 greater than any of the common kinds. 



