and Phenomena of Magnetic Induction. 271 



iluction, in reference to distance, are only such as may reason- 

 ably be ascribed to the errors in the measurements of the dis- 

 tance betwixt the magnet and the iron-bar — these distances not 

 having been adjusted with that attention to perfect accuracy 

 which an absolute coincidence would have required. 



Various as the preceding modes of examining the inductive 

 power of the magnet are, and simple and consistent as the re- 

 sults of the last method have proved, yet we do not find that the 

 law of magnetic induction as to distance is satisfactorily shown 

 by any of them. Whilst the law itself is unquestionably uniform 

 and consistent, the effects of that law are found to vary accord- 

 ing to the mode of directing the inductive energy. When the 

 influence is that of a magnet upon a bar of iron lying in the same 

 straight line, the induced energy, as estimated by the tangents 

 of deviation of the compass, approximates the inverse ratio of the 

 squares of the distance ; but when the influence is that derived 

 from the whole mass of the magnet into the whole mass of the 

 bar, lying in parallel juxtaposition, then the ratio appears to be 

 totally different. For when, in the latter case, measures of an 

 inch are made the integer of distance, then the ratio of energy be- 

 comes I, I, J, ^, and J nearly. But when the thickness of the 

 magnet, or of the bar (both being the same), is made the integer 

 of distance, then, the energy at \ of an inch interval, the first 

 distance, being called 100, the series becomes 100 ; 79.2 ; ii^.5 ; 

 544 ; 47.1 ; 40.5 ; 34.4 ; 29.7; 25.3 ; 21.9; 19.4 ; and 16.1,—. 

 at the 12th interval. 



Unsatisfactory as some of these investigations may appear to 

 be, I have been encouraged to proceed thus far with the subject 

 — not only because of the interest necessarily belonging to any 

 of the laws with which the wisdom of God has endowed the va- 

 rious forms of created elements; but because, likewise, of an im- 

 portant practical application of the influence under considera- 

 tion, to which it appears to be peculiarly adapted. I refer to 

 the employment of magnetism of induction for estimating the 

 quality of iron, as to the strength or ductility of which we have 

 no satisfactory test but that of positive trial. 



The ductility of iron being that property which most chiefly 

 determines its value in commerce and manufactures, it is as a 

 measure of this quality that the magnetic influence, I consider, 



