Reply to Mr. Barnes on Magnetic Polarity. 123 



verting the bar." The experiments were repeated by Mr. 

 Barnes with the same results. 



Now surprising as these results may have appeared to 

 these gentlemen, they were first made known about the year 

 1600, and have been distinctly stated in almost every work 

 on magnetism since that time. Mr. Barnes indeed appears 

 to have been, in part, aware of this, as he says — " Dr. Gilbert 

 has mentioned the fact that opposite ends of an iron bar 

 equally affect the magnet, and in the same way, and he ac- 

 counts for the fact, by supposing that the earth magnetizes 

 the bar instantaneously." But still the whole subject is to 

 him a mystery, as is evident from the following questions, 

 which he gravely propounds — viz. " Is then the common re- 

 mark, in the books, that a bar becomes magnetic by long 

 standing in a vertical position strictly true ? And will not 

 any such bar instantly change its polarity by being inver- 

 ted r 



In answer to these questions, I would remark, that the dif- 

 ference depends wholly upon the kind of bar employed. A 

 bar of hard iron or steel, like a bar of soft iron, when held 

 in a vertical position, becomes magnetized by the influence of 

 the earth. But in the bar of soft iron, the magnetism is im- 

 pressed instantaneously ; — in the bar of hard iron or steel, 

 time is necessary for its development. In the bar of soft iron, 

 the magnetism is transient ; in the bar of hard iron, it is per- 

 manent. In the bar of soft iron, therefore, a change of po- 

 sition produces an instantaneous change in polarity ; — in 

 the bar of hard iron, a change of poles would require the 

 same time that was necessary to render it magnetic. If 

 Mr. Barnes is still in doubt upon these points I would refer 

 him to Biofs Traite Precis, third edition, vol. 2d, pp. 8th 

 and 9th. Cavallo's Philosophy, second American edition, 

 vol. 2d, pp. 279 and 280, and indeed to almost any work in 

 which magnetism is treated of. 



The " Notice 1 ' is chiefly made up of experiments to ascer- 

 tain what is denominated the " neutral point, or medium be- 

 tween polarities directly opposite.'" From the manner in 

 which their result is announced, I should infer that the author 

 was not aware, that several years since, Mr. Barlow in his 

 researches upon the method of correcting the local varia- 

 tion of a ship's compass, ascertained the fact — that in every 

 mass of iron there is a plane of no attraction — a plane, in 

 which a compass being placed, the iron has no effect upon 



