-v>-v 



MAGNETISM. 



113 



The Dutch navigators, in 1599, also constructed accurate tables of varia- 

 tion. 



In the year 1576, Robert Norman, a mathematical instrument maker in 

 London, discovered the dip. He found that the card of the compass near the 

 north point was always depressed or inclined downward, so that he was 

 obliged to put a counterpoise on the southern pole of the needle, to keep it 

 level. 



Mentioning this circumstance to some scientific friends, he was advised to 

 construct a needle on a horizontal axis, and to observe the position to which 

 this downward inclination would bring the northern pole. He accordingly 

 constructed the first dipping needle, and found the dip to be about seventy-one 

 and a half degrees. 



The variation of the needle was accurately observed at London by Burrough, 

 the friend of Norman, who found that in the year 1581 it was eleven degrees 

 and fifteen minutes east. In the treatises extant by Norman and Burrough, no 

 reference is made to any change, periodical or otherwise, either in the varia- 

 tion or the dip. 



In the following century, the change to which the variation is subject was 

 observed by Mair, Gunter, Gellibrand, and Bond. In the year 1599, Edward 

 Wright wrote a work on the compass, which was published by Prince Mau- 

 rice, lord high admiral of the United Provinces, in which the advantage of 

 keeping registers of the variations observed on all voyages is urged. Thus 

 the variation of the variation, not only as to time, but as to place, had at this 

 period begun to receive the attention of those engaged in navigation. 



When the influence of magnets on ferruginous matter came to be examined, 

 it was soon apparent that they not only enjoyed the property of attraction, but 

 that soft iron, so long as it remained within the sphere of their influence, actu- 

 ally acquired their own nature, and became magnetic also. When withdrawn 

 from the influence of the magnet, the iron was found to return to its natural 

 state. If, however, the iron, while influenced by the magnet, were twisted, 

 filed, hammered, or submitted to other violence affecting its structure, it was 

 then found to preserve the magnetism it had acquired, even when withdrawn 

 from the magnet. 



When iron filings were scattered over a sheet of paper under which a mag- 

 netic bar was placed, it was found that the metallic powder arranged itself in 

 a particular manner, indicating different intensities of attraction in different 

 parts of the bar. At a point near the centre the attraction seemed to cease, 

 and to be augmented in each direction toward the extremities. The polarity 

 of the magnet was consequently apparent. The points where the attraction 

 seemed to be most intense were called the poles. 



When a magnetic bar was broken in the middle, or at the neutral point, each 

 part was found to acquire separate polarity, and, like the original magnet, to 

 have two poles with neutral points intermediate. When magnetism was im- 

 parted by a magnet to a bar of iron, the former lost none of its own magnetic 

 force. Hence it was inferred that, in giving magnetism, the magnet lost none 

 of the magnetic fluid. 



When a magnet was brought in contact with a piece of steel, the effect was 

 first discovered to be feebly but gradually increased, until the steel itself be- 

 came a permanent magnet, but that this might be effected suddenly by friction. 

 Bars of steel, thus magnetized, were called artificial magnets. 



Gilbert, in his work already referred to published in the sixteenth century, 

 mentions that the fact of magnetism being imparted to a bar of iron by the earth 

 itself, was first discovered by examining the rod of the weathercock of the 

 church of the Augustines at Mantua. 

 VOL. II. 8 



