CHAPTER I. 



ON THE POLARITY OF THE EARTH'S MAGNETISM. 



Persons who have not paid attention to this interesting ques- 

 tion, may consider this term as simply applied to magnetic 

 needles, but we may observe that it is a property connected 

 with all matter, and that it has an important influence on the 

 general economy of nature. It is well known that if a mag- 

 netic bar be supported in such a manner as to have entire 

 freedom of motion in a horizontal plane, it will, after a few oscil- 

 lations, finally settle in a position directed more or less north 

 and south. If disturbed from this situation and placed in any 

 other direction, it will, as soon as it is again at liberty to move, 

 resume its former position. The two ends are called its poles ; 

 the one which turns to the north being distinguished as the 

 north and the other as the south pole ; the tendency of the mag- 

 net to assume the above described position is called its polarity. 

 On this property is founded the mariner's compass, which is of 

 such essential use in navigation, and by the aid of which the 

 mariner, however distant from land, amidst cloudy skies, or in 

 the darkest nights, is enabled at all times to steer his course with 

 certainty, and traverse in all directions the wide expanse of the 

 ocean which separates the countries and continents of our globe. 

 In exploring subterranean excavations, and, in a word, in all in- 

 vestigations which have reference to the meridian, its aid is 

 equally available. When we reflect on the great benefit mankind 

 has derived from the polarity of magnetism alone, and which 

 has so much contributed to advance the civilization of the human 

 race, it must necessarily create a desire to learn something of its 

 general character. 



It was considered for a long time that the only substance sus- 

 ceptible of magnetism was iron and its oxides, but recent disco- 

 veries have proved that there is no substance but which, under 

 suitable circumstances, is capable of exhibiting unequivocal signs 

 of the magnetic virtue, i. e. radial or lateral attraction, and the 

 usual polar phenomena. The crystalline rocks, constituting the 



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