ggO ' LECTURE LV. 



atttacted towards the globe than if the influence of magnetism were removed ; 

 except when the small needle is placed very near to one of the poles of the 

 artificial magnet, or, on the other hand, when the dipping needle is employed 

 in the neighbourhood of some strata of ferruginous substances, which, in 

 particular parts of the earth, interfere materially with the more general effects, 

 and alter the direction of the magnetic meridian. 



A bar of soft iron, placed in the situation of the dipping needle, acquires 

 from the earth, by iuduction, a temporary state of magnetism, which may 

 be reversed at pleasure by reversing its direction; but bars of iron, which 

 have remained long in or near this direction, assume a permanent polarity ; 

 for iron, even when it has been at first quite soft, becomes in time a little 

 harder. A natural magnet is no more than a heavy iron ore, which, in the 

 course of ages, has acquired a strong i)olarity from the great primitive mag- 

 net. It must have lain in some degree detached, and must possess but 

 little conducting power, in order to have received and to retain its mag- 

 netism. 



We cannot, from any assumed situation of two or more magnetic poles, 

 calculate the true position of the needle for all places; and even in the same 

 place, its direction is observed to change in the course of years, according 

 to a law which has never yet been generally determined, although the varia- 

 tion which has been observed, at any one place, since the discovery of the 

 compass, may perhaps be comprehended in some very intricate expressions; 

 but the less dependence can be placed on any calculations of this kind, as 

 there is reason to think, that the change" depends rather on chemical than on 

 physical causes. Dr. Halley indeed conjectured that the earth contained a 

 nucleus, or separate sphere, revolving freely within it, or rather floating in 

 a fluid contained in the intermediate space, and causing the variation of the 

 magnetic meridian ; and others have attributeil the efliect to the motions of 

 the celestial bodies: but in either case the changes produced would have 

 been much more regular and universal than those which have been actually 

 observed. Temporary changes of the terrestrial magnetism have certainly 

 been sometimes occasioned by other causes ; such causes are, therefore, 

 most likely to be concerned in the more permanent effects. Thus, the erup- 

 tion of Mount Hecla was found to derange the position of the needle consi- 

 derably; the aurora borealis has been observed to cause its north pole to move 



