58 MAGNETISM. 



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 polarity, from the great primitive magnet. 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 gene- 

 rally determined, although the variation 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 withiu it, 

 or rather floating in a fluid contained in the intermediate space, and 

 causing the variation of the magnetic meridian ; and others have 

 attributed the effect to the motions of the celestial bodies : but in 

 either case the changes produced would have been much more regu- 

 lar 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 eruption of Mount Hecla was found to derange the position of 

 the needle considerably; the aurora borealis have been observed to 

 cause it-, north pole to move six or seven degrees to the westward of 

 its iiMial position; and a still more remarkable change occurs con- 

 tinuiiliy in the diurnal variation. In these climates the north pole of 

 the needle moves slowly westwards from about eight in the morn- 

 ing till two, and in the evening returns again ; a change which has 

 with great probability been attributed to the temporary elevation of 

 the tempi-ratiin- of the earth, eastwards of the place of observation, 

 where the sun's action takes place at an earlier hour in the morning, 

 and to the diminution of the magnetic attraction in consequence of 

 the heat thus communicated. in winter this variation amounts 

 to about seven minutes, in summer to thirteen or fourteen. 



Important as the use of the compass is at present to navigation, 

 it would be still more valuable if its declination from the true meri- 



