530 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



the equator ; it is also subject to diurnal variations, which in Europe 

 draw the north-seeking end of the needle westwards from sunrise to 

 one or two o'clock in the afternoon, and bring it back again towards 

 its original position till ten o'clock at night. The amplitude of these 

 variations oscillates between five or six minutes and twenty or twenty- 

 five minutes. 



The perturbations or irregular variations of terrestrial magnetism 

 consist in sudden changes which show themselves in the position of 

 the needle of the compasses. Some are very evidently connected 

 with natural phenomena, such as the aurora borealis, and possibly 

 the eruptions of volcanoes, and the shocks of earthquakes; others 

 are from unknown causes. 



We do not enter further into details on this interesting subject 

 which is to be found developed in original memoirs and in works on 

 physical geography. Our object has been to point out the importance 

 of its applications. 



