204 ON MAGNETISM. 



at the Kew Observatory, and has not remarked 

 any sensible difference. In most cases, but not in 

 all, the disturbances in the east or west direction are 

 comparable with those in the north or south direction, 

 and greater than those in the vertical direction. The 

 periods of great disturbance sometimes occupy a portion 

 of a single day, sometimes several days in succession: 

 they are familiarly known by the name of 'magnetic 

 storms.' They are not connected with thunder-storms 

 or any other known disturbance of the atmosphere ; but 

 they are invariably connected with exhibitions of 

 Aurora Borealis, and with spontaneous galvanic cur- 

 rents in the ordinary telegraph-wires : and this connec- 

 tion is found to be so certain that, upon remarking the 

 display of one of the three classes of phenomena, we can 

 at once assert that the other two are observable (the 

 Aurora Borealis sometimes not visible here, but certainly 

 visible in a more northern latitude). 



But when the ordinates are picked out from the 

 different sheets for the same hour of the day on every 

 day through a year or through several years, the 

 irregularities neutralize each other in a great degree : 

 and the mean laws of inequality of the magnetic 

 elements for different hours of the day have a very 

 close resemblance, as deduced from different years. 

 They are not however precisely the same : the change 

 in their type is gradual, but it does not recur in any 

 cycle of years or according to any other law yet estab- 

 lished. 



Having ascertained, from the mean of all the photo- 



