of Terrestrial Magnetism. 109 



in producing the disturbances in different parts of the globe which 

 have a systematic aspect, and are not unlikely to lead to import- 

 ant generalizations. What has been here stated in respect to 

 the declination appears also to be the case in the other two mag- 

 netic elements, though requiring more labour and patience in 

 the deduction. 



There is yet one other and most notable distinction with refer- 

 ence to geographical relations : the aggregate amount of dis- 

 turbance varies greatly and instructively in different parts of the 

 globe ; it is small in the intertropical regions, and augments in 

 the middle latitudes, but by no means in a ratio dependent on 

 the increase of latitude; for as the higher latitudes are ap- 

 proached, the disproportion of disturbance in different meridians 

 becomes excessive, and leads to the inference that in both hemi- 

 spheres there are localities indicated by the peculiar magnitude 

 of the disturbing influence as being those where that influence 

 may possibly enter on the terrestrial surface, and from whence 

 it may be propagated with progressively decreasing intensity. 

 Such a locality is found in the north-western part of the Ame- 

 rican Continent. Hourly observations made during seventeen 

 months in 1852, 1853, and 1854, at Point Barrow, show an 

 aggregate amount of disturbance unparalleled elsewhere, so far 

 as our knowledge extends — greatly exceeding the amount of 

 disturbance at other stations of hourly observation in the 

 Canadian and Hudson's Bay territories or in high arctic lati- 

 tudes, which amount is itself much greater than is found, or 

 than may be inferred to exist, in corresponding latitudes on this 

 side of the Atlantic. 



Point Barrow is no less distinguished for the extraordinary 

 nrevalence of Aurora, which is now known to be a concomitant 

 phenomenon with magnetic disturbances. During six winter 

 months, three in 1852-53, and three in 1853-54, Aurora was 

 present and visible at nearly one-third of the times of hourly ob- 

 servation, a proportion far beyond any that has been recorded else- 

 where, marking, in common with the excessive amount of mag- 

 netic disturbance, the striking magnetic peculiarity of the locality. 



II. The next cosmical feature which I desire to bring under 

 your notice has been made known to us by discovering the 

 existence of a "semiannual inequality" in the diurnal varia- 

 tion of the declination. When hourly observations have been 

 made at any station on the globe, and the magnetic disturb- 

 ances (or at least the greater portion of them) have been elimi- 

 nated by the process already referred to, the hourly positions of 

 the magnet (i. e. its mean position at each of the twenty-four 

 hours) may be collected into monthly Tables, or Tables corre- 



