398 MAGNETIC STORMS. 



disk, derived by Schwabe from his own systematic observations com- 

 menced in 1826, and continued thenceforward. The decennial varia- 

 tion of the magnetic storms is based on the observations of the four 

 widely distributed colonial observatories, and is concurred in by all. 

 This remarkable correspondence between the magnetic storms and 

 physical changes in the sun's photosphere, of such enormous magni- 

 tude as to be visible from the earth even by the unassisted eye, must 

 be held to terminate altogether any hypothesis which would assign to 

 the cause of the magnetic disturbances a local origin on the surface or 

 in the atmosphere of our globe, or even in the terrestrial magnetism 

 itself, and to refer them, as cosmical phenomena, to direct solar influ- 

 ence, leaving for future solution the question of the mode in which 

 that influence produces the effects which we believe we have thus 

 traced to their source in the central body of our system.* 



We may regard as a step towards this solution the separation of the 

 disturbances of the declination into two distinct forces acting in differ- 

 ent directions and proceeding apparently from different foci: the phe- 

 nomena of distinct (though in so many respects closely allied) variations 

 exhibit the same peculiar' features at all the stations to which the 

 analysis has hitherto extended, and have been exemplified by the ob- 

 servations at Kew, as shown in the early part of this paper. A similar 

 separation into two independent affections, each having its own distinct 

 phenomenal laws, has followed from an analysis of the same description 

 applied to the disturbances of the magnetic dip and force at the colonial 

 stations ; thus placing in evidence, and tracing the approximate laws 

 of the effects of six distinct forces (two in each element) contemporane- 

 ously in action in all parts of the globe, and pointing in no doubtful 

 manner to the existence of two terrestrial foci or sources in each hemis- 

 phere from which the action of the forces emanating from the sun and 

 communicated to the earth may be conceived to proceed. Such an as- 

 cription naturally suggests to those conversant with the facts of terres- 

 trial magnetism the possibility that Halley's two terrestrial magnetic 

 foci in each hemisphere may be either themselves the localities in ques- 

 tion, or may be in some way intimately connected with them. The 

 important observations which we owe to the zeal and devotion of Cap- 

 tain Maguire, K. N., and the officers of H. M. S. Plover, have made 

 us acquainted with Point Barrow as a locality where the magnetic 

 disturbances prevail with an energy far beyond ordinary experience, 



*The existence of a decimal period of the magnetic storms was not, as some have sup- 

 posed, a fortuitous discovery; but a consequence of process of examination early adopted 

 and expressly devised by the employment of a constant separating value, to make known any 

 period of longer or shorter duration which might fall within the limits comprised by the ob- 

 servations. The period being decennial and the epoch of minimum occurring at the end of 

 1843, or beginning of 1844, the epoch of maximum was necessarily waited for in order to 

 ascertain the precise duration of the cycle. The maximum took place in 1848 and 1849, 

 the observations in 1850 and 1851, showing that aggregate value of annual disturbances was 

 again diminishing as it had been in 1842 and 1843. The process of determining the propor- 

 tion of disturbance in different years is a somewhat laborious one, and requires time: but in 

 March, 1852, I was able to announce to the Royal Society the existence of a decimal varia- 

 tion, based on the concurrent testimony of the observations at Toronto and Hobarton, deem- 

 ing it proper that so remarkable a fact should not be publicly stated until it had been 

 thoroughly assured by independent observations at two very distant parts of the globe. 



