400 



and advantage receive the appellation of " occasional" are, in their 

 mean or average effects, subject to periodical laws of a very system- 

 atic character ; placing them, as a first step towards an acquaintance 

 with their physical causes, in immediate connexion with the sun 

 as their primary exciting cause. They have 1, a diurnal variation 

 which follows the order of the solar hours, and manifests therefore 

 its relation to the sun's position as affected by the earth's rotation 

 on its axis ; 2, an annual variation, connecting itself with the sun's 

 position in regard to the ecliptic ; and 3, a third variation, which 

 seems to refer still more distinctly to the direct action of the sun, since, 

 both in period and in epochs of maximum and minimum, it coincides 

 with the remarkable solar period of about ten, or perhaps more nearly 

 eleven, of our years, the existence of which period has been recently 

 made known to us by the phenomena of the solar spots ; but which, 

 as far as we yet know, is wholly unconnected with any thermic or 

 physical variation of any description (except magnetic) at the surface 

 of the earth, and equally so with any other cosmical phenomena with 

 which we are acquainted. The discovery of a connexion of this re- 

 markable description, giving apparently to magnetism a much higher 

 position in the scale of distinct natural forces than was previously 

 assigned to it, may justly be claimed on the part of the Colonial Ob- 

 servatories, as the result of the system of observation enjoined (and 

 so patiently and carefully maintained), and of the investigation for 

 which it has supplied the data ; since it was by means of the disturb- 

 ance-variations so determined, that the coincidence between the phe- 

 nomena of the solar spots and the magnitude and frequency of mag- 

 netic disturbances was first perceived and announced (Phil. Trans. 

 1852, Art. VIII.). 



The extent and mutual relation of the disturbance-variations of the 

 three elements, even at a single station, supply a variety of points of 

 approximation and of difference, which are well suited to elucidate 

 the physical causes of these remarkable phenomena; but valuable as 

 such aids may be when obtained for a single station, their value is 

 greatly augmented when we are enabled to compare and combine the 

 analogous phenomena, as they present themselves at different points 

 of the earth's surface. To give but a single example : there are cer- 

 tain variations produced by the mean effects of the disturbances 

 which attain their maximum at Toronto during the hours of the 



