Proceedings of the British Association. 263 



in a direction opposed to that of the regular day movement. From 

 these remarkable relations, which hold also between the changes, 

 regular and irregular, of the horizontal intensity, it seems evident 

 that the two classes of phenomena are physically connected. Without 

 entering into the question of the mode of this connection, Dr. Lloyd 

 said that he regarded the disturbance of the two elements (in part at 

 least) as an irregular re-action from the regular day movement, and 

 dependent upon it both for its periodical character and for its 

 amount. If this hypothesis be a just one, it will of course follow 

 that the magnitude of the mean disturbance will vary, in some direct 

 proportion to the daily range, and should, therefore, be greater 

 in summer than in winter. Now this (which is contrary to the 

 results deduced by Prof. Kreil and Col. Sabine, with reference to the 

 frequency of disturbances, extending a certain limit) appears to be 

 the fact. If we calculate the mean disturbance of the declination for 

 the several quarters of the year, we find it to be as follows — 

 Spring. Summer. Autumn. Winter. 



2'-66 3'-02 2'-52 l'-80. 



From these results, it appears that the mean disturbance observes an 

 annual as well as diurnal period ; its maximum occurring in summer, 

 its minimum in winter, while in spring and autumn its values are 

 nearly equal. This important relation appears to confirm, in a re- 

 markable manner, the views above given. It by no means necessarily 

 follows, from the results above stated, that the periodical character 

 necessarily belongs to all disturbances. It may be that there are 

 two classes of disturbances, the results of distinct physical causes, of 

 which one observes a period, while the other is wholly irregular ; for 

 it is obvious that on such an hypothesis, the period of the former 

 would necessarily be impressed upon the resultant disturbance, and 

 that the latter would have no effect in effacing it, provided the obser- 

 vations from which it was inferred were sufficiently numerous. There 

 are many circumstances which seem to render this supposition highly 

 probable, and if it be established, the next step in the investigation 

 will be to distinguish these two kinds of disturbances by their exter- 

 nal characters, and to resolve the complex resultant, where they 

 happen to be combined into its more simple elements. Dr. Lloyd 

 stated that he had commenced a series of observations in Dublin, 



2 M 



