246 



however, much more considerable than the latter, and the 

 difference reaching a maximum about 10 p. m. 



It thus appears that the mean daily disturbance observes 

 a regular period, both in magnitude and direction ; and this 

 period, it is worthy of remark, is precisely the reverse of that 

 of the regular diurnal movement, — the mean position of the 

 magnet being nearly constant during the night, the mean dis- 

 turbance during the day ; — the principal oscillation of the 

 magnet, in the regtdai' movement, being to the west during 

 the day, while that of the irregular movement is to the east 

 during the night. From these remarkable relations it seems 

 evident that the two classes of phenomena are physically con- 

 nected ; and Dr. Lloyd is led to regard the disturbance which 

 prevails about 10 p.m., as an irregular reaction 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. This (which is con- 

 trary to the results deduced by Professor Kreil and Colonel 

 Sabine, with reference to the Jregtiency of disturbances ex- 

 ceeding a certain limit) appears to be the fact. The mean 

 disturbance, deduced from the observations of 1843, is, for 

 the summer six months, 2'.9, and for the winter 2'. 2 ; so that 

 it observes an annual as well as a diurnal period. 



It by no means necessarily follovps, from the results now 

 stated, that all disturbances have a periodical character. 

 There probably are tzvo classes of disturbances, the results 

 of distinct physical causes, of which one observes a period, 

 while the other is wholly irregular ; and it is manifest that, 

 in such a case, the period of the former will necessarily be 

 impressed upon the resultant mean disturbance. Dr. Lloyd 

 stated that he had instituted, during the last year, a scries of 

 observations at short intervals, which seem to aftbrdthe means 



