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, itis 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 regular movement, being to the west during 
the day, while that of the trregular 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 trregular 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 frequency 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 follows, from the results now 
stated, that ali disturbances have a periodical character. 
There probably are fwo 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 reswléant mean disturbance. Dr. Lloyd 
stated that he had instituted, during the last year, a series of 
observations at short intervals, which seem to afford the means 
