IN THE MEAN EFFECTS OF THE LARGER, MAGNETIC DISTURBANCES. 
363 
evidence of the periodical affections to which the magnetic disturbances were subject ; 
but the work to be accomplished for the elaboration of that evidence was consider- 
ably greater than in the case of the Declination, from the necessity of eliminating the 
influence of changes of temperature on the magnets employed in measuring the vari- 
ations of the horizontal and vertical forces, before those disturbances could be sepa- 
rated for analysis. The labour required in the different processes of reduction has 
now been gone through ; and it remains to bring together in one view the evidence 
which the three observational and the two derived elements furnish of the periodical 
laws, decennial, annual and diurnal, which regulate the occurrence of the larger 
disturbances. 
In respect to the decennial period, it must be regarded as a fortunate circumstance, 
that the five years of hourly observation, which were commenced before the exist- 
ence of any inequality of longer duration than a year was suspected, began with 
1843 , the year of minimum, and closed with 1848 , the year of maximum disturbance, 
so that the variation has been followed through a complete phase. This has been 
strictly the case in the Declination and Horizontal Forces, and with a single excep- 
tion in the Vertical Force also, the exception being caused by the interruption of 
the observations of that element, for purposes explained elsewhere, during the 
months of October, November and December 1843 , and January and February 
1844 , These months have been supplied in the year ending June 1844 , from hourly 
observations made with the same apparatus in the preceding year, viz. in the months 
of October, November and December 1842 , and January and February 1843 ; thus 
rendering the five years of the Vertical Force complete for the investigation of the 
annual and diurnal variations, but of course in regard to the decennial period the 
months taken from a different year, even though it be the adjacent one, are not a 
perfect substitute. The effect of this substitution has been in fact to swell the 
aggregate value of the disturbances of the Vertical Force in the year nominally ending 
June 30 , 1844 , but really comprising five months taken from a preceding year, so as 
to make them slightly exceed the aggregate value in the year ending June 30 , 1845 . 
A similar slight excess in the aggregate value of the disturbances of the Horizontal 
Force in the year ending June 1 844 over the aggregate value in the year ending June 
1845 , is found when the same five months of the preceding year are substituted for 
its own months ; but when in the case of the Horizontal Force (the observations of 
which were not suspended as were those of the Vertical Force) the actual observations 
throughout the year ending June 1844 are taken, the true progression is restored, and 
the apparent anomaly disappears. 
The variations of the three magnetic elements being measured by instruments 
wholly distinct and unconnected with each other, each element affords an independent 
evidence of the progressive increase in the aggregate values of the larger disturbances 
during the period under examination. The sum of the aggregate values of each 
