IN THE MEAN EFFECTS OF THE LARGER MAGNETIC DISTURBANCES. 365 
for that element, which being taken=r00 and compared with the actual monthly 
disturbance-values, gives the ratios in the following Table : — 
Table II. 
Months. 
Declination. 
Horizontal 
Force. 
Vertical 
Force. 
Mean. 
July 
0-94 
0*61 
0-71 
0-75 
August 
M6 
0-75 
1-08 
0-99 
September 
1-62 
1-71 
1-61 
1-64 
October 
1*31 
1-48 
1*29 
1-36 
November 
0-78 
0-98 
0-75 
0-84 
December 
0-76 
0-58 
0-61 
0-65 
January 
0*57 
0-56 
0-57 
0-57 
February 
0-84 
0-94 
0-74 
0-84 
March 
1-11 
0-94 
1-08 
1-04 
April 
1-42 
1-50 
1-49 
1-47 
May 
0-98 
0-90 
1-12 
1-00 
June 
0-53 
0-36 
0-50 
0-46 
The evidence afforded by each of the three observational elements, in regard to 
annual variation, is to one and the same effect, viz. January and June are the months 
of minimum disturbance, September and April the months of maximum disturbance. 
The aggregate value of the disturbances in the equinoctial months is about three 
times as great as in the solstitial months. Of the two equinoctial months the value is 
somewhat higher in each element in September than in April ; and of the two solstitial 
months December is higher than June, also in each of the three elements. 
Fig. 2, Plate X. has been drawn in illustration of the annual variation which has 
been thus described. The broken horizontal line is the mean monthly disturbance of 
each element {i. e. the sum of the disturbances in the 12 months divided by 12). The 
dark continuous line for the Declination, the light continuous line for the Horizontal 
Force, and the dotted line for the Vertical Force, show in each case the variation in the 
proportions which the actual disturbances in each month bear to the mean monthly 
disturbance in the same element. The correspondence of the three elements could 
scarcely be more perfect. 
The annual variation which has been thus deduced has reference exclusively to the 
variable amount in the different months of the aggregate values of the disturbances 
of each element, without distinguishing apart or separating the disturbances which 
cause easterly deflections and those which cause westerly deflections ; or those which 
increase and those which decrease the horizontal and vertical forces. When this 
separation is made, we continue to find that each of the two portions into which the 
disturbances of each element are divided exhibits distinctly and notably the same 
general features which have been derived from their conjoint consideration. The 
equinoxes are in all cases the epochs of maxima and the solstices of minima. 
But when we study more carefully the relative prevalence oi disturbances of parti- 
cular character at different periods of the year, — which we may do by forming tables 
3 c 
MDCCCLVI. 
