214 ON DETERMINATIONS OF THE ABSOLUTE VALUE, SECULAR CHANGE, 
Winter < 
Summer<( 
"January 
Table VI. 
1847 and December 1848 
-•0043" 
December 1847 and January 
1848 
— •0034 
I February 
1847 and November 1848 
-•0039 
November 1847 and February 
1848 
-•0037^ 
"May 
1847 and August 
1848 
— •0006" 
August 
1847 and May 
1848 
+•0042 
June 
1847 and July 
1848 
+ •0084 
J uly 
1847 and June 
1848 
+•0070^ 
S — -0038 
> + •0047 
We have here a still further confirmation of the greater amount of the horizontal 
force in the summer than in the winter months ; the difference between the two 
seasons is in this experiment greater than that shown by the Bifilar observations for 
1842, or than that derived from the more extended absolute series from January 1845 
to April 1849. It is quite conceivable however that, independently of errors of mea- 
surement, the actual numerical difference between the summer and winter months 
may be liable to vary in different years. 
By three independent methods of experiment, therefore, the general fact of an 
annual variation of the horizontal force at Toronto has been shown, the force being 
greater in the summer than in the winter months ; but the question of whether this 
variation, as well as that of the inclination, is progressive from one extreme in mid- 
winter to the opposite extreme in midsummer, and vice versa, the regularity of the 
progression being only interrupted by the complication of irregular disturbances, — or 
whether, as in the case of the diurnal variation, the change from one half-yearly phase 
to the other takes place (subject to the same complication) about the time of the 
equinoxes, — will require a longer period for its determination than that which we 
have at present before us. Upon the latter supposition, we find, from the absolute 
series at Toronto, that the inclination is on the average 0''88 above its mean value, 
and the horizontal force ’0015 below its mean value during the five months when 
the sun is in the southern signs, — and the inclination 0'‘90 below, and the hori- 
zontal force ‘0011 above, their respective mean values, when the sun is in the north- 
ern signs. 
The sum of the differences of the inclination at the opposite seasons (T’78) is equi- 
valent, in the resolution of the total force into its horizontal and vertical components, 
to •0070 of horizontal force. The annual variation of the horizontal force derived 
from the observations, corresponds in direction in each of the seasons to that which 
is indicated for it by the change of the inclination, but the amount falls considerably 
short of that which would be the equivalent of the alteration in the latter element. 
Hence we must infer the probable existence of an annual variation of the total force, 
the force being greatest in the winter months, or when the sun is in the southern 
signs ; and least in the summer months, or when the sun is in the northern signs. 
