HORIZONTAL COMPONENT OF MAGNETIC FORCE. 231 
24 observations minus 9 observations. 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
0-0000 | —36 -61 —69 +24 — 24 +13 +29 — 34 +68 +46 — 06 — 28 
These quantities being applied as corrections to the monthly means, foot of Table VIII., we obtain the 
following :— 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
0:00 | 1708 1697 1502 1686 2404 2769 2682 2753 3044 3206 3784 4179 
From these it appears that the horizontal component diminishes from J anuary to March, increases from 
March to June, diminishes again slightly from June to between July and August, and then increases again 
till December ; the whole range being 0-002677, and the increase of force from January to December being 
0-002471. The annual period can only be rendered distinct when this secular change is eliminated. It has 
been found by a comparison of the monthly means of 1843 with those of 1842 and of 1844, that the mean 
annual change during the 12 months of 18438 is 0002826. When proportional parts of this are subtracted 
from the means after January, we have— 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
0:00| 1708 1462 1031 0980 1462 1592 1269 1105 1160 1087 1429 1589 
We have here a well marked annual period, consisting of maxima near the solstices, and minima near the 
equinoxes. The maximum is rather greater in the winter than in summer, but the difference is less than the 
effect of half a degree Fahrenheit on the bifilar magnet. 
The range of the annual period is 0-000728, or about half the mean diurnal range for the year. 
It may be desirable to examine the annual period for the year 1842 in a similar manner, If we reduce 
the monthly means of four observations, Table IX., Abstracts for 1841-2, to parts of force by the formula 
f = (n — 530) 0:0001021 + 0-003000 
where » is the mean in scale divisions in the Table referred to, we obtain the following values of f, which may 
be rendered comparable with the means, Table VIII., by subtracting 0-002000. The second line below con- 
tains the corrections for each month of the 4 observations to the 24 as deduced from the observations for 1844. 
and the third line contains the corrected means, 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
0-00 0960 0994 1188 0840 1646 1803 1743 1836 2016 2488 2772 3270 
0-00 | —0057 —0045 —0003 +0294 +0245 +0309 +0352 +0390 +0318 +0187 +0068 —0020 
0-00 0903 0949 1185 1134 1891 2112 2095 2226 2334 2675 2840 3250 
If the secular change be assumed the same for 1842 as for 18438, and if this be eliminated, we have— 
Jan. Feb. March. April, May. June. July. Aug. Sept. Oct. Nov. Dec. 
0-000 | 903 714 714 428 949 935 682 578 450 556 485 660 
These means shew the same law as the means for 1843, with more irregularity, it is true, but this may 
be accounted for by the fewness of the observations, and the possible inaccuracy of the corrections. 
