Horizontal Component of Magnetic Force. 
235 
The last line of Table XI. also gives the mean of the horizontal component for the year greater than the 
mean at 22 h by 0'000960, and, from Table VIII., the mean of the horizontal component for the year 1843 
= 0-002624, whence — 
Mean at 20 h less than the mean for the year by 0-000286, or = 0-002338 
Mean at 23 h 0-001030, or = 0-001594 
Mean at 2 h greater 0-000056, or = 0-002680 
Mean at 5 h 0-000586, or = 0-003210 
From Table IX., Abstracts for the Observations of 1842, we obtain the following means in scale divisions : — 
1842. 20 1 ' = 514-56,' 23 h = 510-09, 2 h = 521-82, 5 h = 526-37. 
Converting these means into parts of force by the first formula given after Table VIII., we obtain the 
following results : — 
20 h . 23 b 2 h - 5 h - 
1842. -0-000576, -0-001033, +0-000165, +0-000629 
1843. +0-002338, +0-001594, +0-002680, +0-003210 
Secular Change. +0-002914, +0-002627, +0-002515, +0-002581 
Mean Secular Change, 1842 to 1843, = +0-002658 
Diurnal Variation of the Horizontal Component of Magnetic Force. 
The means of the nine observations in Table XI. indicate two maxima and two minima in the months of 
January, February, November, and December ; only one maximum and one minimum is visible in the means for 
the remaining months. The means for the four seasons are types of the months which compose them. 
The principal minimum occurs in each month, and in the mean for the year about 9 h a.m. 
The principal maximum occurs at 3 h 10 m p.m. in winter, about 4 h 10 m p.m. in spring and autumn, at 6 1 ' 
10 m p.m. in summer, and at 5 h 10 m p.m. in the mean for the year. 
A secondary maximum occurs in winter about 6 h a.m. 
The principal maximum in December occurs at 6 h 10 m a.m., in all the other months the principal maximum 
occurs in the afternoon. 
In order to determine the periods of the secondary maximum and minimum, the observations made at 0 m 
on the Term-days of 1842 and 1843 have been made use of. The observations in 1842 were corrected by the 
temperature coefficient obtained by the method of deflections, those of which the means are given in the following 
Table were farther corrected by the coefficient — 0-46 Sc. div., the difference between the temperature coefficient 
obtained from deflections and that deduced from comparisons of the daily observations. The means in the 
following Table are then deduced from the observations at 0 m in 1842 thus corrected, and the observations at 
0 m in 1843, as corrected in this volume. The winter solstice includes the months of November, December, 
January, and February, 1842 and 1843 ; the equinoxes, March, April, September, and October, 1842 and 
1843 ; and the summer solstice, May, June, July, and August, 1842 and 1843. 
TABLE XII. — Diurnal Variation of the Horizontal Component of Magnetic Force deduced from the 
Observations at 0 m on the Term-days of 1842 and 1843, the whole Horizontal Component being 
Unity. 
our. 
Winter 
Solstice. 
Equinoxes. 
Summer 
Solstice. 
1842. 
1843. 
Mean. 
Hour. 
Winter 
Solstice. 
Equinoxes. 
Summer 
Solstice. 
1842. 
1843. 
■ 
Mean. | 
O'OO 
0-00 
0-00 
0-00 
0-00 
0-00 
0-00 
0-00 
0-00 
0-00 
0-00 
! 
0-0(1 
10 
0505 
2336 
1547 
1852 
1045 
1449 
22 
0169 
0224 
0005 
0156 
0080 
0119 1 
0430 
1806 
1729 
1417 
1198 
1308 
23 
0042 
0000 
0000 
0000 
0000 
0000 
12 
0349 
1852 
1814 
1328 
1320 
1324 
0 
0000 
0361 
0337 
0342 
0095 
0219 
13 
0614 
0872 
1649 
1010 
1051 
1031 
1 
0250 
0861 
0686 
0656 
0514 
0585 
.4 
0445 
1492 
1425 
1224 
0989 
1107 
2 
0531 
1357 
1420 
1335 
0842 
1089 
15 
0614 
1375 
1174 
1022 
1058 
1040 
3 
0731 
1845 
1520 
1532 
1170 
1351 
6 
0558 
0766 
1242 
0607 
1075 
0841 
4 
0755 
1957 
1881 
1663 
1370 
1517 
7 
0486 
1377 
1190 
0972 
1035 
1004 
5 
0757 
1942 
1996 
1656 
1446 
1551 ' 
8 
0741 
1477 
0784 
0987 
0986 
0987 
6 
0724 
1899 
2197 
1832 
1352 
1593 
I 
0818 
1550 
0482 
0984 
0887 
0936 
7 
0522 
1681 
2466 
1714 
1370 
1542 
0655 
1191 
0324 
0646 
0772 
0709 
8 
0636 
1376 
2224 
1416 
1380 
1398 
1 
0517 
0861 
0234 
0526 
0520 
0523 
9 
0556 
1404 
2140 
1356 
1349 
1353 
