HorizontaL Component OF MAGNeEtIc 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% by 0:000960, and, from Table VIII., the mean*of the horizontal component for the year 1843 
=0-002624, whence— 
Mean at 20" less than the mean for the year by 0:000286, or = 0:002338 
Micambstnd. 3! irae ee reete tere ee ca 0:001030, or = 0:001594 
WMearyate2" creator / pends heedeswcacsisserlss 0:000056, or = 0:002680 
IM Meariee ty Oe tis ah me Rare eae tt Se satarclatafard varc{c is ele/s'eie's «ale 0:000586, or = 0:0038210 
From Table IX., Abstracts for the Observations of 1842, we obtain the following means in scale divisions :— 
1842. 20 = 514-56, 23 = 510-09, 2 = 521-82, 5% = 526°37. 
Converting these means into parts of force by the first formula given after Table VIII., we obtain the 
following results :— 
20. 23h 2h. 5h, 
1842. —0:000576, —0-001033, +0:000165, +0-000629 
1843. + 0:002338, +0-001594, +0°002680, +0-003210 
Secular Change. +40:002914, +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" a.m. 
The principal maximum occurs at 3" 10™ p.m. in winter, about 4 10™ p.m. in spring and autumn, at 6” 
10™ p.m. in summer, and at 5" 10™ p.m. in the mean for the year. 
A secondary maximum occurs in winter about 6" a.m. 
The principal maximum in December occurs at 6" 10™ 4.M., in all the other months the principal maximum 
occurs in the afternoon. 
Tn order to determine the periods of the secondary maximum and minimum, the observations made at 0™ 
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" in 1842 thus corrected, and the observations at 
0™ in 1848, 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 18438. 
TABLE XII.—Diurnal Variation of the Horizontal Component of Magnetic Force deduced from the 
Observations at 0™ on the Term-days of 1842 and 1843, the whole Horizontal Component being 
Unity. 
Winter : Summer Winter . Summer 
Solstice. | Solstice. 1842. 1843, Mean. Hour. Solstice, Equinoxes. Solstice. 1842. 1843. 
! 
0-00 0-00 0-00 0-00 \ Rae 0-00 0-00 0-00 0-00 
1547 1852 1045 1449 22 0169 0224 0005 0156 0080 
1729 1417 1198 1308 23 0042 0000 0000 0000 0000 
1814 1328 1320 1324 0 0000 0361 0337 0342 0095 
1649 1010 1051 1031 1 0250 0861 0686 0656 0514 
1425 1224 0989 1107 2 0531 1357 1420 1335 0842 
1174 1022 ‘1058 1040 3 0731 1845 1520 1532 1170 
1242 0607 1075 0841 4 0755 1957 1881 1663 1370 
1190 0972 1035 1004 5 0757 1942 1996 1656 1446 
0784 0987 0986 0987 6 0724 1899 2197 1832 1352 
0482 0984 0887 0936 7 0522 1681 2466 1714 1370 
0324 0646 0772 0709 8 0636 1376 2224 1416 1380 
0234 0526 0520 0523 9 0556 1404 2140 1356 1349 
