Horizontal Component of Magnetic Force. 



23i 



The last line of Table XI. also gives the mean of the horizontal component for the year greater than the 

 mean at 2211 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 



Mean at 23^ 0-001030, or = 0-001594 



Mean at 2^^ greater 0-000056, or = 0-002680 



Mean at Si^ 0-000586, or = 0-003210 



From Table IX., Abstracts for the Observations of 1842, we obtain the following means in scale divisions : — 

 1842. 20i» = 514-56, 231^ = 510-09, 2i> = 521-82, 51" = 526-37. 



Converting these means into parts of force by the first formula given after Table VIII., we obtain the 

 following results : — 



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^ 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™ 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™ 

 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 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™ on the Term-days of 1842 and 1843, the whole Horizontal Component being 

 Unity. 



Winter 

 Solstice. 



Equinoxes. 



Summer 

 Solstice. 



1842. 



1843. 



Mean. 



Hour. 



AVinter 

 Solstice. 



Equinoxes. 



Summer 

 Solstice. 



1842. 



1843. 



Mean. 



0-00 



0-00 



0-00 



0-00 



0-00 



0-00 





0-00 



11-00 



0-00 



0-00 



0-00 



(-00 



0505 



2336 



1547 



1852 



1045 



1449 



22 



0169 



0224 



0005 



0156 



0080 



0119 



0430 



1806 



1729 



1417 



1198 



1308 



23 



0042 



0000 



0000 



0000 



0000 



0000 



0349 



1852 



1814 



1328 



1320 



1324 







0000 



0361 



0337 



0342 



0095 



0219 



0614 



0872 



1649 



1010 



1051 



1031 



1 



0250 



0861 



0686 



0656 



0514 



.0585 



0445 



1492 



1425 



1224 



0989 



1107 



2 



0531 



1357 



1420 



1335 



0842 



1089 



0614 



1375 



1174 



1022 



1058 



1040 



3 



0731 



1845 



1520 



1532 



1170 



1351 



0558 



0766 



1242 



0607 



1075 



0841 



4 



0755 



1957 



1881 



1663 



1370 



1517 



0486 



1377 



1190 



0972 



1035 



1004 



5 



0757 



1942 



1996 



1656 



1446 



1551 



0741 



1477 



0784 



0987 



0986 



0987 



6 



0724 



1899 



2197 



1832 



1352 



1593 



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 



0517 



0861 



0234 



0526 



0520 



0523 



9 



0556 



1404 



2140 



1356 



1349 



1353 



