VERTICAL CoMPONENT OF MAGNETIC FoRCE. 243 
The mean at 18 in January is rendered comparable with the means at the other hours, thus :— 
Mic. Div. 
Mean of balance magnetometer readings, January 9—31. = 768°8 
shack ghueslSbaadHe ease mic re Raa EREn a CRP tas January 16—81. = 7645 
Difference, 3 4 ‘ A =n 4:3, 
The mean for 185 deduced from the observations January 16 to 31 was therefore corrected by + 4:3 
micrometer divisions, in order to reduce it to the mean from January 9 to 31, as for the other hours. 
TABLE XVIII.—Diurnal Variations of the Vertical Component of Magnetic Force in 18438, the whole 
Vertical Component being Unity. 
8h, 104, 
i 0-00 A i 0-00 0-00 
January 0059 0189 | 0168 
February 0049 | 0215 | 0155 
March 0060 0256 | 0134 
April 0060 0423 | 0213 
May 0158 0272 | 0072 
June 0180 0220 | 0162 
July ( 0078 0260 | 0133 
August 0122 0230 | 0103 
September 0158 3 0169 | 0045 
October 0098 0182 | 0116 
November ~ 0026 0151 | 0126 
December 0017 0263 | 0168 
Spring 0055 0297 | 0166 
Summer 0110 0222 | 0093 
Autumn 0116 0184 | 0078 
Winter 0034 0201 | 0154 
Winter Solstice 0038 0204 | 0154 
Equinoxes 0093 0257 | 0126 
Summer Solstice 0111 0222 | 0095 
The Year 0070 0217 | 0114 
DIURNAL VARIATION OF THE VERTICAL COMPONENT OF MAGNETIC FORCE. 
An examination of the monthly means will shew, that they may be arranged into three very distinct groups, 
namely, the months January, February, November, and December, about the winter solstice; the months 
March, April, September, and October, about the equinoxes ; and the months May, June, July, and August, 
about the summer solstice. A consideration of the monthly means, however, will shew, that the diurnal curves 
pass gradually from the type of the one group to that of the next following. 
Tn the means of the months about the winter solstice there is but one maximum and one minimum dis- 
tinctly shewn ; the diurnal curve is single. 
The minimum occurs before 5" 10™ a.m., Makerstoun mean time. 
The maximum occurs between 6% and 7" p.M., .....-..2.seeeeees 
There is a slight inflexion in the curve about 11 a.m., tending to a minimum; this minimum is more or less 
shewn in the separate monthly means, but never very distinctly excepting in February. 
In the mean of the months, about the equinoxes, there are two maxima and two minima shewn ; the diurnal 
curve is double :— 
The minimum oceurs between 9" p.m, and 5" a.m., Makerstoun mean time. 
The maximum occurs at 52 10™ p.m., 
A secondary maximum occurs about 9? 10™ a.m., 
A secondary minimum ............ 115 407 a.m., 
