xlvi GENERAL RESULTS OF THE MAKERSTOUN OBSERVATIONS. 
83. The numbers in the last column of Table 39 have been obtained by comparing the mean of the first six 
months of the year, for which the secular change is sought, with that for the corresponding six months of the 
following year, and the mean for the last six months.with the corresponding months of the preceding year ; the 
mean of the two is taken for the secular change during the year : only one comparison could be made for 1842 
and for 1849. 
84. The vertical component has diminished from year to year ; the value of the secular change has also 
diminished since 1842; the greatest diminution occurring in the year 1846, after which year the value of the 
secular change has not varied greatly : it was least in 1848, and appears as large in 1849 as in 1846. It is 
probable that the apparent secular change is partially, if not wholly, due to loss of magnetism in the needle, 
especially in the first four years. (See the Section Magnetic Dip.) 
85. When we deduce the yearly value of the vertical component of magnetic force from the days selected as 
nearly free from irregular disturbance in 1844 and 1845 (see volume for 1844, p, 384, and Table XXXVII., 
p. 21, of this volume), and compare these values with those obtained from all the ordimary observations in 
each year, we find, that the yearly mean of the vertical component deduced from the nearly undisturbed days 
In 1844 is greater than that deduced from all the hourly observations of the year, by 0:000021 
i eel 3): a age aa Baan haa an BeacreddecoocunbSddudecodarica: 07>: oA agbS ae aeBE Boose asaraccaadlad 0-:000089 
The effect of disturbance in both years was to diminish the mean value of the vertical component on the 
average by 0000030. See Nos. 38 and 54. 
86. Annual Period of the Vertical Component.—In the discussion of the observations for 1844 it was found 
that the result for the annual period differed considerably from that obtained for the previous year; separate 
discussions of the observations for the different years shewed so many differences, that it was concluded that 
the instrument was incapable of exhibiting the law, if any such existed; this conclusion was quite in accordance 
with that previously made by those who had examined the instrument with the greatest care. A combination 
of the results for several years, however, has rendered it probable that the errors of the instrument or irregu- 
larities in the law have been eliminated to a considerable extent, and that the true law has been obtained ; this 
will appear from the following discussion. 
TABLE 40.—Monthly Variations of the Vertical Component of Magnetic Force, free from Regular 
Secular Change. 
Years March. June. | July. Aug. | Sept. Oct. Noy. Dee. 
Prefix. 0-000 0-000 0000 0-000 0-000 0-000 0-000 | 0-000 0-000 0-000 0-000 0-000 
1843-6 |+086 —030 |—064 |—054 |—060 |+094 |+031 |+016 |—009 |—054 |+013 |+028 
1842-7-8-9 ||—014 |+027 |—071 |—097 |—121 |—113 |+028 |—088 |—127 |4+137 |+229 |+215 
1842-9 +036 |—001 |—068 |—075 |—090 |—010 |+030 |= 036 — 068 |+041 |+121 |4+122 
1842-7 —007 —046 |—099 |—052 = 80 +065 |+011 |—005 |—002 |+008 |+088 |+079 
| | 
Table 40 has been formed in the following manner: The monthly means for the different groups of 
years having been obtained from Table 38, the means for each group were corrected for secular change in the 
manner described No. 8, the secular change employed being the mean for the respective years in the last 
column of Table 39 ; the numbers in Table 40 are the variations about the yearly mean for each corrected group. 
87. The four years 1843-6 are those best fitted for exhibiting the annual law, the diurnal series of observa- 
tions being sufficiently large to give the monthly mean without any considerable error. The result from this 
group is as follows: The vertical component is a maximum in June and January, and it is a minimum in 
April and October. The remaining four years, especially the years 1848 and 1849, though but indifferently fitted 
to exhibit a law liable to so many irregularities, (owing to the fewness of the observations made daily, and to 
the great magnetic disturbances in the last two years,) yet they exhibit a rough approximation to the same 
result : for this group, the vertical component is a maximum in November and December, a secondary maxi- 
mum occurring in July; andit isa minimum in May and September. The group of six years, 1842-7, includes 
all the years during which a sufficient number of daily observations were made to give moderate approximations 
to the monthly means, and this group indicates a law similar to that from the four years 1843-6. Neglecting 
at present the considerations in favour of the law obtained from the four best years, the chief source of doubt as 
to the value of the result, is to be found in the irregularity and great variation of the secular change to be elimi- 
nated. In order to examine the monthly means free from this objection the following Table has been formed. 
