Annual Variations for the Horizontal Component, 
XXXIU 
considerable distinctness in each of the six years ; the variations from it are not greater than might be expected 
when we take into account the large effect of disturbances, the irregular value of the secular change in some 
years, and in others the fewness of the daily observations, and consequent imperfect nature of the corrections. 
These corrections, as deduced from the observations for 1844 and 1845, vary so much as to account fully for 
many of the minor differences fi-om the mean law in the years 1842 to 1847, and for even the larger differences 
in the years 1848 and 1849. Upon the whole the summer maximum appears rather greater than tlie winter 
maximum, and the autumnal minimum than the spring minimum ; although as this is not the case in the mean 
for the first four years (column 10, Table 24), and as very large disturbances occurred in the end of the years 
1846 and 1847, which probably increased the autumn minimum of these years, this difference in the minima 
is perhaps accidental, and might be removed or considerably diminished in a larger series of observations. 
57. When we compare the monthly means, as deduced ft-om the 10 days selected in each month as least 
affected by irregular disturbances, with those deduced from all the hourly observations in the same months, we 
find the latter less ( — ) or greater ( + ) than the former, by the follo\ving quantities : — 
Year. Prefix. .Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 
1844, 0-000 -109 -346 -399 -315 -029 +018 -021 -108 -062 -507 -164 -234 
1845, 0-000 -301 -160 -260 -143 -032 +021 +003 -119 -109 -251 -111 -377 
Mean, 0-000 -205 -253 -329 -229 -030 +019 -009 -113 -085 -379 -137 -305 
In each month, with the exception of June, the mean deduced from the undisturbed days is greater than 
that deduced from all the observation days ; and the excess is greatest in March and October, the months of 
greatest disturbance : the effect of disturbance on the means for the months of May, June, and July, is very 
small, nearly zero. 
58. As the above effects of disturbance seem to obey a law similar to that of the annual period of the 
mean, it will be interesting to consider the law for the latter, as deduced from the nearly undisturbed 10 days 
selected in each month of the years 1844 and 1845. The following are the mean variations, deduced from all 
the daily observations in 1844 and 1845, as in Table 24. 
I Jan. Feb. March. April. May. .Tune. July. Aug. Sejit. Oct. Nov. Dec. 
Mean of all, 0-000 I +082 +044 -224 -280 +132 +494 +373 +022 -314 -294 -065 +028 
Correcting these by the mean quantities in No. 57, we obtain the variations of the monthly means from tlie 
nearly undisturbed days of 1844 and 1845. 
^20 Tay«^ } ^'^^^ I +^^- 
The monthly means, deduced from 10 nearly undisturbed days in each month of the two years, give the 
same law as has already been deduced from the means of all the observations (as in Table 24) : in the rmdis- 
turhed means, the maximum at the winter solstice is rendered more marked, and it appears probable that the 
difference between the values of the two maxima may be wholly a result of disturbance, which appears to 
diminish the winter means considerably, while it rather tends to increase those at midsummer. Wlien a more 
careful selection of undisturbed days is made, as in that of Jive days in each month of 1844, (see p. 365 of the 
volume for that year) it is found, that the effect of disturbance in diminishing the winter means, and in increas- 
ing the summer means, is even more considerable than that found above No. 57. 
Differences of the Daily Means of the Horizontal Component from the Means for the corresponding Mouths. — 
The discussion for 1844, will be found in the volume for that year, page 357, the results for 1845 and 1846, 
are obtained from Tables XVIII. and LVII. of this volume. 
Lament's observations at Munich (1843-5), although by the combinations which he had employed he had failed in detecting the fact. 
The following are the variations of the Munich numbers as corrected for secular change in the note cited, the horizontal component 
at the place being taken as unity. 
Prefix. I Dec. Jan. Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. Nov. Dec. 
0-000 j +172 -029 -086 -029 -158 +265 +238 +280 +136 -421 -328 -045 +172 
The value of this confirmation of the Makerstoun law is increased by the fact, that the Munich instrument has a unifilar suspen- 
sion, and that the processes of observation, reduction, and correction, are completely different from those employed at Makerstoun. 
