Annual Variations fok the Vertical Component or Magnetic Force, xlvii 



Table 41. — Mean Change of the Value of the Vertical Component from Month to Month for 

 different Groups of Years, as deduced from Table 38. 





Dec. 



Jan. 



Feb. 



March 



Ajiril 



May 



June 



July 



Aug. 



Sept. 



Oct. 



Nov. 





Years. 



to 



to 



to 



to 



to 



to 



to 



to 



to 



to 



to 



to 



Mean. 





Jan. 



Feb. 



IVfarch. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 





Prefix. 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



0-000 



1843-6 



-219 



-261 



-178 



-135 



-150 



+ 008 



-208 



-160 



-170 



-190 



-078 



-129 



-156 



1842-9 



-205 



-166 



-194 



-137 



-144 



-048 



-090 



-194 



-161 



-019 



-049 



-128 



- 128 



1842-7 



-194 



-183 



-197 



-097 



-132 



-039 



-196 



-159 



-141 



-134 



-064 



-153 



-141 



88. Considering the numbers for the years 1843-6, we find that the mean change of the vertical component 

 from one month to the next = 0*000156, that the diminution in the months from December to March and from 

 June to October was greater than the mean, while those from March to June and from October to December 

 were less ; the other groups give nearly the same result, which is quite in accordance with that from Table 40. 

 From both Tables we feel entitled to state the following as the annual law, — That the vertical component of 

 magnetic force is a maximum near the solstices and a ininiin.um near the equinoxes . It will be observed that 

 this is precisely the law already deduced for the horizontal component No. 56 ; had it not been for this remark- 

 able coincidence in a law with two maxima and two minima, obtained from two instruments of the most different 

 principles, the conclusions deduced from the observations of the balance magnet would have been left with 

 whatever weight they might appear to physicists to deserve ; but it is conceived that the agreement is too con- 

 siderable and too remarkable to omit adducing it as evidence in estimating the accuracy of this result for 

 the vertical component. 



89. If we compare the monthly means deduced from the days selected in each month as nearly free from in- 

 termittent disturbance, with those deduced from all the hourly observations in the corresponding months, we 

 find the latter less ( — ) or greater ( + ) by the following quantities. 



Year. Prefix. 



1844, 0-000 



1845, 0-000 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 



-039 +096 -124 +003 -071 -031 -031 -020 -048 -043 +006 +051 

 -06§» -036 -039 -086 -061 +018 -049 -091 -049 -046 +008 +024 



Mean, 0-000 -053 +030 -081 -041 -066 -006 -040 -055 -048 -044 +007 +037 



The numbers differ considerably in some cases for the same month in the two years ; a greater number 

 of partial results are therefore evidently required for a good mean. The mean of both years shews, that the dis- 

 turbed means were greater than the undisturbed in November, December, and February (or, about the winter 

 solstice), and less in all the other months, the diminution being greatest in March, May, and August. 



Differences of the Daily Means of the Vertical Component from the Means for the corresponding Months. — 

 The discussion for 1844 will be found in the volume for that year, p. 374, the results for 1845 and 1846 

 4re obtained from Tables XXXI. and LXIII., pages 18 and 35 of this volume. 



90. The conclusions from Table 42 are : — 



\st, That the positive departures of the daily mean vertical component from the monthly mean value are 

 greatest in September and in February, and that they are least in January and June. 



2d, That the negative departures of the daily mean from the monthly mean are greatest in September, 

 January, and May, and least in March and July. 



3(i, That the mean positive departure is most in excess of the mean negative departure in February, while 

 the latter is most in excess of the former in January, and in the months from March to June ; with the excep- 

 tion of February, the mean negative departure is greater than the mean positive departure in the first six months 

 of the year ; and, with the exception of August, the reverse is the case for the last six months. 



Ath, That the mean departure of the daily mean from the monthly mean (without reference to sign) is 

 greatest in September, and least in March. 



bth, Themeandepartureof the daily mean vertical component from the monthly mean for 1844 = 0-000105 



1845 = 0-000117 



1846 = 0-000131 



3 years = 0-000118 



MAG. AND MET. OBS. 1845 AND 1846. m, 



