XXXll 



Geneeal Results of the Makekstoun Observations. 



53. The mean yearly secular change of the horizontal force in absolute measure, 



By the observations of the bifilar magnetometer, = + 0-00446 



By the observations for the absolute force (Table 21), = + 000388 



Such a near agi'eement is, perhaps, more than could have been expected: if the observations with the small 

 deflecting bar were neglected (No. 49), the agreement would be even greater. 



54. It has been shewn, No. 38, that when we deduce the yearly mean declination from the days which were 

 selected as little affected by intermittent disturbances, the result is almost precisely the same as that deduced 

 from the whole ordinary observations, and therefore from the days disturbed ; a similar comparison being made 

 for the horizontal component, we find as follows : — 



The yearly mean of the horizontal component, as deduced from the 120 days selected as nearlyfree from disturbance, 



In 1844, is greater than that deduced from all the hourly observations of the year by 0000189. 

 In 1845, 0000154. 



The effect of disturbance in both years was to diminish the mean value of the horizontal component on the 

 average by 0-000172 of the whole component. It was found for 1844 (see the volume for that year, p. 365) 

 that a more careful selection of 60 days (5 in each month) shewed even a greater effect of disturbance, namely 

 0-000251 for that year. 



Table 24. — Monthly Variations of the Horizontal Component of Magnetic Force, free from 



Regular Secular Change. 





















1842 



1846 



1842 



1842 



Month. 



1842. 



1843. 



1844. 



1845. 



1846. 



1847. 



1848. 



1849. 



to 

 1845. 



to 

 1849. 



to 

 1849. 



to 

 1847. 



Prefix. 



0-000 



0-000 



0-000 



0-0(10 



0-000 



0-00 



o-ooo 



0-00 



0-000 



0-000 



0-000 



0-00(1 



January 



+ 100 



+ 470 



+ 050 



+ 114 



+ 213 



+ 0409 



-219 



-0339 



+ 183 



+ 016 



+ 100 



+ 339 



February 



-015 



+ 232 



-003 



+ 091 



+ 204 



-0007 



-500 



-0271 



+ 076 



-143 



-034 



+ 125 



March 



+ 092 



-327 



-422 



-027 



+ 128 



+ 0162 



-210 



-0413 



-171 



-084 



-127 



-098 



April 



-386 



-468 



-342 



-218 



+ 014 



+ 0023 



+ 007 



-0141 ' 



-353 



-024 



-189 



-344 



May 



+ 377 



+ 224 



+ 126 



+ 139 



+ 262 



-0021 



+ 530 



+ 0462 



+ 216 



+ 308 



+ 262 



+ 277 



June 



+ 381 



+ 371 



+ 637 



+ 351 



+ 002 



+ 0619 



+ 667 



+ 1207 ; 



+ 435 



+ 624 



+ 530 



+ 590 



July 



+ 019 



-075 



+ 561 



+ 185 



+ 257 



+ 0246 



+ 531 



+ 0667 



+ 172 



+ 425 



+ 299 



+ 298 



August 



-189 



-239 



+ 118 



-073 



-408 



+ 0052 



+ 217 



+ 0070 



-096 



-017 



-056 



-185 



September 



-221 



-271 



-133 



-495 



-677 



-0171 



-472 



-0364 



-280 



-421 



-350 



-492 



October 



-083 



-356 



-384 



-205 



-544 



-nil 



+ 081 



-0631 



-257 



-551 



-404 



-671 



November 



-163 



+ 132 



-223 



+ 092 



+ 023 



-0572 



-245 



-0356 , 



-041 



-287 



-164 



-178 



December 



+ 094 



+ 301 



+ 014 



+ 043 



+ 523 



+ 0371 



-389 



+ 0109 : 



+ 113 



+ 153 



+ 133 



+ 336 



55. Annual Period of the Horizontal Component. — Table 24 has been formed in the same manner as Table 3 

 (see p. xiii.) The secular changes employed in the reduction for each year, obtained in the same manner as for 

 the magnetic declination in 1847-8 (No. 8), are as follow : — 



Yearly increase. 



1842. 



0-003480 



1843. 

 0-003804 



1844. 



0-002820 



1845. 



0-001116 



1846. 



0-001368 



1847. 

 0-000768 



1848. 



0-001500 



1849. 

 0-001920 



56. The mean result for the six years 1842 to 1847, given in the last column of Table 24, is probably to 

 be most depended on for an accurate exhibition of the mean annual law, the means for 1848 and 1849 being 

 deduced from too few observations. The mean for the six years 1842 to 1847 shews, that the horizontal 

 component at Makerstoun was a maximum at the summer solstice, and also at the winter solstice ; that it was 

 a minimum shortly after the autumnal, and shortly after the vernal equinox* This result is shewn with 



* This law, as deduced from the Makerstoun Observations for 1842, was stated to the Physical Section of the British Association 

 iu June 1845, confirmed by a rediscussion of observations made at Toronto in 1842 : it has since been confirmed by the observations 

 made in the successive years at Makerstoun, and, as has been shewn in the Makerstoun Observations for 1844 (foot-note p. 357), by l)r 



