240 Abstracts of the Makerstoun Observations, 1843. 



It will be obvious from a consideration of tlie above weekly means, tliat this correction cannot be far from 

 the truth; indeed, the mean of the micrometer readings for November 10 and 11 is 819"6, and for November 

 14 and 15 is 860*1, which, supposing the change for two days small, will give nearly the same difference. 



2d, To connect the observations after September 6, with those before August 22. 



This has been a matter of greater difficulty than in the previous case, both on account of the greater in- 

 terval elapsed, and also on account of considerable changes occuri'ing in the daily means before August 23 ; 

 for this reason, the weekly changes could not be compared with any confidence, and the following method was 

 adopted -. — 



Mean of balance magnetometer readings corrected, January 1842 — August 1842, = 920-3 

 January 184 3— August 1843, = 693-0 



Annual change corresponding to the beginning of May 1842-43, . . . •=227-3 



Mean of the balance magnetometer readings corrected, September 1843 — April 1844, = 783»0 

 September 1844— April 1845, = 585-0 



I 



Annual change corresponding to the beginning of January 1844-1845, . . = 198-0 



Hence, mean annual change corresponding to the beginning of March 1843-1844, = 213-0 



Mean of balance magnetometer readings corrected September 1842- August 1843, "I TQft.n 



corresponding to the beginning of March 1843, .... J "~ 



Therefore the mean in the beginning of March 1844 should be .... = 623-0 



But the mean, September 1843 to August 1844, corresponding to the mean in the! _ ijAf^.n 

 beginning of March 1844, was ..'.... J ~ 



Whence the micrometer readings after September 1843 are more than those before 1 990-n 



September 1843, by ......... J 



After the corrections in the first case had been made, all the means after September 5, 1843 were cor- 

 rected by —222-0 micrometer divisions, which, it is believed, is not far from the truth. The correction for the 

 six daily means, August 25-31, was obtained by interpolating between the means of the preceding and succeed- 

 ing weeks for the mean of the six days, the correction applied was —160 micrometer divisions. 



As it has been found from the observations in 1844, that, in the month of January, the mean of the 4 daily 

 observations corresponding to those made in the first week of January 1843, and of the 8 corresponding to those 

 made in the second week, differ little from the mean of the whole 24, no correction has been applied to the 

 means of the 4 or 8 daily observations. 



Annual Period of the Vertical Component op the Magnetic Force. 



Differences of the monthly means deduced from the observations in 1844, at the hours 18, 20, ... 10, 

 from those deduced from the 24 hourly observations made on each day (excepting Sundays) in that year : — 



24 observations minus 9 observations. 



Jan. Feb. March. April. 



-0-0000 I 16 16 82 70 



These quantities being applied as corrections to the monthly means at the foot of Table XV,, we obtain 

 the following : — 



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



0-01 I 6924 6742 6375 6083 5909 5984 5853 5691 5339 5299 5277 5193 



From these it appears that the vertical component diminishes considerably from January till May, increases 

 from May till June, diminishes slowly from June till August, more rapidly till September, and slowly from Sep- 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



59 



12 



22 



66 



66 



74 



55 



14 



