240 ABSTRACTS OF THE MAKERSTOUN OBSERVATIONS, 1843. 
It will be obvious from a consideration of the above weekly means, that 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 occurring 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, = 9203 
Pamicaae coseantsiecemd dese sence emetehetcmetertenis wea. January 1843—August 1843, = 693°0 
Annual change corresponding to the beginning of May 1842-43, : : 0 = 227°3 
Mean of the balance magnetometer readings corrected, September 1843—April 1844, = 7830 
Basdorcdosgagcoue soveaseeacsuateccvesestecvanesssressseeeves HEptember 1844—April 1845, = 5850 
Annual change corresponding to the beginning of January 1844-1845, 3 : =) 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, — 736-0 
corresponding to the beginning of March 1843, 5 5 0 Pa 
Therefore the mean in the beginning of March 1844 should be. = 523:0 
But the mean, September 1843 to August 1844, eae to the n mean in the 
eginning of March 1844, was 5 2 . 
Whence the micrometer readings after September 1843 are more than those before : 
g 222:0 
September 1843, by 
After the corrections in the first ease had been made, all the means after September 5, 1843 were cor- 
rected by — 222-0 micrometer divisions, which, it is believed, is not farfrom 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 1643, 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 OF 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 :— 
* . 4 = 
24 observations minus 9 observations. 
Jan. Feb. March. April. May. June, July. Aug. Sept. Oct. Nov. Dec. 
—0-0000 | 16 16 82 70 59 12 22 66 66 74 55 14 
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 | 6924 6742 6375 6083 5909 5984 58538 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- 
