144 ON THE VARIATION OF THE EARTH’S MAGNETISM, &c. 
values for Sundays were used. These were obtained by interpolation ; the mean of 
the three days preceding, and of the three days succeeding, were taken as the 
means for the Sundays. The want of approximate means for these days would 
tend to destroy the regularity and distinctness of the results, owing to the varia- 
tions, due to different causes, which it is desired to eliminate, and that the blank 
days enter irregularly into the days of the mean monthly periods. 
22. The results of these summations (Curves No. 4, Plate VI.) are, that each year 
indicates maxima of the vertical component near the quadratures, and minima near 
the syzygies. In order to render the fact more distinct, and the curves somewhat 
more regular, eleven days* of greatest disturbance were eliminated in each of the 
‘years 1844 and 1845, namely, those days on which the mean value of the verti- 
cal component was greater than the mean of the previous and succeeding days 
by more than 26 micrometer divisions (about three times the resulting range). 
The principal minimum in 1844 occurs at the period of full moon; in 1845 it 
occurs about three days before the new moon. In the mean of both years the 
principal minimum occurs at the time of full moon, the secondary minimum about 
two days before new moon; the maxima occur between these periods, and they 
are nearly equal. The lowest curve of No. 4 is the projection of the mean for 
the two years without eliminating disturbances. It differs little in regularity from 
the other in which the large disturbances are eliminated. The irregularities in 
these curves may be partially due to the cause of error already stated (17), 
namely, an insufficiency in the temperature correcticn. 
23. Whether due to the cause just mentioned or not is uncertain, but the 
results for the relation of the variations of the vertical component to the moon’s 
declination are neither distinct nor consistent for the two years, and the elimina- 
tion of the days of large disturbance does not improve them. The mean for the 
two years seems to shew something like the law found for the horizontal com- 
ponent, namely, maxima about the periods of greatest north and south declina- 
tion; but I do not place any trust at present in this result. 
Maxerstoun, April 13. 1846. 
‘ 
* Each of these days was actually observed (observations were made) at the time as a day of dis- 
turbance. ; 
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