EARTH’S MAGNETISM TO THE SOLAR AND LUNAR PERIODS. 143 
17. In 1845 the maximum occurs between 14° and 15°, or about 2" after the 
moon has passed the inferior meridian, the value of the vertical component then 
diminishes till 21" when there is a secondary minimum, it then increases consi- 
derably till 2; a secondary maximum occurring about 13", it again diminishes till 
7*, when the principal minimum occurs, after which it increases rapidly to the 
principal maximum. This year’s result is little affected by the elimination of dis- 
turbances ; the curve is rendered somewhat more regular. 
18. The results for the two years differ slightly from each other. The 
principal maximum occurs about an hour later in 1844 than in 1845 ; the princi- 
pal minimum occurs at the same time in both years; but the secondary maximum 
and minimum are not nearly so well defined in 1844 as in 1845. The mean of 
the two years gives the following periods :— 
The principal minimum about 5 hours before the moon’s passage of the inferior meridian. 
Pietprincipal maximum about d hours after ...............-.ceeeserseces caceceeccnerseceseerecs 
A secondary minimum about 4 hours before ............:.ceceeseee cee eee eens superior ......... 
A’secondary maximum about 1 hour after — ...........sescensesess seen eceeeenereseeeeeeeenesens 
19. These periods are surprisingly near those indicated (No. I., 23), for the 
horizontal component, the principal maximum and secondary minimum of the 
vertical component occurring about an hour after those of the horizontal com- 
ponent, while the principal minimum and secondary maximum occur an hour or 
more before those of the horizontal component; seeming to indicate that these 
variations belong chiefly to the total intensity. 
20. As evidence of the accuracy of the results obtained after eliminating 
disturbances, nothing perhaps could be more conclusive than the fact, that in 
those months in which no disturbances occur, the general law is found well 
marked: it would not be difficult to bring this kind of evidence to bear, both 
for the horizontal and vertical components. I shall satisfy myself at present by 
giving the projection of one month’s results, namely, the results for the lunation 
included by December 15. 1845, and January 10. 1846 (Curves No. 3, Plate VL). 
From this single month the same, or very nearly the same, periods of maxima and 
minima are obtained as in the mean for the two years. The maxima have nearly 
equal values, and so also have the minima; the principal maximum and minimum, 
however, occur at the times of the secondary maximum and minimum of the mean 
curve. Such a difference in the values of the maxima might be expected; for, 
though the moon has every declination in the course of one lunation, it is full 
only at one of these declinations. It would not have been difficult to have ob- 
tained a curve from one month’s observations representing the mean curve better. 
21. In order to obtain the variations of the vertical component with respect to 
the moon’s age, summations were made similar to those indicated (No. I., 18) for 
the horizontal component. It should be mentioned (as it should have been before 
for the horizontal component), that in all the results for monthly periods mean 
