EARTH’S MAGNETISM TO THE SOLAR AND LUNAR PERIODS. 105 
the same periods as in 1844; but the branch ascending to and descending from 
the period of greatest N. declination is greatest, the periods of minima being 
nearer the greatest S. declination, namely, about 5 days before it and after it. 
The curve for 1845 is, however, more irregular after the 8. declination maximum 
than in any part of the other curve. Besides the non-elimination of the effect 
connected with varying phase and disturbance, there is another possible cause of 
difference, namely, the varying distance of the moon; the period of perigee is 
about two days before the greatest S. declination in 1844, and two days after it 
in 1845. It should also be remembered that each point in these curves is a mean 
of only 12 or 13 days; as for the minor irregularities in the positions of the 
points, it is obvious that, as there are 27 days between the periods of the moon’s 
greatest N. declination, if the full moon occurs on the day of greatest N. declina- 
tion in one month, it will occur on the second day after the greatest N. declina- 
tion on the next month, the fourth day on the next, and soon. It will be seen 
afterwards that this will cause a slight irregularity. It is on this account that 
I have projected the curves among the points, giving a preference to the mean 
positions of each two points. 
15. The similarity of the positions of maxima and minima in these curves, hav- 
ing the moon’s declination for abscissee to the annual curve, or that having the 
sun’s declination for abscissze, is at once evident; by taking the mean of the two 
lunar curves, however, the cases will be identical, for then the moon’s perigee 
will occur at the time of its greatest S. declination, and its apogee at the time of 
the greatest N. declination; this is the case with us for the sun. The resulting 
means have been projected below the other curves. By comparing the mean 
curves of No. 4 and No. 6, it is at once obvious that the facts are the same for 
both the sun and moon. I conceive, then, that I am justified in stating that the 
same relation exists for the moon as for the sun between the variations of the 
horizontal component of the earth’s magnetic intensity, and the variations of de- 
clination and parallax. 
16. We have, then, a law connected with two periods, namely, distance and 
declination. To which does it belong, or does it belong to both? It will take a few 
years’ observations to determine this for the moon: it may be determined for the 
sun by observations for the annual period made in the Southern Hemisphere. Is 
there a maximum at the greatest N. declination, and also at the greatest S. de- 
clination ; or have changes of declination no effect? and are the maxima due to 
the moon’s or sun’s distance solely? The supposition that at first sight seems 
most probable is, that these variations are due to both; that a maximum occurs 
at the time of perigee, a minimum at the apogee, a maximum at the greatest N. 
declination, and a minimum at the greatest S. declination. It may easily be 
shewn that two regular curves having these arguments, when superposed, would 
