387 
maxima occur about the 3rd and 18th days of the moon’s age, 
and the minima about the 10th and 24th days.* The mean 
amount of the variation is 15, which is consequently double 
of the effect due to the lunar action. The mean magnitude 
of the range itself, from which the lunar variation is elimi- 
nated, is 96; so that the effect of the moon is to that of the 
sun as 1 to 13 nearly. 
<‘ It is obvious that the numbers above given are the resul- 
tants of two oscillations, whose periods are different, and 
which therefore combine in every variety of phase, namely, 
the diurnal changes produced by the two luminaries; and, 
on account of the doubling of the lunar change, they are well 
fitted to establish its existence. The next step in our inquiry 
will be to ascertain the law of the variation, whose total amount 
has been here deduced. This can be done only by an ana- 
lysis of the hourly observations in reference to the moon’s 
hour-angle, which will form the subject of another communi- 
cation.” 
The author concluded by some remarks upon the supposed 
ten-year periodical variation of the mean yearly range, indi- 
cated by Professor Lamont, and upon the corresponding va- 
riation of the mean disturbance noticed by Colonel Sabine. 
Although quite prepared for a correspondence between these 
_ phenomena, upon grounds stated in a paper formerly read to 
‘ the Academy,t he doubted the existence of a true period in 
either. 
* It would seem as if there were two minima, with a small intervening 
maximum, in the fourth quarter. These results differ altogether from those 
of Mr. Broun, by whom the effect of the moon’s age upon the range of the 
 magnetical declination has been also examined. According to him, the 
diurnal range of the magnetic declination has but one maximum and one 
‘minimum during the month, the maximum occurring two or three days after 
full moon, and the minimum about three days before new moon. 
+ Transactions of the Royal Irish Academy, vol. xxii. p. 94. 
