387 



maxima occur about the 3rd and 1 8th days of the moon's age, 

 and the minima about the 10th and 24th days.* The mean 

 amount of the variation is l' # 5, 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 9'*6 ; 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,f 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 mffon'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. 



t Transactions of the Royal Irish Academy, vol. xxii. p. 94. 



