438 



The foregoing results agree in their main features with 

 those obtained by Professor Kreil* and Mr. Broun, from the 

 discussion of the Prague and Makerstoun observations. The 

 chief difference is in the winter lunations. In the Prague ob- 

 servations the lunar variation is extremely small in winter, and 

 its law is apparently masked by irregular changes ; while at 

 Makerstoun there is but one maximum and one minimum in the 

 winter months, and the magnet deviates but once to the east 

 and once to the west in the course of the day. It seems difficult 

 to reconcile such influences of season with any physical cause. 



It now remains to examine the consistency of the fore- 

 going results with those already obtained, on the dependence 

 of the diurnal range of the declination upon the moon's age. 



It is obvious that as the periods of the oscillations caused 

 by the sun and moon respectively, in the position of the freely 

 suspended magnet, are different, they will combine in every 

 variety of phase; so that the resultant oscillation will vary with 

 the moon's age in the course of the month. Let the variation 

 of the declination at any hour, caused by the sun and moon 

 respectively, be denoted by Aw and Su ; then m and n being 

 the solar hours of greatest and least declination, and p the in- 

 terval (in hours) between the sun and moon's meridian pas- 

 sage, m -p and n -p will be the corresponding lunar hours, 

 and the resultant range will be 



A m U- & n U + $m-p U-$n-p «• 



The values of this quantity are given in the following Table, 

 — in the first column of which are the days of the moon's age ; 

 in the second the corresponding hours (p) of the moon's re- 

 tardation ; in the third and fifth the calculated values of 



* The author takes this opportunity of stating that, in referring to Pro- 

 fessor Kreil's labours on this question in his former communication, he omit- 

 ted to notice the elaborate memoir, " On the Influence of the Moon on the 

 Magnetic Declination," read to the Imperial Academy of Sciences in Vienna, 

 in 1850, and published within the last year. Had he read that paper before 

 he had written his own, he could not have questioned the sufficiency of the 

 evidence for the lunar action which it contains. 



