Magnetic Declination. 



341 



The means of the observations made on the 60 days selected, representing the mean divimal variation for 

 the year, give the following result: — 



The north end of the declination magnet is farthest west at O*" 45" p.m. ; it then moves easterly, the 

 velocity of motion being nearly constant till S*" p.m., after which it moves more slowly in the same direction 

 till ll"" 10™ P.M., having moved altogether through 5''5; from ll"" 10™ p.m. till 1'^ a.m., it moves westerly 

 through O'-IO ; after l"" a.m. it again moves toward the east, attaining its farthest easterly position at 7*" 0" a.m., 

 having moved l'-6 between 1^ and 7"" a.m. After 7^ it returns 7'-0 to its farthest westerly position at C" 45'° 

 pja. It will be observed, that this result differs from that obtained by means of the whole series, in placing 

 the principal minimum in the morning, and in nearly obliterating the secondary maximum and minimum. 

 The mean for the 120 days gives nearly the same result as that obtained from the 60 days' observations ; the 

 former places the secondary maximum about l*" 40" a.m., and gives 0'15 as the amount of motion from the 

 position of secondary minimum to that of secondary maximum. Although the secondary maximum were 

 wholly wanting, there would still be distinct evidence of the action of a secondary cause in the inflections of 

 the curve. Other differences between the results for the 60 days, and for the whole series, will be indicated 

 immediately. 



The following Table contains the epochs of maxima and minima, deduced from the monthly means for 

 the 10 days and 5 days respectively. 



TABLE X. — Epochs of Maxima and Minima of Magnetic Declination, obtained from the Ten-Day 



and Five-Day series of observations. 



Month. 



January 



February 



March 



April 



May 



June 



July 



August 



September 



October 



November 



December 



10 Days. 



Max. 



h. in. 



10 p. 



30 



1 10 

 1 

 1 10 

 1 20 

 1 10 



30 

 20 

 

 20 

 30 



Min. 



-10 P.M, 



-10 ■-. 



(6 O)?-- 



6 

 6 



9 0?- 



8 O?-- 



12 ■• 



■ 9 10 •■ 



■10 10 •. 



Max. 



? 



A.M. 



3 



Min. 



?A.M, 



? 



-8 10 

 -7 30 

 -6 20 

 -6 10 

 -6 40 

 -6 40 

 -6 40 

 -8 

 7 

 9 10 



5 Days. 



Max. 



Min. 



10 











40 







20 



20 



40 



30 



50 



30 



1 



P.M.| - 9 40 P.M. 

 ... l_io 10 ... 



11 ■•• 



8 O?— 



12 ••• 



• 9 10 ••• 



•10 10 •■• 



Max 



h. in. 



2 0?A.M, 



1 ... 



1 



1 

 4 



2 



3 



-5 



-8 

 -7 

 -6 



0?A.M, 





 10 

 10 

 10 



-6 20 

 -6 50 

 -6 40 

 -7 20 

 -8 10 

 7 10 

 9 10 



The epochs for both series are nearly the same ; considering the 5-day series as most free from the effect 

 of intermittent disturbances, we find the same law to hold with respect to the varying epoch of the principal 

 maximum as has already been found from the whole series. The principal minimum occurs between O*" and 

 IC" P.M. in the four months of November, December, January, February, and it is only in these months that 

 two maxima and two minima are distinctly marked ; in November and February the minima are nearly of 

 equal value. The minimum in the remaining 8 months occurs between 6*" and 9'^ a.m. 



Ranges of Mean Diurnal Variation. — The following are the ranges of the monthly mean diurnal 

 variation, as deduced from all the hourly observations, ft-om the hourly observations in the 10 days and in 

 the 5 days least affected by disturbances. 



In each case, the ranges for the months from March to October, both inclusive, vary but little, and that 

 irregularly. The ranges for the undisturbed variations change considerably between February and March, 

 and October and December ; and the ranges for the four months, November, December, January, and February, 

 do not differ greatly. The range for November from the whole observations, is considerably more than its 

 average value at Makerstoun. 



mag. and met. obs. 1844. 



4r 



