DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC, 583 
37. Differences in the Decennial Variations at different Stations.—In order 
to show these differences, the variations of the yearly mean ranges for different 
stations have been projected Plate XXXIX. for the years near the minimum 
1844. It will be seen there that the minimum appears to have been attained 
earliest at the stations nearest to, and south of, the equator; that an increase of 
the oscillation occurred 1845-46, at the two widely separated stations, Munich 
and Hobarton; while a diminution is shown at Toronto, and the oscillation 
remains nearly constant at Dublin and Makerstoun. These differences are suffi- 
ciently marked to prove that the decennial variation is affected by local causes, 
_a result already shown by the differences of the ratios at the same time.* 
38. Comparison of the Yearly Mean Diurnal Oscillation of the Needle and 
Sun-spot Area.—Having treated the sun-spot areas given by Messrs De La 
Rue, Stewart, and Lavy, in the same way as the magnetic ranges, so as to 
have the yearly mean corresponding to the beginning of each month, these 
were projected above the curves of yearly mean diurnal range, Plate XX XIX. 
In the period, 1836-1848, we find that the maximum spot-area occurred at 
1836-75, and that it diminished with considerable regularity thereafter; whereas 
the diurnal magnetic oscillation attained the maximum at 1836°9, which was fol- 
lowed by a nearly equal maximum at 183825, when the spot-area had diminished 
by one-third of its whole value. 
In the next period the maximum spot area occurs again earlier than that of 
the diurnal oscillation; while after 1850°5, the former increases and the latter 
diminishes, 
* Dr Lamont has given the diurnal ranges for different years at several places, deduced from obser- 
vations at two hours only, which he points out are thus affected by disturbances, and are therefore only 
“preliminary approximations.” He has concluded from the general agreement of the increase from 
minimum to maximum that— 
where 7, and 7’, are the diurnal ranges for the same (n) year at any two stations, Dr Lamont has 
not, however, given the ratios which result from his data; these scarcely satisfy the equation even 
approximately. Thus taking St Helena and Munich, for which the ranges are given by him, we-find— 
Sera 0Q * oD) 
Eitan” Figgg Oe agg eed = 90g ekg) = B99. 
St Helena ’ 2°55 2°81 3°48 
I believe, however, that Dr Lamonv’s conclusion is true, “ that the cause of the ten-yearly period 
is to be found in the sun, or more generally, in a cosmic force acting from a great distance.” Indeed, 
the preceding equation is a general form of that given Art. 36. And when we compare the migi- 
mum*and maximum at Munich for 1850 and 1860 with those for Trevandrum near the same times, we 
find— 

Munich , 1856, 08 _ 3.76 1860, WT _ 3°75. 
188 2 98 
; Trevandrum 88 d 
These ratios are not quite accurate, since only the minimum for 185675, and the maximum for 
1859°5 at Munich are known to me, but the true minimum and maximum values cannot be very 
different; these ratios then will satisfy Dr Lamonr’s equation nearly, and confirm his conclusion, See 
“inige Bemerkungen iiber die zehnjahrige Period, Sitzungsberichte der K. Akad. der Wischenschat- 
ten,” Miinchen, 1864, II. S. 21. 
VOL. XXVII. PART IV. P 
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