DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 591 
which satisfy the preceding equation nearly. We have, also, by the original 
equation, Art. 50, 
nf B47 —n/'335 = 341 
J'527 — f'141 = “350 
agreeing nearly with the constant previously obtained. In a similar way, if we 
determine the exact values of the maxima and minima for the upper curves in 
Plate XL., showing the decennial variation of the disturbance at any hour, we 
shall have approximately, 
rf Din — n/m = Constant. 
53. We may conclude, then, that the increase of the disturbance from mini- 
mum to maximum in the diurnal variation follows the same law of the con- 
stant difference of the square roots of the deviations in each year of the decen- 
nial period ; and that the increase of the mean deviation for any hour in the 
year of minimum to the same hour in the year of maximum disturbance obeys 
a similar law. 
54, Hypotheses as to the Cause of the Decennial Period.—The only periodic 
movement of the sun with which we are acquainted is that of his rotation on 
his axis. We are induced then to seek, without the sun, for some phenomenon 
which may occupy a like period with the variations in the areas of his spotted 
surface. If any such could be found, we might then inquire whether the 
former could be the cause of the latter; and if so, how? The planetary motions 
offered themselves at first as possible causes. The sun and the planets attract 
each other. The relation of the spot area, the amplitude of the magnetic oscil- 
lation, and ‘the frequency of the aurora borealis, indicate an electrical cause in 
the latter cases; and without any exact knowledge of the forces which might 
be required so to disturb the equilibrium of the gaseous envelope of the sun, it 
did not appear impossible that electrical actions passing between the sun and 
the planets might suffice for the production of the solar spots. The only 
periodic movement, however, approaching the decennial period is that of the 
revolution of Jupiter, which occupies nearly 1} year more than the decennial 
period. 
Dr Wotr has endeavoured to represent the latter period by formule which 
are functions of the masses, distances, and periods of ‘revolution of the planets. 
Such an effort may have some appearance of success when confined to a single 
period of eleven years; but it fails completely when extended to two or three 
periods. 
55. The most interesting result with which I am acquainted is that due 
to the investigations of Messrs DE La Ruz, Stewart, and Lawy;; from which it 
appears that the mean area of spots (1854-60 and 1862-66) is greatest on the 
side opposite to Venus and Mercury, and least on the sun’s surface presented 
PART XXVII. VOL. IV. Ge 
