592 MR J. A. BROUN ON THE 
to these planets.* This result is all the more marked that it appears to hold 
even for different solar meridians, so that the meridian passing through Venus 
and Mercury shows a lesser spot area (on the side next the planet), and a 
greater spot area (on the opposite side), than the meridians on either side of 
this principal meridian. 
56. At the same time, I have made, for the purposes of this paper, a variety 
of discussions of sun-spot area relatively to the planetary periods, including 
the synodical revolution of Venus, and the conclusion has been that diffe- 
rent periods of eleven years (1833-44, 1844-55, and 1855-66), give different 
results.t 
57. When we examine the curves representing the three-monthly mean-spot 
area given with the valuable memoir of Messrs Dre La Rue, STEWart, and 
Lawy, in the “ Philosophical Transactions” (1870, Plate xxxi.), we remark 
that there is not a regular increase from minimum to maximum, nor diminution 
from maximum to minimum in the decennial period; but that there are series 
of well-marked secondary maxima and minima. We cannot fail to perceive, 
also, that the most marked of these maxima (one of which, in each decen- 
nial period, is also the principal maximum) occur generally within the months 
October to March. Only one marked exception to this rule appears in the 
period 1834 to 1844, when it occurs in September; no exception appears in 
the period 1844 to 1856. In the third period (1856 to 1867) there is one 
marked exception in July 1862 (see Plate XL.), and two irregular years. The 
variation of spot area during some of these secondary periods is so great that it 
amounts to nearly four-fifths of the range for the whole decennial period. The 
exceptional cases repeat themselves in the same month in different periods. 
58. It follows, from these considerations, that a discussion for a period of 
twelve months gives a maximum of spot area when the earth is nearest the sun 
in all the three decennial periods, but in the last a maximum is also shown 
when the earth is farthest from the sun. 
59. It has, however, been already pointed out, that the great maximum of 
sun-spot area, which occurred for July 1862 (see Plate XL.), had no cor- 
responding maximum of magnetic disturbance, nor was there any corresponding 
increase in the amplitude of the diurnal oscillation. If we suppose for the 
moment, that electrical actions passing from the sun to the earth are connected 
with spot production, we may also assume that when great variations of sun- 
spot area are unaccompanied by similar variations of terrestrial magnetic dis- 
turbance, the earth itself is not in question ; and that such cases as that of July 
* Proc. Roy. Soc. London, vol. xx. p. 210, March 1872. 
+ It should be remarked, however, that the discussion of Messrs Dz La Rus, Stewart, and 
Lawy, is founded on more exact and detailed measurements than those for the other periods included 
in my own investigations. 


