1864. ] Hersouen on the Solar Spots. 227 
of solar activity (as evinced by the production of spots), with the inter- 
vals elapsed between each, viz. :— 
Minima, | : | Mitae, 
at _ Intervals. | ane Intervals, 
Se Sra i eee ay Years, 
1610°8 8-9? SS a 
1619:0 : 17450 i 
24. 15°0? 10°7 
1634°0 11:0 1755°7 Tee 
1645-0 10-0 | 1766-5 as 
aoe eet Oe wali Wee ee moo 
1666°0 1784°8 
37 12-5 | 98° 13-7 
1679°5 10:0 | 1798°5 12-0 
1689°5 8-5 1810°5 12°7 
1698-0 . 1823°2 j 
14-0 29, 10°6 
1712-0 1833°8 
92. 11:0 : 10°2 
1723:°0 10-5 1844°0 122 
V733=5 1856°2 
The mean interval is 112, or, considering that the two first epochs 
are necessarily somewhat uncertain, very nearly 11” 3th, or nine 
complete periods in a century ; and the mean epoch 1799:24, which is 
so nearly 1800-0, that as a convenient date for memory the commence- 
ment of the terminal year of each century may be taken as a starting 
point. The comparison of epochs of maximum activity leads to a 
similar conclusion as to the length of the average period; but these 
epochs are less definitely marked, and subject to greater deviations from 
their average places than the minima which, themselves, as is evident 
from the above synopsis, are subject to pretty considerable irregu- 
larities. Generally speaking there appears a tendency in the maxima 
to anticipate the middle time between the consecutive minima, the 
interval 11°11 being divided into two unequal sub-intervals of 4°-77, 
and 6°34. 
Professor Wolf estimates the solar activity on any day by adding 
together the number of individual spots counted on the disc and ten 
times the number of groups. This is to a certain extent arbitrary. 
But some rule must be adopted for calculation, and it would not be 
easy to propose one less open to objection. Taking the total so obtained 
for each day for the measure of that day’s activity, and thence calcula- 
ting the mean yearly activity, and the mean during each period, he has 
arrived at some very striking and remarkable conclusions, which 
may be thus stated. Ist. If a series of equal distances be marked 
off in a line to represent years, and on the middle of each an ordinate 
erected representing the mean annual activity, their extremities be- 
ing joined by a curve; this will, of course, exhibit a series of waves 
averaging 11-11 years in breadth. Now it is found that the summits 
of these waves (and also their depressions) are of very unequal heights, 
and that (regarding their summits only) the curve connecting these 
exhibits again a series of larger waves, occupying, from summit to 
summit, a breadth of about 56 years, or (?) five times the length of the 
smaller period, the maximum value of 7s ordinate being nearly double 
of the minimum. In other words, besides the shorter period of 11-11 
