90 
PROFESSOR A. SCHUSTER ON THE PERIODICITIES OF SUNSPOTS. 
gives the sum of four successive intervals. Starting with the last figure so as to 
make use of all the best determined maxima, the first must be omitted when the 
intervals are collected in groups of four. It will be noticed that the average interval 
of four periods is approximately constant from 1675'to 1848, or during a period of 
over 170 years; the average of the periods which took place during that time is 
consistently about 10‘8 years. It follows that, independently of the observations in 
the last 60 years, it is only the long intervals between 1626 and 1675 which bring up 
the average to 11 years. The latter part of this interval was one of great scarcity of 
sunspots, very few being observed between 1640 and 1670, and the whole of the 
second part of the seventeenth century seems to have been quite anomalous, as 
regards the behaviour of spots.* 
Table XIII. 
Maxima. 
A. 
Interval. 
Sums of 
four successive 
intervals. 
Maxima. 
A. 
Interval. 
Sums of 
four successive 
intervals. 
1615-5 
-0 
8 
__ 
1761-5 
+ 1 
0 
11-2 
43-3 
1626-0 
- 1 
4 
10-5 
_ 
1769-7 
-1 
9 
8-2 
v — 
1639-5 
+ 1 
0 
13-5 
— 
1778-4 
-4 
2 
8-7 
— 
1649-0 
-0 
6 
9-5 
— 
1788-1 
-5 
6 
9-7 
— 
1660-0 
-0 
6 
11-0 
— 
1805-2 
+ 0 
4 
17-1 
43-7 
1675-0 
+ 3 
3 
15-0 
49-0 
1816-4 
+ 0 
5 
11-2 
— 
1685-0 
+ 2 
2 
10-0 
— 
1829-9 
+ 2 
9 
13-5 
— 
1693-0 
-0 
9 
8-0 
— 
1837-2 
-0 
9 
7-3 
— 
1705-5 
+ 0 
5 
12-5 
— 
1848-1 
- 1 
1 
10-9 
42-9 
1718-2 
+ 2 
1 
12-7 
43-2 
1860-1 
-0 
2 
12-0 
— 
1727-5 
+ 0 
3 
9-3 
— 
1870-6 
-0 
8 
10-5 
— 
1738-7 
+ 0 
4 
11-2 
— 
1883-9 
+ 1 
4 
13-3 
— 
1750-3 
+ 0 
9 
11-6 
— 
1894-1 
+ 0 
5 
10-2 
46-0 
[The assumed grouping is admittedly arbitrary, but serves to show how difficult it 
is to devise a satisfactory measure of a period from the observations ol maxima which 
are unequally spaced .—February 18, 1906.] 
Assuming the permanency of the 11 years’ period, both Newcomb and Wolfer 
formed tables giving the deviation in time of the observed maxima with those ol a 
mean period derived from the whole of the available material. These deviations, 
according to Wolfer’s estimate, are given in the second column of Table XIII. It 
will be seen at once that though differences occur amounting in one case to 5‘6 years, 
yet the maxima seem on the whole to group themselves in pretty close proximity to 
the calculated times. Professor Newcomb sums up his conclusions in the sentence :— 
“ Underlying the periodic variations of spot activity there is a uniform cycle, 
unchanging from time to time, and determining the general mean of the activity.’ 
A convincing feature of the second column of Table XIII. is that when maxima such 
* According to Sporer there is no record of a sunspot being seen on the Northern Hemisphere of the 
Sun between 1672-1705. 
