CYCLES. 107 
averaging near the larger figure. It should more properly be called a 
“broken” period perhaps, since it is made up of different periods for 
different intervals, first one and then another predominating. The 
methods used in the search for this 2-year period have revealed frequently 
asolar cyclealso, and there seems to besome obscure connection between 
the two. 
PERIODOGRAPH ANALYSES. 
Differential patterns.—The periodograms, as already shown, indicate 
the different cycles operating within a certain range. Any one cycle 
together with others close to it may be studied more minutely on the 
differential pattern produced when the instrument is set at the desired 
period. Plates 12 and 10, B, are arranged to illustrate this and at the 
same time show the solar cycles in several of the groups. A periodic 
effect equal in length to the setting of the instrument is indicated by a 
vertical row of light crests or dark spaces. These rows may be seen in 
any of the patterns. If the row of crests points downward to the 
right, its period is greater than the setting of the instrument; if to the 
left, the period is less. The straightness of the row indicates the 
regularity of the period. Plate 12, a, is made from the standard 5-year 
period with a setting at 5.0 years. The first pattern in plate 12 shows 
the regularity of the sunspot period since 1610 A. D. The interval in 
the latter part of the eighteenth century, when the cycle was reduced 
to less than 10 years, is distinguished by a bending of the row toward 
the left. This is followed by a deflection toward the right during the 
interval of readjustment from 1790 to 1830. The direction of any 
row becomes an exact measure of its period. 
If a period is constant, the row of crests is straight. A zigzag row 
made up of short, straight parts means that one period after another 
becomes predominant. A curved row means a constantly changing 
period. Some examples of apparent curved rows may be picked out 
in the sequoia pattern. A curved row may indicate some other func- 
tion than a simple period. Pattern R in plate 12 is made to illustrate 
a logarithmic variable, beginning at the top as a 5-year period and 
changing by a constant percentage increase to a 10-year period at the 
bottom. The instrument is set at 8.0 years. 
The 11-year cycle—tThe first 6 patterns in plate 12 illustrate this cycle. 
The first gives the sunspot numbers from 1610 to 1910, including the 
uncertain ones from 1610 to 1750. Pattern B gives the fine vertical 
row shown by the 6 groups of trees from north Europe. This was 
shown as a curve in figure 25, page 77. The qualitative test of the 
entire 80 European trees is shown in pattern C. This may be seen as 
a curve in figure 26. The small secondary maxima at several of the 
minima show as light crests between the main rows. Pattern D shows 
a 12-year period in south Sweden during the past 50 years, preceded by 
