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 

 a solar cycle also, and there seems to be some 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 tune 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, Q, 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. The 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 



