and the 90 years of record in the wet-climate Scotch pines near the Baltic Sea give the finest examples of rhythmic growth yet found in the trees.
Fig. 32.—Smoothed curve of Arizona pines showing the half-sunspot period for 120 years.
In order to test for possible variations in the sunspot curve during these 500 years, the tree record from 1420 to 1909 has been divided into 8 periods of approximately 60 years each and the form of the 11-year period obtained in each. This is shown in figure 33. From this it appears that the 11-year cycle is not uniform throughout the whole 490 years covered by the curve. In general the cycle shows 2 maxima and 2 minima. From 1420 to 1660 the second minimum is generally the deeper. For the next 60 years the curve flattens out in a striking manner. From 1730 to 1790 the curve again shows variations, but they are not well related to this cycle. After 1790 there are again 2 minima, but on the whole the first is more conspicuous.
The 11-year cycle in sequoia. — The question of agreement between the sequoia and the yellow pine is a vital one. Although the sequoias grow in a locality some 450 miles distant, there is a similarity in the rainfall of the two places. Some attempt has been made to cross-identify the rings in the two groups, and the puzzling fact was revealed that from 1400 to about 1580 no certain identity could be found, though after that date it was evident in many places. The difficulty has been partly removed by applying this same method of analysis to the last 500 years of the sequoia. The result is shown in the dotted lines of figure 33. It is evident that from 1420 to 1476 the second maximum of the pines is almost entirely lacking in the sequoias. The same is true of the interval from 1602 to 1658. The sequoias show strikingly the flattening of the curve from 1670 or 1680 to 1727. In the remainder of the curves the sequoias show better rhythm in the sunspot cycle than do the pines.
Taking the evidence as a whole, it seems likely that the sunspot cycle has been operating since 1400 A. D., with some possible interference for a considerable interval about the end of the seventeenth century.