120 DOUGLASS— CHARACTERS IN ANNUAL RINGS OF 



periods. The first line of evidence naturally is to compare the cycles 

 in a tree sequence with cycles in rainfall records near by. This is 

 done successfully with the Prescott trees, but the period over which 

 such a comparison can be made is under half a century, and that is 

 too short for satisfactory results. These trees show a 7.7-year cycle 

 and the eleven-year sunspot cycle. The rainfall shows the sunspot 

 cycle and a 7.7-year cycle, but the latter could be interpreted as any- 

 thing between 5.5 and 8 years. 



The next line of evidence depends upon the area over which cycles 

 may be traced, for the common environment over a large area is 

 climate. A test has been made between ten pines in southwestern 

 Colorado, nine pines in northwestern New Mexico, fifty miles away, 

 and nineteen pines at Flagstafif, Arizona, 200 miles southwest of the 

 other groups. These three groups are largely identical in their cycles 

 for the last two hundred years or more. This gives us much confi- 

 dence that these cycles are real and are climatic in origin. 



But still further evidence comes from a purely historical source 

 and is of a kind full of interest on its own account. Professor E. W. 

 Maunder, of England, in a recent letter, called attention to the pro- 

 longed dearth of sunspots between 1645 ^^^ 171 5 and judged that if 

 there were a connection between solar activity and the weather and 

 tree growth, this extended minimum should show in weather condi- 

 tions and in the trees. On receipt of the letter this period was imme- 

 diately recognized as the interval in which the greatest difficulty had 

 been found in tracing solar effects. In fact, in 1914, when the writer 

 was trying to trace the history of the solar variations in the yellow 

 pines, the difficulty between those dates almost led to the view that 

 the trees were not giving this cycle. A present review of the eleven- 

 year period in those trees confirms its well-marked existence from 

 before 1400 to the middle of the seventeenth century. Soon after 

 1700 it reappears, but not in complete form until the latter part of 

 that century. 



The test was then carried to the sequoias and two difficulties were 

 encountered. First, it was found that the slow-growing, sensitive 

 upland trees were the ones which displayed the solar cycle, and, 

 second, the interference by other cycles was such that the double 



