6 CLIMATIC CYCLES AND TREE GROWTH 



have reason to hope will compensate in substantial measure for the tempor- 

 ary character assigned to climatic cycles, and will contribute to the final 

 climatic theory that opens the way to scientific long-range prediction. 



Period and Cycle — Some remarks should be given on the use of the words 

 period and cycle. In general, the word period is extensively used in astron- 

 omy and refers to a repetition that comes at fairly exact intervals or whose 

 variations from exactness are due to fully assignable causes. The best 

 examples of periods are found in the motions of the planets. To cycle stu- 

 dents the word has commonly meant continuous and unending operation at 

 closely equal intervals. In these pages the terms discontinuous, or frag- 

 mentary or temporary are introduced before period to indicate fairly exact 

 repetitions that seem to have a beginning or an end. The word cycle seems 

 to us a more general term with fewer requirements as to duration and to the 

 stability of its length, phase and amplitude. Thus we refer to the sunspot 

 cycle as a series of discontinuous periods in order to describe the fact that it 

 remains steadily at one period length through a number of repetitions and 

 then changes slightly to some other value, to which it clings for a time. The 

 sun's persistence in maintaining a slightly irregular fluctuation in the number 

 of spots through the centuries is to some students a most suggestive connota- 

 tion in the word cycle. The application of the methods described in this book 

 gives the best apparent chance of demonstrating the existence of this char- 

 acter in climatic cycles. 



Acknowledgments — Acknowledgment is made of the generous assistance 

 given by the Carnegie Institution of Washington for our climatic studies 

 since 1918. Aid from that source has been augmented in recent years, and 

 at the present time a special grant is making possible our statement of results. 

 This grant becomes effective by cooperative arrangement with the University 

 of Arizona. The latter has given its assistance for many years in the way 

 of laboratory rooms, and for a time, while extensions of the Arizona ring 

 chronology were pending, made it possible for the writer to work without 

 being subject to class schedules. Great obligation is due the National 

 Geographic Society for its financial aid and moral support during several 

 years when the Arizona chronology was being extended. To the Society, 

 that extension brought about the successful determination of the age of the 

 splendid ruin called Pueblo Bonito and other ruins of the Southwest; to us, 

 it was, in addition, the extension of a climatic history into far past times in a 

 favorable region. The publication of my report of the dating work is now 

 made, and I appreciate particularly the gracious willingness of Dr. Grosvenor 

 that the important collections made for that purpose may be freely used in 

 climatic and other scientific studies. 



Acknowledgments are also most cordially made to Dr. H. S. Colton and 

 his colleagues of the Museum of Northern Arizona; to the American Museum 

 of Natural History and other institutions through Dr. Clark Wissler and 

 Mr. Earl H. Morris; to the Research Corporation of New York; and to many 

 others, among whom the students of "tree-ring interpretation" should be 



