10 
THE SECRET OF THE BIG TREES. 
Manifestly it was necessary to devise some new line of research 
which should not only furnish dates, but should prove positively the 
existence or nonexistence of changes of climate, and should do it in 
such a way that the investigator’s private opinions, his personal 
equation, so to speak, should not be able to affect his results. The 
necessary method , was most opportunely suggested by an article 
published in the Monthly Weather Review in 1909 by Prof. A. E. 
Douglass, of the University of Arizona. In regions having a 
strongly marked difference between summer and winter it is well 
known that trees habitually lay on a ring of wood each }^ear. The 
CROSS SECTION OF A SEQUOIA SHOWING THE GROWTH RINGS. 
wood that grows in the earlier part of the season is formed rapidly 
and is soft in texture, while that which grows later is formed slowly 
and is correspondingly hard. Hence each annual ring consists of a 
layer of soft, pulpy wood surrounded by a thinner layer of harder 
wood which is generally of a darker color. Except under rare con¬ 
ditions only one ring is formed each year, and where there are two 
rings by reason of a double period of growth, due to a drought in 
May or June followed by wet weather, it is usually easy to detect 
the fact. In the drier parts of the temperate zone, especially in 
regions like Arizona and California, by far the most important factor 
in determining the amount of growth is the rainfall. Prof. Douglass 
measured some 20 trees averaging about 300 years old. He found 
that their rate of growth during the period since records of rainfall 
