ESTIMATES OF TIME 825 



The sun-spot cycle with an average length of 11.4 years is well known. 

 Newcomb and others have recognized a slight effect in the mean annual 

 temperatures, and Hunting-ton and Douglass have shown that it finds 

 terrestrial record in certain regions by fluctuations in the growth rate 

 of trees.^* The variation in time of culmination of sun-spots is, how- 

 ever, considerable, from 7 to 16 years, and the climatic effects are slight, 

 the geological effects perhaps negligible. 



Bruckner detected a cycle of 35 years in the variations of glaiciers and 

 in the level of the Caspian Sea- — a rhythm which has since been found to 

 have its origin in the sun and corresponds nearly to three of the sun- 

 spot cycles.^^ 



The work of Douglass has brought to light a 21-year cycle in rainfall 

 and tree-growth in Arizona, and less definitely a larger cycle of about 

 150 years.«« 



H. W. Clough, from the evidence contained in the catalog of early 

 observations of sun-spots and auroras, compiled by Fritz, holds there is 

 evidence extending back to about 300 A. D. of a solar cycle of about 

 300 years in length.^^ 



As to the existence of long-term changes of climate during historic 

 time, investigators as yet are not agreed. There are those who hold, from 

 the distribution of crops and the dates of harvest, and from the freezing 

 of rivers and harbors, that no appreciable change has taken place. This 

 appears to be nearly true for the past thousand years, in so far as a 

 regular or progressive change of climate is concerned; but the evidence 

 seems to be good that within that time there have been centuries which 

 departed appreciably from the mean. On comparing the conditions pre- 

 vious to the dark ages with later times, however, more notable changes 

 begin to appear. These have been investigated in recent years obiefiy by 

 Huntington, and, although there are those who do not accept the results, 

 the evidence in many respects seems to the present writer compulsory 

 in its nature. 



Huntington concludes from several methods of attack, but chiefly from 

 the degree of habitability of arid lands, that the climate of such critical 

 regions has varied notably and repeatedly during the historic period. 

 Such changes might pass undetected in the regions of equable rainfall 

 and temperature, except by means of precise scientific observations, but 



«* E. Huntington : The climatic factor. Carnegie Institution of Washington, 1914, 

 Publication No. 192. 



«" W. J. S. Lockyer : The solar activity, 1833-1900. Proc. Royal Soc, vol. 68, 1901, 

 pp. 285-300 ; Nature, vol. 64, 1901, pp. 196, 197. 



«« Huntington : Loc. cit, p. 117. 



«■ Astrophysical .Tonrnal, vol. xxii, 1905, pp. 42-75. 



