250 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



of some maxima and the accentuation of others. Hence, if it be true that solar changes 

 influence terrestrial climate, we should expect that in some places the results would be 

 immediately and clearly visible, while elsewhere they would be masked in such a way as 

 to be invisible when comparison is made between such phenomena as the sun-spot curve 

 and the curve of growth of great trees like the sequoias; yet even here, if we examine long 

 periods and obtain averages of many sun-spot cycles, we should expect to find some trace 

 of the influence of the sun. And this is exactly what we find; the trees of Germany (which 

 depend upon summer rains and have only a slight conservation factor) vary their rate of 

 growth in close harmony with the sun-spot cycle. The sequoias of California, on the 

 contrary, with their winter precipitation and large conservation factor, show in their 

 growth the same general periodicity as the sun-spots, but individual maxima or minima 

 by no means agree with those of the sun. Yet when the growth of a century or two is 

 considered the trees are found on an average to grow relatively fast when the sun-spots 

 are at a maximum, and slowly when they are at a minimum. 



The work of Arctowski does not exhaust the recent contributions to our knowledge of 

 ways in which the effect of solar radiation upon terrestrial climate maj^ be modified. 

 Wliile the proof of this volume was being read there came to hand a paper by Abbott and 

 Fowle,* two students of solar physics who have done much to demonstrate the existence 

 of a relationsliip between solar radiation and terrestrial temperature. They now show that 

 during the summer of 1912 volcanic dust from the volcano of Katmai in Alaska seems to 

 have filled the upper air to such an extent that it decreased the amount of solar radiation 

 received on the earth's surface by about 10 per cent of the normal solar constant. More- 

 over, they present a certain amount of evidence indicating that other volcanic eruptions, 

 such as that of Ivrakatoa in 1883 and Bandai-San in Japan in 1888, have produced similar 

 effects. The most important part of their paper, however, is a diagram which is not 

 reproduced, but which in all essentials is almost identical with figure 85. This shows a 

 previously published set of curves comprising the sun-spot curve from 1880 to 1909, the 

 curve of departures from the mean temperature at 15 stations in the United States, 

 and a similar curve of departures for the whole world. These three curves, to quote 

 Abbott and Fowle, show "a considerable degree of correspondence — yet it is not hard to 

 see that there is also much discordance." They are among the pieces of evidence referred 

 to on a previous page which on the whole lead to the conviction that terrestrial tempera- 

 ture varies in accordance with fluctuations in the spots of the sun. Our authors now add 

 to their previous diagram a curve showing recorded variations in the intensity of the sun's 

 direct radiation as measured at the earth's surface and as modified by such terrestrial 

 phenomena as the dust of volcanic eruptions. They then combine this curve with that of 

 the sun-spots in such a way that the sun-spot curve still predominates, but is considerably 

 modified. The correspondence between this modified curve and the temperature curves, 

 particularly that of the United States, is, as they truly say, "most striking." 



After the preceding paragraph was written and when the page-proof of this volume 

 was being indexed, still another important article on the same subject came to hand.f 

 In this Professor W. J. Humphreys follows a line of reasoning almost identical with that of 

 Abbott and Fowle, and comes to the same conclusion, but carries it farther. In an un- 

 pubKshed manuscript, which he has kindly placed at my disposal, he appUes his results 

 to the climatic changes of geological times, and emphasizes the importance of changes in 

 continental form, oceanic currents, and related phenomeiia in a way which differs little 

 from that employed by Professor Schuchert and myself in the lemaining portions of this 

 volume. The main points of difference between his ideas and those here presented are 

 that in the first place he regards variations in solar activity as of negligible importance 



* Volcanoes and Climate, by C. G. Abbott and F. E. Fowle, SmitlLSonian Mis. Coll., CO, No. 29. Wa.shington, 1913. 

 fW. J. Humphreys, "Volcanic Dust and Other Factors in the Production of Climatic Changes, and their Possible 

 Relation to Ice Ages." Bulletin of the Mount Weather Observatory, vol. 6, part 1, August 20, 1913, pp. 1-26. 



