90 



ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1913. 



physical ObseiTatory, shows that we must attribute the changes to 

 the sun itself and not to the interposition of matter between the 

 earth and the sun. Thus we may conclude that the sun is variable, 

 having not only a periodicity connected with the periodicity of sun 

 spots, but also an irregular, nonperiodic variation, sometimes run- 

 ning its course in a week or 10 days, at other times in longer periods, 

 and ranging over irregular fluctuations of from 2 to 10 per cent of 

 the total radiation in magnitude. 



i.es I. so .^ 



MONTH 



91 .92 S3 a-* .SS .36 S7 ISO 



LY SOLAR-CONSTANT VAUUE.5, CAUORlES. 



3. THE EFFECTS OF VOLCANIC ERUPTIONS. 



Violent eruption of Mount Katmai, Alaska, occurred on June 6, 

 7, and 8, 1912. The solar observations made at Bassour, Algeria, 

 and at Mount Wilson, CaL, began to indicate the presence of dust 

 in the upper air from this volcano about June 20, 1912. The effects 

 of this dust became more and more considerable, so that in August 

 the direct radiation of the sun was reduced by the interposition of 

 the dust cloud by about 20 per cent, both at Bassour and Mount 

 Wilson. A study of the influence of Mount Katmai and other 

 volcanic eruptions was published by Messrs. Abbot and Fowle in 

 the Smithsonian Miscellaneous Collections, volume 60, No. 29, 1913. 

 It was shown that not only the volcano of Mount Katmai, but also 

 other great eruptions of former years, have materially decreased the 

 direct radiation of .the sun, and apparently altered the temperature 

 of the earth. Various observers have shown that the presence of 

 sun spots is attended with a decreased terrestrial temperature. In 

 the paper just mentioned it is shown that quite as important an 



