24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 60 



SUMMARY 



The transparency of the atmosphere was much reduced in the 

 summer of 1912 by dust from the volcanic eruption of Mount 

 Katmai, June 6 and 7. 



Evidence of the dust appeared at Bassour, Algeria, on or before 

 June 19, and at Mount Wilson, California, on or before June 21. 



The total direct radiation of the sun was reduced by nearly or 

 quite 20 per cent at each of these stations when the effect reached 

 its maximum in August. 



In the ultra-violet and visible spectrum the effect was almost 

 uniform for all wave-lengths, but was somewhat less in the infra-red. 



From Bassour experiments, including measurements by two 

 methods of the radiation of the sky, it appears that the quantity of 

 heat available to warm the earth was diminished by nearly or quite 

 10 per cent by the haze. There is, however, some indication that 

 this was in part counterbalanced by a decrease in the earth's radia- 

 tion to space, caused by the haze. 



Similar periods of haze followed great volcanic eruptions in 

 former years. The influence of Krakatoa, Bandai-San, Mayon, 

 Santa Maria, and Colima seems to have been recorded by measure- 

 ments of solar radiation, and caused pronounced decrease in the 

 direct solar beam from 1883 to 1885, 1888 to 1894, and 1902 to 1904. 



Evidence is presented that the dust layer of 19 12 affected terres- 

 trial temperatures, especially of high stations. 



A remarkable correspondence is found between the average depar- 

 tures of the mean maximum temperature for 15 stations of the 

 United States and a curve representing a combination of the sun- 

 spot numbers of Wolfer and the departures from mean values of the 

 annual march of direct solar radiation from 1883 to 1909. 



