4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



coming energy from the sun was scattered by this dust or what may 

 be called " dry haziness " in distinction from a somewhat similar con- 

 dition to be discussed later but associated with water vapor and there- 

 fore denoted " wet haziness." During 1912 ' owing to volcanic dust, 

 this scattering by dust particles increased to about 25 per cent on the 

 haziest days. It had decreased, on the average, to 2.6 per cent 

 during 1913, and 1 per cent during 1914 and 1915. During 1916 it 

 increased again to an average value of 3.2 per cent producing a 

 marked streakiness in the sky as seen at dawn at Mount Wilson. 



Between wave-lengths 0.35 and 0.57 n nearly all the loss of light 

 from a beam passing through dry, dust-free air is seen to be due to 

 scattering by the molecules of the air. As has been just noted, 

 during 1910-11 the air was nearly dust free. In the last line of 

 table 1 are given the means of the dry-air coefficients for these two 

 years. They are closely in accord with the values to be expected 

 from Rayleigh's theory. For the first three values, in brackets, 

 theoretical values have been substituted since at these wave-lengths 

 the accuracy of the observed ones is vitiated by field light. 



The water-vapor coefficients will next be analysed. Because of the 

 more normal results, the formula will first be applied to a group of 

 transmission coefficients for liquid water obtained by Kreusler, 2 

 Ewan, s and Aschkinass. 4 



Table 4. — Number of Molecules N derived from Liquid Water 

 Transparency 



The data of the above table were somewhat differently treated in the Attrophysical Journal 

 (38, p. 401, 1913). There the values were graphically reduced using a uniform value of the 

 index of refraction for all wave-lengths. Here n — i is assumed to have the same fractional 

 variation from wave-length to wave-length that liquid water has. However, the observed 

 vaiue of n even for wave-length 0.589 n must be held very doubtful. 



Annals of the Astrophysical Observatory of the Smithsonian Institution, 

 vol. 3, p. 216, 1913. 



2 Annalen der Physik, 6, p. 412, 1901. 



3 Proceedings of the Royal Society, 57, p. H7, 1894. 

 1 Annalen der Physik und Chemie, 55, p. 401, 1895- 



