F. W. Very — Transmission of Terrestrial Radiation. 515 



the reason why it was easier to find suitable occasions for such 

 measures in winter at Allegheny than it is at Westwood. Out 

 of twenty-one observations of sky radiation made here on six 

 days in winter between Jan. 20 and Feb. 3, 1909,* only one 

 was good enough to give a genuine air transmission : but all of 

 the nineteen summer measures on three days in July were 

 good, and man}' more as good could have been given. 



"When the measures of terrestrial radiation, obtained from 

 zenith-sky observations on these suitable occasions, are plotted 

 with the surface pressures of water vapor as abscissae, o they are 

 well represented by the curve which Mr. Anders Angstrom 

 has pnblished.f My measures were all made several years 

 before Mr. Angstrom's paper appeared, and by an entirely 

 different method. They are, therefore, completely independent. 

 This being so, and the agreement between our separate experi- 

 mental work being very nearly perfect, it may well be asked 

 how it is that such diverse conclusions in regard to transmis- 

 sion can be drawn from what are practically the same data ; 

 for, whereas 1 have maintained for many years that the atmos- 

 phere in middle latitudes transmits, on the average, about 40 

 per cent of radiation,;}: the observed values varying between 

 26 per cent in summer and 58 per cent in winter,§ Mr. Ang- 

 strom, on the other hand, states his belief | " that the trans- 

 mission for clear sky seldom is greater than 25 per cent and 

 seldom is less than about 5 per cent. 1 ' If this statement referred 

 to those imperfect skies which, however much they may delight 

 the farmer praying for rain, are the bane of astrophysics, no 

 fault could be found ; but on referring to the accompanying 

 text, this reading is seen to be inadmissible. The discrepancy 

 arises from the aforesaid diverse modes of reduction, but the 

 smaller values have also been supported by further erroneous 

 arguments. 



In his note^f Mr. Angstrom illustrates by assuming a surface 

 temperature of 283° Abs. C. which "should, if no atmos- 

 pheric radiation existed [that is to say, if the atmosphere 



were absolutely transmissive], radiate to space 0*490 — - — '-r- ." 



cm mm. 



The "effective" sky radiation is then taken as — 0*15, which 



* Published in this Journal for April, 1913, pp. 379-380, 



f Astrophysical Journal, vol. xxxvii, p. 319, fig. 2, June, 1913. It is a 

 pleasure to note one more point on which we coincide. This is Mr. 

 Angstrom's recognition that dry dust alone, suspended in air, has almost no 

 influence on the transmission of terrestrial radiation, confirming results 

 reached during my work at Allegheny. 



% See Astrophysical Journal, vol. xxxiv, p. 375, December, 1911, and also 

 earlier papers. 



§See also Science, N. S., vol. xl, p. 420, September 18, 1914. 



|| Astrophysical Journal, vol. xxxviii, p. 200, September, 1913. 



IT Op. cit,, p. 198. 



