﻿FOW 1 E] 



ABSORPTION OF WATER VAPOR 



5 



8 equals the amount of water vapor in the atmosphere. It is 

 tentatively determined according to a formula given by ilann,' where 

 u = g (.23) ; £ is the vapor pressure, in centimeters, at the surface 

 of the earth, as deduced from readings of the wet and dry ther- 

 mometers. Unfortunately water vapor measures were not taken 

 during- the times of taking these holograms, originally made for a 

 purpose not requiring such measures, and it has been necessary to 

 use in place of contemporary data, observations taken at 8 p. m., and 

 kindly furnished by Mr. Willis L. Moore, Chief of the United States 

 Weather Bureau. 2 



Bouguer's formula in its logarithmic form, 



log d— log ." -f md log a, (2) 



with log </ and m as variables, is used in plates 11 and in. shortly to 

 be described ; but in plates iv and V the formula has been further 

 modified, as follows : The fractional transmission due to the water 

 vapor alone, apart from the dust of the air, has been used in place of 

 kd, so that, assuming all the absorbing vapor to be terrestrial, e 

 becomes unity and we have, calling D this fractional transmission, 



! Dr. Julius Harm, Lehrbuch der Meteorologie, Leipzig, 1901, p. 22$. 



- These, with other data incident to the holograms, are tabulated below 

 (Table n) ; the second column gives the range of air masses between which 

 observations were used. In the last column the condition of sky is given ; 1 

 being as good as possible, 3 would be useless for observation : 



Table II 



