EEPOET OF THE SECRETARY. 83 



gone on steadily in charge of Mr. F. E. Fowle, aided by Miss F. A. 

 Graves, computer, and Mr. R. Eisinger, messenger. 



Mr. Fowle completed and published 1 his investigation of the ab- 

 sorption of long wave rays by long columns of air containing known 

 quantities of water- vapor. His results give the relations between ab- 

 sorption and atmospheric humidity, wave-length by wave-length, 

 from the visible spectrum down to waves of more than 20 times the 

 maximum visible wave-length, and for quantities of water ranging 

 from ywo t° three times that which prevails in the vertical thick- 

 ness of the atmosphere above Washington in clear spring weather. 

 Many difficulties which were met required tedious subsidiary investi- 

 gations which are described in the paper. 



Notwithstanding the greatness of this contribution to meteorologi- 

 cal science the subject of the relations of water- vapor and terres- 

 trial radiation demands yet more investigation adapted to cover the 

 range of wave-lengths from 16 microns to 50 microns, where Mr. 

 Fowle was forced to give over the investigation temporarily, because 

 no substance suitable to make a prism for forming the spectrum of 

 these rays was known. 



Mr. Aldrich has since investigated at the observatory a great num- 

 ber of natural crystalline and other substances, including many pure 

 chemical preparations. None was found appreciably more trans- 

 parent than rock salt, which was used by Mr. Fowle, except potassium 

 iodide. Apparently this substance, if it could be procured in large 

 crystals, or fused into a noncrystalline structure, would be suitable 

 to carry the work to much longer wave-lengths. Efforts have been 

 made, as yet unsuccessfully, to procure blocks of this substance of 

 suitable proportions and inner structure for making large prisms. 



Mr. Aldrich has carried on a number of investigations on the ab- 

 sorption and reflection of atmospheric-water-vapor, liquid water, 

 lampblack, gelatin, and other substances to rays emitted by a black- 

 ened reservoir filled with boiling water. In these experiments he 

 has employed rock salt transmission plates to roughly separate the 

 total radiation into two parts, whose wave-lengths are respectively 

 greater and less than about 20 microns, where rock salt ceases to be 

 transparent. The results on water-vapor agree well with what Mr. 

 Fowle's spectrum work would tend to indicate. They also show 

 that an atmospheric layer about 50 meters deep, containing water- 

 vapor equal to 0.05 centimeters of precipitable water, would probably 

 absorb all the rays sent out by the 100° C. radiator which are non- 

 transmissible to rock salt, that is above the wave-length 20 microns. 

 This is in harmony with observations of the sun and of nocturnal 

 radiation made by Mr. Aldrich on Mount Wilson, to which reference 



1 Water Vapor Transparency to Low-temperaturu Radiation. Smith. Misc. Coll., Vol. 

 68, No. 8, 1917. 



