REPORT OF THE SECRETARY. 93 



column over most of the earth exceeds 3.0 cm., it now seems probable 

 that the proper figure should be eight-tenths instead of nine-tenths. 



As regards the absorption of carbonic-acid gas Mr. Fowle finds 

 that one-fortieth part of the amount of this gas found in a vertical 

 atmospheric column produces the maximum possible effect. This* 

 does not lead to any modification of our conclusions as to the effect of 

 atmospheric carbonic-acid gas as stated above. 



With ordinary humidity, at sea level a layer of air 10 meters long, 

 according to Fowle, will absorb 50 per cent of the radiation of a 

 perfect radiator at terrestrial temperatures. Similarly the layer of 

 air above 11 kilometers, or 6 miles, altitude contains enough water 

 vapor to absorb 50 per cent of such radiation. 



In view of what has been said and remembering the presence of 

 clouds, only about one-tenth of the radiation of the solid and liquid 

 surface of the earth escapes directly to space. The atmosphere above 

 11 kilometers apparently contributes more than half of the radiation 

 of the earth viewed as a planet and prevents half of the radiation of 

 lower layers from escaping. Nearly the entire output of radiation 

 of the earth to space, certainly more than three-fourths, arises from 

 the atmosphere and its clouds as its source. The " effective radiat- 

 ing layer," meaning a layer which if perfectly radiating to space 

 would equal in radiation the actual earth viewed as a planet, may still 

 be thought of as at several kilometers altitude and at a temperature 

 well below freezing. 



The subject of atmospheric absorption is so difficult both theoreti- 

 cally and experimentally that much more investigation ought still to 

 be done on it. Mr. Fowle's long experience has well fitted him for 

 making further advances. It is hoped to put at his disposal soon 

 the necessary means to make new researches. These include bolo- 

 metric apparatus of greatly increased sensitiveness, such as recent 

 studies now enable us to construct. The one obstacle to complete 

 success which now seems insuperable is the lack of any means to 

 form an intense unabsorbed spectrum free from stray light, extend- 

 mg from 15 to 50 microns in wave length. 



2. AT MOUNT WILSON. 



The expedition of 1916 continued solar-constant and other observa- 

 tions at Mount Wilson until late in October. The expedition was 

 renewed late in June, 1917. Improvements in the supply of electric- 

 ity and water to the station were completed in June, 1917. 



In 1916 many observations of the sky by day and by night were 

 made at Mount Wilson with the pyranometer. The plan was fol- 

 lowed from August to October of measuring with this instrument the 

 total solar radiation at a fixed zenith distance of the sun, and almost 



