Hutchins and Pearson — Air Radiation. 277 



Akt. XXXI. — Air Radiation ; by C. C. Hutchins and J. C. 



Pearson. 



In 1892, one of the present writers carried out, at the request 

 of Professor Cleveland Abbe, some experiments for finding the 

 radiation constant of atmospheric air. The radiation was 

 measured from a hot moving column of air of one centimeter 

 depth in the line of sight, and as close as possible, consistent with 

 proper screening, to the heat-recording apparatus. Owing 

 doubtless to defective surroundings, the results obtained from 

 day to day showed considerable variation ; in fact, more than 

 could be produced artificially by changing the normal constit- 

 uents of air in a closed room, such as dust and moisture, between 

 wide limits. An average of the best results gave "00000114 

 small calories per second per square centimeter per degree, for 

 a thickness of l cm of the radiating layer. 



In 1900, Professor Very published an extended monograph 

 on the subject, in which very numerous experiments of his own 

 and others are collected and discussed with the utmost skill and 

 patience. Yery's result, stated in the terms given above, at 

 100° excess temperature, was '00000036, or three times smaller 

 than what we had obtained. The large difference led to a 

 reexamination of our figures and methods without finding any- 

 thing that could account for it. An entire change of apparatus 

 and method often leads to unexpected results, and may cause 

 us to modify our views as to the probable error of our former 

 consistent figures. In 1902, we constructed an entirely new 

 apparatus, containing nothing that belonged to the old. A 

 radio-micrometer after Boys was the heat-receiving instrument. 

 AVe avoided all suspicion of air contamination by taking air 

 from out-of-doors. A box some 6 feet long, 2*5 feet wide, and 

 3 inches deep, containing a sheet-iron bottom about half-way 

 up, and covered with a single sheet of glass, was set at an angle 

 of about 45° outside a south window. The upper end of the 

 box was extended by a wooden chimney that projected through 

 a slit in the window shutter. The box had trunnions at the 

 sides upon which it could be tilted by pulling a string. The 

 radio-micrometer was mounted inside the shutter, so that in the 

 lowest position of the chimney the current of air that streamed 

 up through came opposite the radio-micrometer opening. Sun- 

 light falling upon the exposed glass cover heated the sheet-iron 

 bottom, and this in turn heated the air in contact with it, and 

 a current of hot air was delivered through the chimney. The 

 temperature of the hot air was obtained by a thermal junction 

 of two thin copper and iron wires inserted in the stream, the 

 circuit being completed through a calibrated galvanometer. 



Am. Jour. Sci. — Fourth Series, Vol. XVIII, No. 106.— October, 1904. 

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