NO. 5 PYRHELIOMETRY — ABBOT, ALDRICH AND FROILAND 3 



measures are made day after day on the same high mountain, under 

 very clear skies, this consideration can hardly be serious in studies 

 of solar variation. 



The remaining question about the standard quality of the water- 

 flow pyrheliometer concerns the loss of infrared radiation to space 

 and the gain of infrared radiation from space, plus the gain of solar 

 radiation reflected from the sky immediately surrounding the sun, 

 when the shutter is opened. 



Radiation from within the receiving chamber is mainly intercepted 

 by the blackened walls and diaphragms of the vestibule. As this is 

 bathed by the water stream, its temperature is the same as that of 

 the chamber, and such losses from the chamber as are thus inter- 

 cepted in the vestibule are exactly replaced by radiation from the 

 vestibule, A part of the radiation from the chamber that reaches the 

 outer end of the vestibule goes on toward the sun and sky when the 

 shutter is opened, through the measured aperture above described. 

 A cone of radiation from the chamber at other times is intercepted 

 by the triple-leaved shutter. This shutter is blackened below, bright 

 above. Its lower leaf certainly is at air temperature. When the 

 shutter is opened, about half of this formerly intercepted part goes 

 on outward through the measured aperture. The outer end of the 

 vestibule has a blackened diaphragm 2.9 centimeters in diameter, 

 situated 24 centimeters from the receiving cone. 



All the sky rays that reach the receiving cone, either infrared from 

 the atmosphere or reflected rays from the sun, enter through the 

 smaller accurately measured aperture above described. All others 

 from these sources are intercepted in the vestibule and cannot appreci- 

 ably alter the measurement, because whatever warmth they contribute 

 raises the temperature of both incoming and outgoing water equally. 

 We think the incoming sky radiation, scattered sunlight and infrared 

 combined, will generally exceed the outgoing infrared radiation, when 

 the shutter is opened. On high mountains, under very clear and dry 

 atmosphere, the result may be the opposite. 



Froiland has recently investigated experimentally, at Table Moun- 

 tain, Calif, (elevation 7,500 feet), the magnitude of the gain or loss 

 of radiation on exposure of the water-flow standard pyrheliometer. 

 The instrument was deflected away from the sun just far enough to 

 avoid direct sun rays when exposed. But as the polished surface sur- 

 rounding the two outer apertures reflected sun rays up onto the lower 

 leaf of the shutter, and some scattered reflection of solar rays into 

 the instrument might occur, he dispensed with the shutter. To take 

 its place he used a cork, which he rapidly exchanged back and forth 



