RADIATION OF THE SUN ABBOT. 



157 



atmosphere. We attempt, there- 

 fore, to make the observations hi 

 such a manner as to permit a 

 correct estmiate of the atmos- 

 pheric losses, so that the result 

 can be expressed as if the meas- 

 urements were made in free space 

 beyond the atmosphere. But of 

 course our actual work must be 

 done at the earth's surface. 



We express solar radiation in 

 heat units called calories. As the 

 bolometer (pi. 2) is not of itself 

 capable of giving true calories we 

 have devised an instrument shown 

 in figm'e 3, a standard pyrheh- 

 ometer, so called. A is a cham- 

 ber of nearly the dimensions of a 

 large test tube, whose walls are hol- 

 low and adapted for the circulation 

 of a stream of water. The stream 

 entere at E, bathes the walls and 

 rear of the chamber and the cone- 

 shaped receiver of rays H, and 

 passes out at F, carrying the heat 

 developed by solar rays which 

 enter the chamber by the meas- 

 ured orifice C. At D^ and D^ are 

 platinum coils adapted to measure 

 the rise of temperature of the wa- 

 ter due to solar heating. Know- 

 ing the weight of water flowmg per 

 minute, the rise of temperature 

 and the area of the opening C for 

 solar rays, their intensity is deter- 

 mined. As a check, heat may be 

 produced electrically in the cham- 

 ber, and the proof of the accuracy 

 of the instrument consists in find- 

 ing the known quantity of elec- 

 trically introduced heat correctly 

 measured. Another simpler in- 

 strument for everyday use is the 

 silver disk pyrheliometer shown in 



I'la. 3.— Standaed water-flow pykheliometer. 



