SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 



(3) Let D ji be the deflection due to radiation, Dc the deflection 

 due to heating current, C the strength of current, and^ the constant 

 of the pyranometer. ( For our pyranometer, A. P. O. No. 6, i^ = 2.54.) 



Then the intensity of radiation R — K j~ C^ . 



For the observation of nocturnal radiation we sometimes use the 

 same method, but generally we effect an exact compensation of the 

 nocturnal radiation by adjusting" the heating current to reduce the 

 galvanometer deflection to zero. In this way the temperature of 

 the strips is brought back to the temperature of the air, and experi- 

 ence shows that even on windy nights the galvanometer is steady 

 'under these conditions. When the glass hemisphere is removed 

 for night work, the constant of the pyranometer is decreased from 

 2.54 to 2.41. (See our earlier paper.) 



When in the use of the pyranometer deflections are observed 

 too large for convenient reading, a suitable resistance is used in 

 series with the galvanometer. In new instruments a little switch is 

 provided for this purpose and marked G. It provides three degrees 

 of sensitiveness according as open or closed on one side or the other. 



SAMPLE OBSERVATIONS MADE ON MOUNT WILSON 



On the proportion borne by the defections observed after fixed 

 intervals to the final deflections due to different heating currents. 

 Glass hemisphere on. 



A. With German galvanometer. Resistance 58 ohms. 



I. Current strength, 0.200 amperes. No extra resistance in circuit, 

 and no definite first swing. 



Time in seconds. . . . 

 Deflection in cm . . . 

 Proportion of finaL 



• 34 



ID 

 8.93 



./Sii .950 



IS 

 9 24 

 .983 



20 

 9-32 

 991 



25 

 9-35 

 •993 



30 

 9.36 



40 

 9^37 



.9961 .997 



50 

 9-38 



60 

 9^39 



75 

 9.40 



.99911.000 



2. Current 0.400 amp. Extra resistance 60 ohms. Deflections 

 20 to 23 cm. 



Proportion of finaL .869 



.967 



• 987 



• 994 



.998 .999 ^999 



• 999 



1. 000 



1. 000 



3. Current 0.605 amp. Extra resistance 260 ohms. Deflections 

 20 to 23 cm. 



Proportion of final. 



i70 



.966 



• 9871 -993 ^995 ^999 



.998 .999 .999.1.000 



