8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 



On the comparison of solar readings by the pyranomcter and 

 pyrheliometer as observed on Mount Wilson. 



Observations made with pyranometer A. P. O. No. 6 and secondary 

 j^yrheliometer A. P. O. No. IV. Results in calories per cm.- per min. 



August 6, 1916. Hour Angles i''S2'" to i"37"' 



Secant Z 



Pyranometer X secant Z 



Pyrheliometer 



Difference 



August 7, 1916. Hour Angles 5''35'" to s''23" 



.Secant Z 



Pyranometer X secant Z. 



Pyrheliometer 



Difference 



3.448 



1. 109 



1. 118 



+ .009 



From these experiments it appears that the pyranometer gives 

 values of solar radiation comparable in accuracy with those observed 

 with the pyrheliometer. As the results are of satisfactory accuracy 

 at both great and small zenith distances, the pyranometer may be 

 supposed to give accurate results on the sky, which involves all zenith 

 distances. 



DIRECTIONS FOR OBSERVING AND REDUCING OBSERVATIONS 



Employ a galvanometer of not more than 60 ohms resistance giving 

 a first swing within 5 seconds. If too sensitive diminish its deflection 

 by a suitable resistance in series. Employ a heating current adjust- 

 able from zero to 0.8 amperes. If a storage battery is available it 

 will be found the most satisfactory source to furnish the current, 

 but dry cells may be used. A simple slide wire rheostat is required for 

 nocturnal work. Employ an accurate ammeter for reading the 

 current strength. 



Daylight work. — Place the pyranometer on a level surface in the 

 place where the intensity of radiation is to be measured. If the sun 

 is sometimes to be shaded ofif, adjust the flat arc (which is the sun- 

 shade support) to He north and south, set the arc to the latitude of 

 the place, and set the shade to cast its shadow centrally on the pyrano- 



