NO. TI USE OF THE PYRANOMETER ABBOT AND ALDRICH 9 



meter. The shade is to be turned forward as the sun goes westward. 

 Employ the glass hemisphere in measurements of direct or scattered 

 sun light. Remove it for nocturnal measurements. Be sure the 

 glass screen has no dirt or finger marks upon it. ( The glass may be 

 cleaned by breathing upon it and while damp wiping with clean cloth 

 or cotton.) When ready to observe, read the position of the galva- 

 nometer scale, open the shutter, read the first swing, close the shutter, 

 wait a half minute, read the galvanometer, throw on a current suit- 

 able to give about the same swing, read the first swing, and read the 

 current strength. 



Let the deflection due to radiation be Z>^, that due to current be Dc. 

 the constant of the instrument be K ( =2.54 for pyranometer A. P. O. 

 No. 6 with glass on). Then the result in calories per cm.- per min. 



is K ^ C'-. Where there is non-uniformity of the g^alvanometer 



scale, as here, it is of course necessary that D^ shall not dififer greatly 



(72 

 from Dji. We generally form the quotient j^ and take the mean of 



several values of it to use for neighboring values of radiation. 



Night ivork. — The glass hemisphere is removed. When ready to 

 read note the zero of the galvanometer. On opening the shutter a 

 negative deflection occurs. The zero of the galvanometer is then to 

 be restored by throwing on a suitable current, and adjusting its 

 strength by means of the slide wire rheostat until exact compensa- 

 tion is reached. This should occupy not less than 30 seconds to 

 enable the apparatus to reach a steady state. Read the current, C. 

 The intensity of radiation is given by A'C^, where A' is the constant 

 of the pyranometer with glass removed. (For pyranometer A. P. O. 

 No. 6, A' is then 2.41.) 



SUMMARY 



Test experiments tend to verify the accuracy of the pyranometer. 

 A new method of observing is described which conduces to more' 

 accurate results and to quicker operation. Sample observations are 

 given, as made with the two-strip pyranometer No. 6 on Mount 

 Wilson. The new method of observing is applicable, however, to 

 the one-strip form of pyranometer described in our former publica- 

 tion,' and if used in the new way it is possible that this simpler form 

 of pyranometer will prove equally accurate as well as more sensitive 

 than the two-strip form. 



' Smithsonian Misc. Coll., \'ol. 66, No. 7. 



