2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 



procedure the constant of such instruments is detennined by exposing 

 them within inclosures of constant-lower-temperatured walls. 



It might appear that the pyrgeometer could also be employed in 

 daytime to determine the radiation scattered from sunlight by the 

 sky, if at such times the sensitive strips were covered by a hemi- 

 sphere of glass to cut off exchange of rays of long wave lengths. In 

 such a case the heating current w^ould require to be applied to the 

 bright strips rather than the black ones. This use of the instrument 

 is, however, defeated by the fact that the absorption coefficient of the 

 bright strips for sky-light is not even approximately zero, and varies 

 greatly with the wave length, especially in the blue and violet and 

 ultra-violet parts of the spectrum. 



It was our purpose to devise a standard instrument for measuring 

 the solar radiation scattered inward by the sky in daytime, and it 

 was our hope that the instrument suitable for this purpose should also 

 be applicable to the measurement of nocturnal radiation as well. We 

 began experiments for this purpose in 1912; and now, after having 

 devised and constructed six different forms of instrument, we have 

 satisfied ourselves that the last two types are very satisfactory for 

 the purpose. 



The name Pyranometer,^ selected for the instrument we have 

 devised, is taken from Greek words ( Trvp, fire; uvd, up; fxirpov, a 

 measure) signifying that which measures heat above. The name 

 was chosen with reference to the fact that the instrument is designed 

 to measure the energy of radiation to or from a complete hemisphere 

 lying above the measuring surface. 



PYRANOMETER A. P. O. 6 



Referring to the accompanying illustration, figure i is a side view ; 

 figure 2, looking down from above ; figure 3, an attachment not used 

 in measuring total sky radiation, but employed when it is desired to 

 restrict the measurements to the sun alone ; figure 4, a cross section 

 taken at right angles to the view presented in figure i and omitting 

 the wooden base and apparatus for shading the sun. In figure 5 

 are details showing the arrangement of the sensitive strips and 

 thermo-couples. The instrument shown is the sixth form we have 

 devised. In the fifth form there is but one sensitive strip instead of 

 two as shown here. The fifth form of instrument is more sensitive 

 than the sixth form, but has a certain source "of error which was to a 



^ We make our acknowledgments to Miss M. Moore and to Dr. Casanowicz 

 for advice in selecting this name. 



