4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



areas of the solar apertures, on the adjustment of the heating current 

 to reproduce exactly the observed zero reading of the galvanometer, 

 and on an exact determination of the electric heating current required. 

 The solar apertures were turned truly circular and equal, and a plug 

 exactly fitting them was made. This plug was inserted from time to 

 time during observations to surely clear the aperture of dust particles. 

 It was measured at numerous diameters with two different Browne & 

 Sharpe micrometers. The mean value was 2,312 centimeters. The 

 computed area of the aperture was thus found to be 4.198 square 

 centimeters. Two milliammeters in series were used to measure the 

 currents. One of them was a more satisfactory instrument than the 

 other, and its results were used exclusively in the final computations. 

 The others dift'ered from them only very slightly. These electric in- 

 struments were calibrated several times during the measurements 

 against new Weston standard cells. It is believed that the errors of 

 individual current measurements did not exceed o.i per cent, so that 

 the errors of individual energy measurements, due to inaccuracy in 

 reading the current values, did not exceed 0.2 per cent. In the mean of 

 many observations given below this error would be sensibly eliminated. 



We computed the loss of solar radiation by reflection through the 

 entrance of the receiving chamber and made a plus correction of o.i 

 per cent for the imperfect " blackness " due to it. 



In our earlier days of observing in June, 1932, the sky was very 

 exceptionally clear at Mount Wilson. Observed values of radiation 

 reached 1.53 calories according to silver-disk pyrheliometer S.I.5bis 

 notwithstanding that the earth was then in aphelion, and besides, the 

 temperature being high, the atmospheric humidity was considerable. 

 At that time we used the water-flow pyrheliometer No. 5 with exactly 

 the same sky exposure as that described for No. 3 in Volume 3 of 

 the Annals, which had an angular aperture as viewed from the limit- 

 ing diaphragm of about 16°. The silver-disk pyrheliometer S.I.5bis, 

 on the other hand, was of our modern improved type, exposing only 

 5° 48' angular diameter as viewed from the silver disk, or 0.0013 

 hemispheres. Fearing that the June measurements of the two instru- 

 ments would not be quite comparable, despite the exceptional clearness 

 of the sky, we subsequently erected a double highly reflecting screen 

 in front of pyrheliometer No. 5 designed so as to give it exactly the 

 same sky exposure as S.I.5bis- We repeated the comparison on July 8 

 with these arrangements but obtained almost exactly the same results 

 as before. 



The following table includes all the comparative observations of 

 pyrheliometer S.I.5bis and water-flow pyrheliometer No. 5, except a 



