4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IIO 



in producing a constant water stream. Air bubbles were carried 

 along, and local fluctuations in temperature occurred owing to air 

 currents affecting the short rubber tubes which had to be introduced 

 to allow free movement. These irregularities, both of mechanical 

 and heat natures, caused accidental differences of successive measure- 

 ments so appreciable that great numbers of comparisons with silver- 

 disk pyrheliometers had to be made to obtain accurate results. What 

 with this source of error, and the effect of sky radiation from near 

 the sun, which was minimized by using the longer vestibules of the 

 silver-disk pyrheliometers after the year 1925, we found that the 

 earlier determinations of the constants of silver-disk pyrheliometers 

 were too high by 2.3 percent. This correction we published in the 

 year 1934.' Nevertheless, so as not to upset the world's system of 

 pyrheliometry, and the comparability over a long term of years of 

 Smithsonian solar-constant results contained in volumes 2 to 6 of 

 Annals of the Smithsonian Astrophysical Observatory, while we admit 

 that the 191 3 scale of pyrheliometry is 2.3 percent too high, we and 

 those who follow us still use the Smithsonian scale of 1913. 



The variability of the brightness of the sky may still slightly affect 

 silver-disk pyrheliometry. However, as stated at pages 53 to 55, 

 Annals, volume 6, we now eliminate variations of sky brightness as a 

 source of error in solar-constant measurements. 



- Abbot, C. G., and Aldrich, L. B., The standard scale of solar radiation. 

 Smithsonian Misc. Coll., vol. 92, No. 13, 1934. 



