8 SMITHSONIAX MISCELLANEOUS COLLECTIONS VOL. 87 



Pyrheliometer S.I. 5 after these experiments was altered early in 

 August, 193 1, as explained on a preceding page and called S.I.5bis- 



Other comparisons support the view that pyrheliometer A. P.O. 

 8bis bis still gives fairly the standard scale of 1913. Thus the com- 

 parisons referred to on a preceding page made in Potsdam in October 

 1931 on exceptionally clear days indicate through S.I.Sbis and A. P.O. 

 8bis bis for S.I. 12 the constant 0.3624. The original calibration made 

 in September 1912 resulted 0.3631. Again a long series of compari- 

 sons between A.P.O.Sbis and S.I.i from 191 1 to 1920 indicate even 

 greater constancy than the set of comparisons with S.I.5 just quoted. 



On the whole, therefore, we are forced to conclude that the standard 

 scale of radiation as indicated by water-flow pyrheliometer No. 5 

 lies 2.5 per cent below the Smithsonian scale of 191 3. The great sim- 

 plicity and freedom from accidental error in the measurements of 

 standard No. 5 warrants very high weight for its results compared 

 to those obtained from pyrheliometers No. 3 and No. 4. In confirma- 

 tion it may be added that a long series of unpublished measurements 

 made by Messrs. Abbot and Aldrich with No. 3 at Mount Wilson in 

 the year 1920 also tended to give results below the scale of 1913. We 

 hope, however, to make additional experiments on this question next 

 summer. 



In our solar-constant values hereafter to be published, we regard 

 comparability with preceding ones as more important than absolute 

 scale. Hence we shall not introduce our new scale into such future 

 publications. We may remark, however, that if applied to our mean 

 value, 1.940 calories per square centimeter per minute, for the solar 

 constant of radiation, it would become as corrected 1.893. ^^^^ have 

 admitted, however, our belief that a contrary small correction of un- 

 determined magnitude should be applied to allow for extreme ultra- 

 violet rays not observed, some of which cannot enter the atmosphere 

 through the ozone layer. 



In his publication " Ein neues Pyrheliometer f iir Absolutmes- 

 sungen," C. Tingwaldt gives a preliminary result obtained by experi- 

 ments at Davos. This indicates a plus correction of 1.9 per cent to the 

 Angstrom scale, and according tO' Tingwaldt indicates a minus correc- 

 tion of 1.8 per cent to the Smithsonian scale of 1913. Our own correc- 

 tion is in that direction, but of greater magnitude. We hope that fur- 

 ther experiments on very clear days may be made by Tingwaldt and his 

 colleagues, in which silver-disk pyrheliometer S.I. 12 will be the com- 

 parison instrument. We shall then be in a position to evaluate very 

 closely the difference, if any, between our new water-flow pyrheliometer 

 No. 5 and the absolute pyrheliometer used by Tingw^aldt. 



