^0 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1919. 



This new method will hereafter be employed by the Smithsonian 

 observers at Calama in combination with the old, not only for air 

 mass 2, but for air mass 3, and they will check one against the other 

 frequently for a considerable period of time until we are abundantly 

 satisfied of the accuracy of the new method of observation. Hith- 

 erto the new method has enabled us to save at Calama a number of 

 days of observation which, owing to the obvious changes in trans- 

 parency of the atmosphere, due to formation or disappearance of 

 clouds, would otherwise have been lost. 



So far as we have as yet been able to compare the results by the 

 old and the new methods, they are on the average closely identical. 

 For instance, on July 1 three values of the solar constant were com- 

 puted: (1) By the old process; (2) from observation at air mass 2; 

 (3) from observations at air mass 3. The results obtained were as 

 follows: 1.948", 1.940, 1.955, all agreeing within less than 1 per cent, 

 and the mean of the results by the new process agreeing identically 

 with the result by the old. 



The new process requires but two or three hours of work, where 

 the old required about 15, so that if it continues to appear as satis- 

 factory as now a very great gain in labor will result from it. Not 

 only is this so, but a still greater gain we think will come in accu- 

 racy, for we have now eliminated the fruitful source of error, de- 

 pending on the variability of the atmospheric transparency during 

 the observations. 



The new method of determining the solar constant of radiation is 

 not applicable to other stations than Calama without a new series 

 of contemporaneous solar constant determinations by the old method 

 and pyranometer observations at air mass 2 and air mass 3 to use 

 with them. We have not at present available the necessary pyra- 

 nometer observations at Mount Wilson, but we shall undertake to 

 obtain them at the earliest practicable moment, and hereafter it is 

 probable that the new method of determination will be employed 

 there as well as in South America. 



On the whole, the expedition to South America was unexpectedly 

 fruitful. First, satisfactory observations were made of the eclipse, 

 including both photographic observations of the eclipse phenomenon 

 and pyranometer observations of the brightness of the sky during 

 its progress. Second, a very interesting conference was held with 

 the chief and chief forecaster of the Argentine meteorological serv- 

 ice, in which they explained their investigations of the application 

 of solar radiation measurements to weather forecasts and indicated 

 their high sense of the value of solar radiation work for weather 

 forecasting. Third, investigations at Calama based upon the obser- 

 vations there indicated a new empirical method of determining the 

 solar constant of radiation, which appears to be equally as accurate 



