REPORT OP THE SECEETARY 63 



discusses the sources of error inherent in the use of the bolometer, the 

 pyranometer, and the pyrheliometer, and their application to solar- 

 constant determinations. It explains the methods now in use at the 

 several stations for measuring solar radiation; gives long tables of 

 pj^rheliometry and results of bolometry, pyrheliometry, and pyra- 

 nometry combined in daily determinations of the solar constant. 

 Finally, it gives a discussion of the results of all stations for the in- 

 terval 1920-1930. The agreement of the stations, the best results on 

 the variability of the sun, and the apparent periodicities in solar 

 variation, and their reflection in weather changes are set forth. 



As delegate from the Smithsonian Institution, Doctor Abbot at- 

 tended the Geographical Congress at Paris and the one hundredth 

 anniversary meeting of the British Association for the Advancement 

 of Science, the Faraday celebration, and the Maxwell celebration in 

 London and Cambridge. He delivered a paper before the British 

 Association entitled " Twenty-five Years' Study of Solar Variation." 



At the conclusion of the meetings he went to Berlin, and with 

 Doctor Martens at Potsdam he made an accurate comparison between 

 silver-disk pyrheliometers S. I. 5bis, carried abroad with him, and 

 S. I. 12, the property since 1912 of the meteorological observatory at 

 Potsdam. Apparently no appreciable change of scale in the read- 

 ings of pyrheliometer S. I. 12 has occurred in the intervening 19 

 years. This result, confirming the stability of scale of the Smith- 

 sonian silver-disk pyrheliometers, is very gratifying. 



Owing to long-continued illness, Mr. Fowle's work was practically 

 confined to the preparation for publication of a new edition of the 

 Smithsonian Physical Tables. 



Messrs. Aldrich and Kramer spent much time on instruments for 

 pyrheliometry. After long-continued efforts to perfect a new form 

 of secondary pyrheliometer of much promise, the instrument was 

 laid aside for a time. Following the suggestion of the Russian 

 physicist, V. M. Shulgin, a new 3-chamber water-flow pyrheliometer 

 was put in construction. The instrument comprises two pyrheliom- 

 eters, each nearly like that described in the Annals of the Astro- 

 physical Observatory, Volume III, page 52. A common current of 

 distilled water, carefully guarded against temperature changes, 

 divides into two nearly equal branches to operate the two instru- 

 ments. Solar heating in the one is compensated by electrical heat- 

 ing in the other, interchanging the two instruments at intervals of 

 two minutes. The measurement consists only in adjusting and ob- 

 serving the required electric current to exactly compensate the solar 

 heating, so that the two water currents issue at exactly equal tem- 

 peratures. Equality of their temperatures is indicated by eight 

 thermoelectric elements connected in series with their junctions alter- 

 nately immersed in the two issuing water currents. 



