MAEca 6, 1914] 



SCIENCE 



339 



mass zero he preferred to find its tangents 

 and thus derive the subsidiary curve of 

 tangents from which he derived a formula 

 for extrapolating his observations. In this 

 way he obtained results corresponding to 

 the value 2.85 calories per square centi- 

 meter per minute for the solar constant. 

 Thus Forbes cut loose entirely from 

 Bouguer's exponential formula of atmos- 

 pheric transmission. 



Pouillet observed in the years 1837 and 

 1838 at Paris. His work was published be- 

 fore that of Forbes, although made later. 

 He found transmission coefficients by 

 means of Bouguer's formula. He appar- 

 ently did not investigate the defects of this 

 formula as thoroughly as Forbes did. His 

 result for the solar constant of radiation is 

 1.7633 calories per square centimeter per 

 minute. This value, on account of the non- 

 homogeneity of the solar rays, is necessarily 

 too low. 



Quetelet observed with a Robinson actin- 

 ometer similar in form to Herschel's, at 

 Brussels from the year 1843 to 1853. 

 These experiments might well repay a 

 critical examination now, not for their 

 value in determining the absolute measure 

 of the solar constant of radiation, but iu 

 connection with the variation of the aver- 

 age intensity of the solar radiation from 

 year to year as influenced by volcanic erup- 

 tions. 



Desains employed a thermopile, and 

 compared the transmissibility of the rays 

 of the sun through a water cell at different 

 stations. He found the transmissibility of 

 solar rays through the water cell always 

 increased by a long preliminary course 

 through moist air. This result is essentially 

 the same as that of Forbes, although ob- 

 tained in a different manner. 



VioUe observed at many different sta- 

 tions, including Mont Blanc. His instru- 

 ment apparently read much too high, as 



noticed by Langley in the report of the 

 Mount Whitney expedition. He used a 

 somewhat complicated empirical formula of 

 extrapolation, as he was fully cognizant of 

 the defect of Bouguer's formula, as indi- 

 cated by Forbes. He obtained the follow- 

 ing values : 



These values should be reduced about 

 one fourth to make them comparable with 

 observations made in recent years at high 

 elevations by many observers. In such a 

 case the value outside the atmosphere would 

 become about 1.9 calories per sq. cm. per 

 minute. 



Crova made many observations at Mont- 

 Pellier with his alcohol aetinometer stand- 

 ardized against the Tyndall pyrheliometer. 

 He made some attempts to extrapolate his 

 observations to the limit of the atmosphere, 

 but these, like other solar constant values 

 obtained by pyrheliometry alone, are not 

 definitive. Great value, however, attaches 

 to the long series of direct observations con- 

 tinued from the year 1883 to 1900 at Mont- 

 Pellier. These show plainly the influence 

 of the volcano Krakatau and others. 



K. Angstrom observed with the electrical 

 compensation pyrheliometer at several sta- 

 tions at different altitudes on the island of 

 Teneriffe in the years 1895 and 1896. 

 Some of his measurements were made at 

 the altitude of 3,700 meters, and give 

 direct readings of solar radiation as high 

 as 1.63 calories per square centimeter per 

 minute. Ang'strom declined to determine 

 from these a value of the solar constant of 

 radiation, recognizing that this demanded 

 observations of the solar spectrum as well 

 as pyrheliometric work. In later years he 



