KEPORT OF THE SECEETARY. 101 



Mount Wilson observations of 1915, including both the solar-con- 

 stant work and the tower work, have been almost all reduced. 



Mr. Fowle has continued at intervals between other work the re- 

 duction of his numerous observations of the transmission of rays of 

 great wave length through long columns of air of known humidity. 

 Many sources of error have required to be considered and eliminated, 

 and the reading and reduction of the curves of observation was ex- 

 tremely tedious. The results are at length reaching such a stage that 

 it can be seen that they fall into excellent agreement and will be of 

 high interest in connection with studies of the earth's temperature 

 as dependent on its radiation outward toward space. In fact, the 

 results of Mr. Fowle's work are expected to be ready for publication 

 within a short time. 



For some years we have endeavored to design and construct an in- 

 strument capable of measuring accurately the intensity of sky light 

 by day and of radiation outward toward the whole sky by night. 

 At last success seems to be reached in an instrument devised by 

 Messrs. Abbot and Aldrich and constructed by IMr. Kramer. The 

 instrument is called the p3a'anometer, from the Greek words TrOp, 

 fire, ava, up, (xtTpov, a measure; thus designating an instrument 

 adapted to measure heat coming from or going to space above. 

 The pyranometer is somewhat after the principle of the Angstrom 

 pyrheliometer, in that the intensity of radiation is measured by 

 electrical compensating currents, whose strength is adjusted with 

 reference to the indications of a delicate thermocouple. A full ac- 

 count of the instrument has been published by the Smithsonian 

 Institution,^ including the tests which have been made to determine 

 its accuracy by comparisons in solar measurements with the pyrheli- 

 ometer. Complete accord between the two instruments is found at all 

 altitudes of the sun when due regard is paid to the fact that the 

 pyranometer presents a horizontal surface. The pyranometer seems 

 to be suitable for botanical investigations, for it is capable of measur- 

 ing the radiation even in deep shade, as in forests and greenhouses, 

 as well as in full sun. In short, it can measure radiation in all situa- 

 tions where plants are accustomed to grow, except under water. 



The consideration of the pyranometer has led us to undertake 

 the determination of the constant ordinarily called " sigma " of 

 Stefan's formula of radiation, according to which the emission of 

 a perfect radiator per square centimeter per second is equal to the 

 fourth power of the absolute temperature multiplied by " sigma." 

 In recent years a good deal of disagreement has arisen as to the 

 value of " sigma." We require to use it for certain tests of the py- 



^ The pyranometer — an instrument for measuring sky radiation : Smithsonian Misc. Coll., 

 Vol. CO, No. 7, May, 1916. 



73839°— SM 1916 S 



