268 PROCEEDINGS OP THE AMERICAN ACADEMY 



Ihvesttoations on Light and Heat, made and published wholly or in part with 

 Appropbiation from the Rumford Fund. 



XV. 



ATMOSPHERIC REFRACTION. 



By Edward C. Pickering. 



Communicated May 26, 1885. .' 



PART I. 



During the summer of 1876 several thousand observations of the 

 altitudes of the "White Mountains were made by the writer.* The 

 method of zenith distances was employed, the instrument used being 

 the micrometer level. Great difference of opinion exists among ge- 

 odetists regarding the value of the zenith distance of a distant object 

 as a means of determining its height, on account of the uncertainty 

 in the atmospheric refraction. A large number of measures of this 

 quantity were accordingly made in subsequent years, to show to what 

 extent the observations of 1876 were likely to be affected by this 

 error. Four micrometer levels have been employed, which will be 

 designated as A, B, C, and D. 



The first of these, A, was shown to this Academy on January 11, 

 1876,t and was exhibited at the International Exposition of 1876 in 

 Philadelphia. It consisted of a telescope conveniently mounted, carry- 

 ing a level, and having in the eyepiece a spider-line micrometer. 



Instrument B, which was that used in 1876, was made out of an 

 architect's level. It consisted of a telescope having an aperture of 

 3 cm. and a focal length of 25 cm., with an erecting eyepiece magnify- 

 ing eighteen diameters. A delicate spirit-level was attached directly 

 to the telescope, which rested in two wyes, 19 cm. apart. One end 

 of the bar carrying these wyes rested upon a bearing, and the other 

 upon the pointed end of a micrometer screw. The bearing and the 

 nut of the screw were carried upon a bar which rotated around a ver- 

 tical axis. A horizontal circle divided into degrees was added, for 



* Appalachia, I. 138. t Proc. Amer. Acad., XI. 256. 



