120 



THE PRESSUKE DUE TO RADIATION. 



Farther down oiw/ /> a .small .silvered plane mirror w/j was made fa.st 

 at right an«-les to the plane of C and D. This mirror was polished 

 bright on the silver side, so that the scale at 85 (tig-. 2) could be read 

 in either face, A small brass weight /y/., (tig-. 1), of 452 mg. ma.ss and 

 of known dimensions, was attac-hed at the lower end of a 1>. The cover- 

 glasses which served as vanes were silvered and brilliantly polished 

 on the silvered sides, and so hung on the small hooks that both silver 

 faces or both glass faces were presented to the light. A (juartz liber 

 f^^ 3 cm. long, was made fa.st to the upper end of a 7-», and to the lower 

 end of a tin(^ glas.s rod d^, which carried a horizontal luagnet iiu^. The 



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rod (l^ was in turn su.spendcd by a short liber to a steel pin t, which 

 could 1)0 raised or lowered in the bearing //. The whole was carried 

 b\' a bent glass tube /, firmly fastened to a solid brass foot F, resting 

 on a plane ground-glass plate P, cemented to a brass platform mounted 

 on three leveling screws not shown. A bell jar B, 25 cm. high and 

 11 cm. in diameter, covered the balance. The flange of the bell jar 

 was ground to tit the plate P. A ground-in hollow glass stopper 

 fitted the neck of the bell jar, which could thus be put in connection 

 with a .s3^stem of glass tubes leading to a Geissler mercury pump, a 

 MacLeod pressure gauge, and a vertical glass tube dipping into a 



