NICHOLS AND HULL. 



PRESSURE DUE TO RADIATION. 



509 



by means of which the glasses could be hung- on the hooks on the cross- 

 arm c. On opposite sides of the rotation axis at d two other drawn- 

 glass cross-arms were attached. The cover glassses slipped easily 

 between these, and were thus held securely in one plane. Further 

 down on ab, a small silvered plane mirror m l was made fast at right 



a 



D 



\m t 



X±im 3 



cms. 



S 



Figure 2. 



angles to the plane of C and D. This mirror was polished bright on the 

 silver side, so that the scale at S 5 (Fig. 3) could be read in either face. 

 A small brass weight m 3 (Fig. 2), of 452 mgs. mass and of known dimen- 

 sions, was attached at the lower end of ab. 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 quartz fiber / 2 , 3 cms. long, was 



