576 PROCEEDINGS OP THE AMERICAN ACADEMY. 



gives the mean value of the moment of radiation pressure computed 

 from the data in Table II. Decrease of the deflection with time indi- 

 cates gas repulsion on the warmed silver faces and increase in deflection, 

 gas suction. It will be seen from the curves that beginning at a gas 

 pressure of 66 mm. of mercury, the gas action was repulsion changing to 

 suction in passing from 19.8 to 11.2 mm. In the last two cases the total 

 gas action is small. For lower pressures the suction increases to 

 0.05 mm. At a gas pressure of 0.02 mm. the gas action is again a 

 strong repulsion. 



The curves indicate the existence of two gas pressures, at which the 

 gas action in our arrangement of apparatus should be zero, one between 

 19.8 and 11.2 mm. and the other between 0.05 and 0.02 mm.* The 

 former region was chosen for the ballistic measurements and nearly 

 all of the observations were made at a gas pressure of approximately 

 16 mm. Even for the two pressures where the decrease in the static 

 deflection was most rapid, i. e. at gas pressures of 66 and 0.02 mm., the 

 first throw was always in the direction of radiation pressure. The gas 

 action is strongly influenced by very slight changes in the inclination 

 of the plane of the vanes to the vertical and also by any object intro- 

 duced under the bell-jar anywhere near the vanes. For instance, a very 

 considerable effect was observed when a small vessel of phosphoric 

 anhydride was placed under the jar behind the vanes, though the nearest 

 wall of the vessel was separated from the vanes by a distance of at 

 least 3 cms. 



During the observations, the polished silver coatings on the vanes 

 deteriorated rapidly ; new coatings rarely lasted for more than two even- 

 ings' work. As the balance had to be removed and the mirrors taken 

 from the hooks, silvered, polished, and replaced a great number of times 

 during the entire series of measurements, although great care was taken 

 in setting the plane of the vanes vertical, it is not likely that precisely the 

 same conditions for gas action were ever repeated. The principal value 

 of the static results was in indicating favorable gas pressures for work, 

 rather than affording quantitative estimates of the gas action in short 

 exposures. The dotted parts of the curves are not based on results of 

 observation and might perhaps have been omitted without loss. 



* Crookes in his work with the radiometer discovered certain gas pressures 

 for which the combined gas and radiation forces neutralized, but as he did not 

 discriminate between forces due to radiation and gas forces his results were 

 apparently capricious and his reasoning somewhat confused. See Phil. Trans., 

 p. 519 (1875). 



