1878.] 



Repulsion resulting from Radiation. 



37 



negative ; the speed of rotation is greater as the vanes are further 

 removed from this neutral position on either side. The position 

 of this neutral point varies with the degree of exhaustion; thus 

 at 12 M, the screens must be 3 millims. from the convex side; at 

 •18 M they must be 13 millims. from the convex side. The higher 

 the exhaustion the greater the distance which must separate the convex 

 side of the hemi-cylinders and the screens. 



The author gives explanations of these phenomena, based on the 

 following already ascertained facts : — When thin aluminium vanes are 

 exposed to light the metal rises in temperature and becomes equally 

 warm throughout, and a layer of molecular pressure is generated on 

 its surface. The thickness of this layer of pressure, or the length of 

 the lines of force of repulsion, varies with the degree of exhaustion, 

 being longer as the exhaustion increases. The lines of force appear to 

 radiate from the metal in a direction normal to its surface. The force 

 of repulsion is also greater the closer the repelled body is to the 

 generating or driving surface, and the force diminishes rapidly as 

 the distance increases, according to a law which does not appear to 

 be that of "inverse squares." Diagrams are given illustrating the 

 author's explanation, based on the above data. 



An apparatus is next described not differing in principle from the 

 last, but having, in addition to the aluminium hemi-cylinder and 

 movable mica screen, a small rotating fly made of clear mica, mounted 

 in such a way that it could be fixed by means of an exterior magnet 

 in any desired position inside the bulb. The screen was also capable 

 of adjustment by means of another magnet ; the aluminium hemi- 

 cylinder in this apparatus being fixed immovable. The adjustible 

 indicator being very small in diameter in comparison to the other 

 parts of the apparatus, and, being easily placed in any part of the 

 bulb, was expected to afford information as to the intensity and 

 direction of the lines of pressure when a candle was brought near the 

 bulb. Experiments have been tried, a, with the screen in different 

 positions in respect to the hemi-cylinder ; b, with the indicator in 

 different parts of the bulb ; c, with the candle at different distances 

 from the hemi-cylinder on one side or the other ; d, with the degree of 

 exhaustion varying between wide limits. It would be impossible to 

 give an intelligible abstract of the results obtained with this apparatus 

 without numerous diagrams. It may, however, be briefly stated that 

 they entirely corroborate the theories formed from a study of the 

 behaviour of the instruments previously described. 



The next part of the paper treats of the action of heat employed 

 inside the radiometer. In a previous paper, the author showed that 

 phenomena feeble and contradictory when caused by radiation external 

 to the bulb, became vigorous and uniform when the radiation w r as 

 applied internally by the agency of an electrically-heated wire. It 



