1878.] 



Repulsion resulting from Radiation. 



39 



described in detail. Many different forms of instrument for effecting 

 this rotation are described, and their mode of action explained. 



The reacting inner surface of the envelope being thus proved to be 

 essential to the rotation of the fly, other instruments were made in 

 which this necessary reaction is obtained in a more direct manner. 

 In one, the radiometer is furnished with a fly carrying four flat 

 aluminium vanes, polished on both sides. Three vertical partitions 

 of thin clear mica are fixed in the bulb, with their planes not passing 

 through the axis of rotation, but inclined to it, thus throwing the 

 obliquity off the fly on to the case, and giving three fixed planes for 

 the reaction to take place against. Candles arranged symmetrically 

 round the bulb make the fly rotate rapidly against the edges of the 

 inclined planes. Breathing gently on the bulb gives negative rotation. 

 A hot glass shade inverted over the instrument causes strong negative 

 rotation, changing to positive on cooling. When the fly is furnished 

 with clear mica or with silver flake mica vanes, the same results are 

 obtained as when aluminium vanes are employed. The principal 

 action is produced by dark heat warming the bulb, screens, and vanes. 



The otheoscope is the next subject treated on in the paper. This has 

 already been given in abstract,* and need not be again referred to. 

 Many different varieties of otheoscope are figured and described. 



Fig. 1. 



* " Proc. Boy. Soc," No. 180, April 26, 1877. 



