110 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



tance when the tension exceeded twenty millimeters. They 

 observed, moreover, sensible deviation from the law of in- 

 verse squares at most of the tensions. 



Rood has described some very ingenious experiments on 

 the radiometer, which show most conclusively that the theo- 

 ry which supposes the motion to be due to a reaction be- 

 tween the blackened surface of the vanes and the containing 

 envelope is the true one. A two-vane mill with blackened 

 surfaces of aluminum, and carrying a small magnet, was pre- 

 pared, and before one of these surfaces was placed a screen 

 of mica, also attached to the suspending wire. The whole 

 was placed in a flask, which was exhausted to 0.25 millime- 

 ter. Light falling upon the unprotected vane alone, caused 

 a deflection of 3.23 ; upon the protected, 0.10. When it fell 

 on both there was a deflection of 2.38 in favor of the unpro- 

 tected disk thus proving that when reaction is prevented 

 between the walls and the vanes no revolution takes place. 

 The author also devised an experiment for measuring this re- 

 pulsion. Experiments were also made showing that motion 

 under atmospheric pressure is due to currents. 



Volpicelli has also given the results of some radiometric 

 experiments. He finds, for example, that a freezing mixture 

 applied to the upper half of the globe causes a rotation with 

 the non-blackened face foremost, as when radiant heat is 

 used ; but when applied to the lower half, the rotation takes 

 place in the inverse direction, the blackened faces being in 

 advance. In the latter case, radiant heat brings the mill to 

 rest. The whole globe being plunged in a hot liquid or in 

 a freezing mixture, there is no motion. 



Crookes has given the name Otheoscope to a form of the 

 radiometer in which, regarding the blackened surfaces of the 

 vanes as the heater and the glass the cooler, and deducing 

 from theoretical considerations that the latter rather than 

 the former should be the moving body, a blackened fixed sur- 

 face is so arranged that the stream of molecules driven from 

 it shall impinge upon the transparent vanes and drive them 

 round. In this instrument, unlike the radiometer, the glass 

 envelope plays no part other than a preserver of the rare- 

 faction. At the Royal Society's May soiree six otheoscopes 

 of different forms and thirteen new radiometers were exhib- 

 ited. 



