1877.] 



Movements of Radiometers. 



547 



It will be remembered that, under ordinary circumstances, radiation to- 

 wards either radiometer produces positive rotation. 



1. If a glass tumbler be heated to the temperature of boiling water, 

 and inverted over the mica radiometer, there is little or no immediate 

 motion of the fly ; but quickly a negative rotation sets in, feeble at first, 

 but rapidly becoming lively, and presently dying away. 



2. If after the fly has come to rest the hot tumbler be removed, & posi- 

 tive rotation soon sets in, which becomes pretty lively, and then gradually 

 dies away as the apparatus cools. 



3. If the tumbler be heated to a somewhat higher temperature, on first 

 inverting it over the radiometer there is a slight positive rotation, com- 

 mencing with the promptitude usual in the case of a feeble luminous 

 radiation, but quickly succeeded by the negative rotation already de- 

 scribed. If the tumbler be heated still more highly, the initial positive 

 rotation is stronger and lasts longer, and the subsequent negative rota- 

 tion is tardy and feeble. 



4. If the pith radiometer be treated as in § 1, the result is the same, 

 with the remarkable difference that the rotation is positive instead of 

 negative ; it is also less lively. 



5. But if the tumbler be removed when the fly has come to rest, it 

 remains at rest, or nearly so. 



6. If the tumbler be more strongly heated, positive rotation begins as 

 promptly as with light. In this case the tumbler must not be left long 

 over the radiometer, for fear the vacuum should be spoiled by the evolu- 

 tion of gas from the pith. 



7. If the tumbler be heated by holding it over the spout of a kettle 

 from which steam is issuing, and held there till the condensation of water 

 has approximately ceased, and be then inverted over the pith radiometer, 

 the bulb is immediately bedewed, and a negative rotation is almost 

 immediately set up, though sometimes, just at the very first moment, there 

 is a trace of positive rotation. The negative rotation is lively, but not 

 lasting; and, after 15 seconds or so, is exchanged for a positive rotation, 

 which is not lively, but lasts longer. 



8. If the tumbler be lifted when the negative rotation has ceased, and 

 the dewed surface be strongly blown upon, a lively, but brief, positive 

 rotation is set up. 



9. To produce positive rotation by blowing, it is not essential that the 

 bulb be wet. If it be merely warm, and the circumstances are such that 

 the fly is at rest for the moment, or nearly so, blowing produces positive 

 rotation, though much less strongly than when the bulb is wet. 



10. If the tumbler be heated as in § 7, and inverted over the mica 

 radiometer, the rotation is positive, as when the tumbler is dry. 



11. If the tumbler or a cup be smoked inside (to facilitate radiation), 

 heated to a little beyond the temperature of boiling water, and inverted 

 over the pith radiometer, a positive rotation is produced ; and if, when 



