Radiometers 199 



principle. At each end of a horizontal tube of wrought iron, 

 a similar horizontal tube is fixed, the openings from the one 

 to the other being incomplete and only below their horizontal 

 mid-section. The transverse tubes communicate by a glass 

 tube of small diameter at a higher level than the wrought- 

 iron one. The first tube is filled with mercury, but the 

 second pair only to half their depth ; the remainder being 

 filled with coloured alcohol, except for a bubble of air in the 

 centre of the glass tube. The whole apparatus is mounted 

 on three set screws to maintain its level. When a heavy 

 object is brought near one end of the instrument the mercury 

 is attracted thither and the bubble moves away from the 

 attracting body. Dr. Siemens explained how the sensitive- 

 ness of the instrument might be increased, suggesting the 

 application of it to measuring and recording the attraction 

 of the sun and moon which gave rise to the tides. 



May iSth, 262nd meeting. Sir W. Grove described some 

 experiments which he had made with a modification of 

 Crookes' radiometer, 1 giving a summary of the results. 

 Though these were somewhat negative, they tended, in his 

 opinion, to show that all the effects were due to residual air. 



June i5th, 263rd meeting. Sir W. Grove described 

 experiments with a new electrometer, made for him by 

 Mr. Crookes, and with the old one after being re-exhausted. 

 As a result its action had become normal. The experiments 

 showed the effects of attenuation of air upon discharge and 

 induction not to be the same. When it has commenced and 

 is increasing, the discharge passes more and more rapidly 

 till it becomes a glow, but further attenuation stops it 

 entirely, while induction does not appear to be in any way 

 lessened by extreme attenuation. A radiometer also appears 

 to be a most delicate electroscope. By tilting it till the 

 vanes touch the glass, the interior of this may be electrified, 

 and it will then remain for days in that condition. Every 

 endeavour had been made to discharge or neutralize the 

 electricity on the glass surface, but it remained charged, 



1 The Minutes include a diagram and description of the instrument, 

 but without the one the other would hardly be intelligible. 



