Mr. G. J. Stoney on Crookes's Radiometer. 311 



also pressure on the front of the disk in excess of the pressure 

 on its back. These excesses of pressure are what I have 

 called Crookes's pressure. From the mechanical principle of 

 momentum it follows that their resultants must be in a right 

 line (which is, of course, perpendicular to the disk), equal in 

 amount, and opposite in direction. 



18. It has been urged as an objection to this explanation 

 that the arms of a radiometer often fly round so fast that 

 their revolutions cannot be counted, and that they appear to 

 the eye like a haze. It has been supposed that the vanes, in 

 sweeping round so rapidly, would mix together all strata of 

 air within the instrument, so as to prevent the development of 

 the processions which I have described. But this difficulty 

 will disappear when we appreciate the exceeding activity of 

 molecular motions, and the exceeding shortness of that first 

 stage of the phenomenon which I have called the period of 

 adjustment. It probably occupies, as I have already observed, 

 something like the 25V0 °f a secon( i °f time. After the lapse 

 of some such mere moment of time, the machinery is in full 

 operation, so that the vanes might probably whirl round even 

 hundreds of times in a second without stopping its action. 



19. Again, it has been supposed that the rarefaction in the 

 vacuum-chamber is earried very much further than I have 

 admitted in my former paper. Considerable latitude in re- 

 gard to this is allowable ; and it will therefore be desirable to 

 examine what are the probable limits of tension within which 

 Crookes's pressure is likely to be appreciable. 



20. It must be remembered that when the disk is vertical, 

 as in a radiometer, Crookes's pressure will be accompanied by 

 the forces occasioned by a convection-current. The convec- 

 tion-current will influence the motion of the vane in three 

 ways. It will bring a continuous supply of cold air to the 

 front of the disk, which, being thrown off with augmented 

 molecular motions, will cause a recoil of the disk in the same 

 direction as Crookes's pressure. The convection current will 

 also occasion a defect of tension in front of the disk, by that 

 law of fluids in motion according to which the tension de- 

 creases along a stream wherever the velocity increases. This 

 will occasion a minimum of tension where the velocity of the 

 convection-current is greatest, i. e. in front of the disk, and 

 will thus give rise to a force opposed to Crookes's pressure. 

 And again, the convection-current, after being turned down- 

 wards by the roof of the little chamber, is likely to become a 

 draught blowing upon the back of the disk with a force which 

 is also opposed to Crookes's pressure. The resultant of these 



