THE RADIOMETER. 
175 
procession ; and any member of the swift procession that comes 
into contact with the glass is converted into a member of the 
slow procession. The characteristic of each procession is re- 
tained over a considerable distance, although it is gradually 
effaced as the procession advances. If there is room between 
the disc and the glass for the entire gradient, that is, for the 
whole thickness of those layers which I have described, then 
that part of the slow procession which reaches the disc will not 
only have lost its sluggishness, but will have attained the full 
molecular speed of the beginning of the swift procession; and 
the swift procession, where it comes into contact with the 
envelope of glass, will have become as slow as the beginning of 
the slow procession. In such a case there will be no Crookes’s 
pressure. If there is not room for the full gradient, each pro- 
cession will retain a portion of what characterises it when it 
reaches its destination. It would require the whole gradient to 
produce such a defect of density as would entirely compensate 
for the greater average momentum imparted by each molecule 
that strikes the front, as compared with the momentum imparted 
by each molecule striking the back of the disc.” 
In a vertical disc Crookes’s pressure will be accompanied by 
the forces occasioned by a convection current. This will influ- 
ence the motion of the vanes in three ways — (1) It will bring a 
continuous supply of cold air to the front of the disc, which, 
being thrown off with augmented molecular motion, will cause 
a recoil of the disc in the same direction as Crookes’s pressure, 
(2) The convection current will also occasion a defect of tension 
in front of the disc, by that law of fluids in motion according 
to which the tension decreases 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 disc, and will thus give rise to a force opposed to 
Crookes’s pressure. (3) Lastly, the convection current, after 
being turned downwards by the roof of the little chamber, is 
likely to become a draught blowing upon the back of the disc 
with a force also opposed to Crookes’s pressure. The resultant 
of these three forces seems in all the experiments to have been 
opposed to Crookes’s pressure. At any particular degree of 
exhaustion it is only the difference between this resultant and 
Crookes’s pressure which becomes apparent. It is evident that 
there is a certain tension, depending in some degree upon the 
form of the instrument, in which the above excess will be at a 
maximum. 
In the “ Philosophical Magazine ” for May is a tft Theory of 
the Radiometer,” by Professor Challis. He commences by 
stating that rays of light incident on opaque substances are 
partly reflected without undergoing transmutation, and are partly 
