1876.] The Radiometer and its Cosmical Revelations . 517 
times of truths laboriously established of old ; and this faCt 
is of itself enough to indicate the necessity of searching 
carefully in the scientific heritage of the past after all that 
it may be able to furnish us for the increase of our actual 
knowledge ; for a double discovery, necessarily requiring a 
double effort of human intellect, is an evident waste of that 
creative force which causes the advance of humanity in the 
glorious path of civilisation.” 
VI. THE PHILOSOPHY OF THE RADIOMETER 
AND ITS COSMICAL REVELATIONS. 
By W. Mattxeu Williams, F.R.A.S., F.C.S. 
yC? O much speculation, and not a little extravagant specu- 
vCS) lation, has been devoted to the dynamics of the 
radiometer, that I feel some compunction in adding 
another stone to the heap, my only apology and justification 
for so doing being that I propose to regard the subject from 
a very unsophisticated point of view, and with somewhat 
heretical directness of vision, — i.e., quite irrespective of 
atoms, molecules, or ether, or any other specific preconcep- 
tions concerning the essential kinetics of radiant forces 
beyond that of regarding such forces as affections or con- 
ditions of matter which are transmitted radially in constant 
quantity, and therefore obey the necessary law of radial dif- 
fusion, or inverse squares. 
The primary difficulty which appears to have generally 
been suggested by the movements of the radiometer, is the 
case which it seems to present of mechanical action without 
any visible basis of corresponding reaction ; a visible tangible 
objeCt pushed forward, without any visible pushing agent or 
resisting fulcrum against which the moving body reaCts. 
This difficulty has been met by the invocation of obedient 
and vivacious molecules of residual atmospheric matter, 
which have been called upon to bound and rebound between 
the vanes and the inner surfaces of the glass envelope of the 
instrument. 
How is it that the advocates of these activities have not 
sought to verify their speculations by modifying the shape 
and dimensions of the exhausted glass bulb or receiver ? 
If the motion of the radiometer is due to such excursions 
and collisions, the length of excursion and the angles of 
