THE PHILOSOPHY OF THE RADIOMETER. Ill 



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 colli- 

 sions, the length of excursion and the angles of collision must 

 modify its motions ; and such modification under given con- 

 ditions would form a fine subject for the exercise of the inge- 

 nuity of molecular mathematicians. If their hypothetical data 

 are sound, they should be able to predict the relative velocities 

 or torsion-force of a series of radiometers of similar construc- 

 tion in all other respects, but with variable shapes and diam- 

 eters of inclosing vessels. 



If we divest our minds of all visions of hypothetical atoms, 

 molecules, ethers, etc., and simply look at the facts of radia- 

 tion with the same humility of intellect as we usually regard 

 gravitation, this primary difficulty of the radiometer at once 

 vanishes. The force of gravitation is a radiant force acting 

 somehow between, or upon, or by distant bodies ; and these 

 bodies, however far apart, act and react upon each other with 

 mutual forces, precisely equal and exactly contrary. We con- 

 ceive the sun pulling the earth in a certain direction, and 

 receiving from the earth an equal pull in a precisely contrary 

 direction, and we have hitherto demanded no ethereal or 

 molecular link for the transmission of these mutually attractive 

 forces. Why, then, should we not regard radiant repulsive 

 energy in the same simple manner ? 



If we do this there is no difficulty in finding the ultimate 

 reaction fulcrum of the radiometer vanes. It is simply the 

 radiating body, the match, the candle, the lamp, the sun, or 

 whatever else may be the source of the impelling radiations. 

 According to this view, the radiant source must be repelled 

 with precisely the same energy as the arms or pendulum of the 



* Since this was written some such modifications have been made 

 with equivocal results. 



