456 On the Radiometer, the Otheoscope, and the Telephone. 



speaker. These facts have received experimental explanations 

 on the hypothesis of the generation of electric currents ; but 

 how or why the means employed should produce such currents 

 has not been accounted for. For the following reasons I con- 

 sider that the hydrodynamieal theory of physical force furnishes 

 the required explanation. The component atoms of the plate 

 which the speaker's voice puts in vibration, move to and fro 

 by forces which result from the simultaneous action of atomic 

 repulsion and molecular attraction, varying in direction ac- 

 cording as the one or the other is in excess. Hence, as these 

 forces are due to harmonic vibrations of the aether, according 

 to the general Theorem I. sethereal currents are continually 

 being generated, and coexist with the vibrations of the plate. 

 In order that cognizable effects of these currents may be pro- 

 duced, it is necessary to provide means of circulation by con- 

 ducting wires, or by earth-connexion, inasmuch as all gethereal 

 currents, as already intimated, must satisfy this condition. 

 The usual arrangements in the application of the Telephone 

 do, in fact, fulfil this requirement. This theory of the action 

 of the telephone is especially confirmatory of the electrical 

 character of the sethereal streams to which in the previous 

 explanation of the action of the radiometer the rotation has 

 been ascribed. 



An article " On certain movements of the Radiometer,' ' 

 dated November 1, 1877, is contained in No. 184 of the Pro- 

 ceedings of the Royal Society, p.. 546. The author, Professor 

 Stokes, has said nothing about " theoretical explanations " of 

 the phenomena in the sense in which I use these terms in the 

 title to the present communication.; but by "coordinating" 

 the experiments he has arrived at conclusions of essential im- 

 portance as regards the part which the theorist has to perform. 

 He concludes that we must recognize, as distinct conditions 

 under which motion may be obtained in a radiometer, (1) dif- 

 ference of temperature of the two faces, (2) curved form of 

 the disk, (3) roughness of surface on one face ; but he does 

 not decide whether condition (3) is different from (2). From 

 a theoretical point of view, I should say that it is only required 

 to take account of the effect of difference of temperature when 

 there is no curvature, or the faces are plane, and to take ac- 

 count of the effect of curvature of the faces when there is no 

 difference of temperature. The theories I have proposed 

 account for the movements of the radiometer under the two 

 conditions considered separately ; and hence, on the allowable 

 supposition that these conditions operating conjointly produce 

 independent effects, it may be said that all phenomena of the 

 radiometer are thus accounted for on the principles of the 

 general hydrodynamieal theory of physical force. 



