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LXI. Theoretical Explanations of the Actions of the Radiometer, 

 the Otheoscope, and the Telephone. By Professor Challis, 

 M.A., F.R.S., F.R.A.S.* 



I HAVE included the proposed explanations of the actions 

 of the three instruments in the same communication, 

 because in each case the theoretical reasoning rests on those 

 hypotheses respecting the qualities of the aether and of atoms 

 which I have so often defined in this Journal, and, again, 

 because the explanations viewed in relation to each other serve 

 to confirm the truth of the hypotheses. The constituent atoms 

 of any substance are conceived to be inert, spherical, and of 

 constant form and magnitude ; and the aether is considered to 

 be capable of producing cognizable phenomena only so far as 

 it may be mathematically treated as a simple continuous fluid 

 of unlimited dimensions, susceptible of variation of density, 

 and pressing always and everywhere in exact proportion to its 

 density. The supposed variation of density implies that the 

 aether is atomically constituted; but in the physical theories I 

 have proposed this constitution does not specifically come 

 under consideration. It may be strictly proved on hydrody- 

 namical principles that the motions of such a fluid cannot per- 

 manently alter the quantity of fluid matter on the opposite 

 sides of any fixed plane of unlimited extent, and consequently 

 that the motions are either vibratory, or steady circulating 

 currents. These conditions being understood, the determina- 

 tion of the dynamical effect of the aether in motion on visible 

 and tangible substances depends eventually on the solutions of 

 two very definite hydrodynamical problems — namely, to find 

 the motion of an atom (1) when it is acted upon by a steady 

 current of the aether, (2) when acted upon by a series of har- 

 monic aethereal vibrations. 



The first problem is easy of solution ; and the general result, 

 when powers of the velocity of the fluid above the second are 

 omitted, is, that the acceleration of the atom has a constant 

 ratio to that of the contiguous surrounding fluid (see ' Prin- 

 ciples of Physics,' solution of example viii. p. 313, and the 

 Hydrodynamical Theory of Attractive and Repulsive Forces, 

 in the Philosophical Magazine for September 1872, art. 33). 

 The solution of the second problem, when only the first power 

 of the velocity of the vibrating fluid is taken into account, con- 

 ducts, as might be anticipated, to motion of vibration of the atom, 

 without permanent motion of translation. But when the square 

 of the velocity is included, the pressure by which the motion of 

 the atom is produced contains in its expression a term which has 



* Communicated by the Author. 



