256 THE POPULAR SCIENCE MONTHLY 



attempted a wave theory of gravitation from his observation that bodies 

 floating on water agitated by waves were drawn toward the center of dis- 

 turbance. The action of a body immersed in water was not considered. 

 Since his time various attempts have been made to explain gravitation 

 as due to a wave-motion. At the last meeting of the American Associa- 

 tion for the Advancement of Science, Mr. Brush, of Cleveland, Ohio, 

 presented a paper 2 in which he accounts for gravitation by ether-waves. 

 His theory was doubtless suggested by radiation pressure. His theory 

 demands an ether possessed of intrinsic energy. As before stated, the 

 views of many modern physicists permit this. He assumes that this 

 energy is due to waves; but the frequency is so much less than that of 

 heat or light that molecules will not respond ; and hence bodies do not 

 become warm. He thinks that the energy of the ether is a generalized 

 type whose degradation has been brought about by repeated absorptions 

 and remissions. To substantiate this view he cites the case of the 

 earth and the sun, in which rays of one length are absorbed by the earth 

 and longer ones are emitted. He accepts the view of J. J. Thomson 

 that all energy is kinetic energy of the ether. Before attempting to 

 explain the mechanism of gravitational attraction he paves the way by 

 referring to a well-known phenomenon in light. If we take an opaque 

 body in a room with luminous walls it will experience pressure on all 

 sides because we now know that light-waves have both energy and 

 momentum. If we now introduce a second body each will be in the 

 shadow of the other or will screen the other; and hence the radiation 

 pressure is less on the side of each body which faces the other; and 

 hence there will be a tendency for the bodies to be pushed together. 



Now substitute for the light-waves waves of great length and less 

 frequency, and owing to their low frequency they will affect the interior 

 molecules as well as the surface ones, and hence we will have a volume 

 or mass effect and not a surface effect as in light. This is in accord 

 with Newton's law, for the force is proportional to the mass. " "We may 

 picture . . . molecules of matter buffeted about in every direction by 

 ether-waves in which they are entangled, like a suspended precipitate in 

 turbulent water." Now introduce a second body and the pressure in 

 the direction of the line joining these two is less than in any other 

 direction, as each is in the ' shadow ' of the other; hence they are pushed 

 together. Notice that according to his theory gravity is not a tension, 

 but a pressure. Mr. Brush's theory, like all other theories regarding 

 gravitation, is beset with difficulties. If gravitation is due to a type of 

 radiation transmitted at finite speed it ought to be subject to aberration 

 as is light. To avoid this Brush takes longitudinal waves and assumes 

 the elasticity of ether is such that their velocity is much greater than that 

 of light waves which are transverse. Longitudinal waves in the ether 



2 See Science, March 10, 1911. 



