attempted a wave theory of gravitation from his observation that bodies floating on water agitated by waves were drawn toward the center of disturbance. 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 Association for the Advancement of Science, Mr. Brush, of Cleveland, Ohio, presented a paper 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
- See Science, March 10, 1911.