384 



SCIENCE 



[N. S. Vol. XXXIII. No. 845 



propagated in straight lines in every con- 

 ceivable direction, i. e., the wave energy is 

 isotropic; and that this energy is distributed 

 uniformly throughout the universe except in 

 so far as the distribution is disturbed by the 

 presence of matter, I shall endeavor to explain 

 my conception of the mechanism of gravita- 

 tion. 



For illustration in terms of the known, let 

 us imagine a closed space having uniformly 

 luminous walls of such character that every 

 point on their surface radiates light in all 

 internal directions. The enclosed space may 

 be of any shape, but for the sake of simplicity 

 let it be spherical or cubical, and large, say 

 as large as a lecture room. The space will be 

 filled with isotropic radiant energy uniformly 

 distributed — any cubic centimeter of space 

 containing as much energy as any other. 



Next let us picture a smaU opaque body 

 suspended anywhere in our luminous space. 

 The body may be of any shape we may imag- 

 ine an atom or molecule to have; but, again 

 for simplicity, let it be spherical — say a small 

 grain of shot, and let it be located near the 

 center of the space. 



The small body will absorb the light which 

 falls upon it and will cast a spherical shadow, 

 the depth or intensity of which will vary in- 

 versely with the square of the distance from 

 the center of the body; and the shadow will 

 extend to the confines of the enclosure, how- 

 ever large the latter may be. We can not per- 

 ceive the shadow, but we know it is there. 

 It is true that the body wiU soon acquire the 

 temperature of its surroundings, and radiate 

 as much energy as it receives ; but for the pur- 

 pose of this illustration let us consider only 

 the high-frequency light energy. 



As is well known, the ether waves of light 

 will exert a slight pressure on the body. But 

 in the case supposed, the pressure wiU be 

 equal on all sides and no effort toward trans- 

 lation can result. 



Now let us introduce a second small body, 

 similar to the first, and some distance from it. 

 This, also, will cast a spherical shadow like 

 the first. The two shadows will intersect, and 

 each body will lie within the shadow of the 



other. In other words, each body wiU be 

 partially shielded by the other from the ether 

 waves coming from that direction. Hence the 

 light pressure wiU be less on that side of each 

 body which faces toward the other than on the 

 side which is turned away, and the bodies will 

 be urged toward each other by the excess of 

 light pressure on the sides turned away. This 

 excess of pressure will vary with the inverse 

 square of the distance between the centers of 

 the bodies so long as the ratio of distance to 

 diameters remains large. 



The ether waves concerned in gravitation 

 can not, however, be like the light-waves I 

 have just used for illustration, because light- 

 waves heat bodies on which they fall; and 

 their pressure is almost wholly superficial, it 

 does not reach molecules much below the sur- 

 face, and hence bears little relation to mass. 



But let us substitute for the short and 

 feeble waves of light, powerful waves, still of 

 the radiant kind, but of such great length 

 and slow frequency that, as before explained, 

 they do not excite the moleciilar vibrations 

 which we appreciate as heat, and hence are 

 not absorbed by matter; they pass freely 

 through all bodies, bathing the interior mole- 

 cules as effectually as those on the surface. 



Under these conditions each molecule or 

 atom or unit of a gravitating body will have 

 its own spherical shadow or field of influence, 

 and the gravitative force acting on the body 

 will vary directly with the sum of its units, 

 i. e., with its mass. 



The spherical shadow which I have pictured 

 as the field of influence of each atom or ma- 

 terial imit implies that the atom has caused, 

 principally in its immediate neighborhood, a 

 diminution of the ether's energy. Let us fur- 

 ther imagine this subtracted energy resident 

 in the atom as kinetic energy of translation 

 in many paths, almost infinitesimally short 

 and in every direction, but without collisions 

 because neighboring atoms follow very nearly 

 parallel paths. We may then picture the col- 

 lective atoms or molecules of matter buffeted 

 about in every direction by the ether waves 

 in which they are entangled, like a suspended 

 precipitate in turbulent water. 



