I30 



NATURE 



[March 23, 191 1 



there was a definite arousal of visual sensation by the 

 alternatin{j field ; the effect appeared more like an alternate 

 intensification and inhibition of whatever sensor>' process 

 was already in progress. That is to say, the idio-retinal 

 light which is present before the current is turned on is 

 increased and decreased alternately. 



.4 K!S'EriC THEORY OF GRAVITATION.' 



"pVER since Sir Isaac Newton enunciated the law of 

 ■^' universal gravitation, more than two hundred years 

 ago, philosophers have speculated on the nature of that 

 mysterious agency which links every atom of matter in 

 the universe with every other atom. Newton found him- 

 self unable to offer any adequate e.xplanation. 



Since Newton's time several theories of gravitation have 

 been proposed ; but all, of which I am aware, are open to 

 strong objections, and are not considered even promising 

 by physicists. 



Study of the nature of gravitation is beset with unusual 

 diHiculties, because gravitation is ever with us and about 

 us ; it is the one universal phenomenon, and we cannot 

 escape from its influence — cannot obtain any outside point 

 of view. 



Gravitation is often described as a feeble force ; and so 

 it is, from one point of view. It is difficult to measure, 

 or even to detect, attraction between two small bodies. 

 But when the bodies are of planetary size the aggregate 

 attraction of their molecules is enormous. It is easy to 

 calculate that the attraction between the earth and the 

 moon, which is just sufficient to retain the latter in its 

 orbit, would, if replaced by a steel cable, require that the 

 cable be about five hundred miles in diameter in order to 

 withstand the strain. Between the earth and sun, the 

 cable would have to be nearly as large in diameter as the 

 earth ; and attraction between the components of some 

 double stars is millions of times greater than between the 

 earth and sun (Lodge). So tremendous a phenomenon as 

 gravitation, a phenomenon compared with which all others 

 seem trivial, must have a mighty origin. 



That gravitation is a phenomenon of the all-pervading 

 aether is beyond reasonable doubt. This is so generally 

 conceded that it need not be argued. But how does the 

 gravitative influence originate? How is it transmitted and 

 maintained? What is the mechanism of gravitation? It 

 is the purpose of this paper to attempt an answer to these 

 questions. 



Let us consider what happens to a falling body. We 

 know that it gathers kinetic energy from some source, as 

 evidenced by its acceleration ; that this energy may do 

 external work or develop heat ; that the amount of energy 

 gathered is measured directly by the distance fallen 

 through (within the limits of uniform gravitation), irrespec- 

 tive of the time or rate of falling. When the distance 

 fallen through is of inter-planetary magnitude, and the 

 attracting body large, the gathered energy is enormous, 

 sufficient, if converted into heat, to vapourise the most 

 refractory falling body. 



We are here confronted with the question. Whence 

 comes the energy acquired by a falling bodv? Certainlv 

 it was not inherent in the body before the fall.' as evidenced 

 by the fact that during unimpeded fall none of the phvsical 

 or chemical attributes of the bodv, aside from the acquired 

 motion, changes in the slightest degree. 



We have been taught that before the fall the bodv was 

 endowed with " potential energv of position," which is 

 converted into kinetic energy during the fall. I think 

 "energy of position " is an unfortunate term, because it 

 is so very inadequate. To me it explains nothing. The 

 case is not like that of a flexed spring, where there is 

 internal molecular strain or displacement. 



Let us imagine a pound-weight of iron, for instance, 

 raised from the surface of the earth to a point near the 

 moon in a line joining the centres of the two bodies, the 

 point so chosen that the opposing attraction of the earth 

 and the moon shall exactly balance each other, leaving 

 orbital motion out of consideration. 



On the surface of the earth the pound-weight had some 

 so-called " potential .energy of position," because it was 

 capable of falling into a pit; but in its new position near 



_ Par-er read before the American Associ.-ition for the Advancement of 

 Science, December, igio, by Mr. Charle?: F. Brush. 



NO. 2160, VOL. 86] 



the moon, this potential energ>' not only has not bt. ., 

 augmented, but has disappeared entirely ; the pound* 

 weight, left free to move, remains stationary ; and yet we 

 mukt have expended more than twenty million foot-pounds 

 of energy in overcoming the attraction of the earth and 

 lifting the weight to its new position. This amount o( 

 energy would oe sufficient to impart to the weight a 

 velocity more than ten times greater than that ••' •'"•- 

 swiftest cannon-ball, or, if converted into heat, v 

 many times more than sulTicient to raise the iron ^ 

 to dazzling incandescence and then vapourise it. Now, in 

 lifting the weight, this large amount of energ}' has dj*> 

 appeared utterly. We cannot believe that the whole or 

 any part of it has been annihilated ; it must, in some form» 

 be resident somewhere. 1 think no one will contend that 

 this energy is resident, in any form, in the cold, motionless 

 pound-weight. I believe it was absorbed by, and is now 

 resident in, the aether through which the weight was 

 raised. Conversely, if this be true, a falling body must 

 acquire its energy from the a;ther through which it falls. 

 This is a fundamental idea to which 1 invite attention. 

 Faraday glimpsed it long ago, and others have appreciated 

 it more clearly since his time. But, so far as I am aware,^ 

 no one has realised its significance. 



This view of gravitation implies that the aether it 

 endowed with very great intrinsic energy in some form. 

 Many men of science now hold that the aether is 

 endowed, and that the amount of this intrinsic energs 

 enormous. Sir Oliver Lodge (" The Ether of Space } 

 appears to regard this energy as potential in form, and 

 estimates the intrinsic energy of a single cubic millimf^tr.- 

 of the aether to be almost inconceivably vast. He ^.. 

 " .All potential energy exists in the ether." Sir J. 

 Thomson says, " All kinetic energy is kinetic energy 

 the ether." 



I conceive the ethereal energy involved in gravitaiiun 

 to be kinetic rather than potential, the latter involving. 

 strain or stress. Newton, and later Maxwell, assumed 

 that bodies produce a stress in the aether about them of 

 such nature as to account for gravitation, but they wer-, 

 unable to imagine any physical cause for the stress. 



All the past theories of gravitation of which I am aw;^. 

 except the corpuscular theory of La Sage, appear to regard 

 gravitating matter as the seat of the gravitative influence, 

 the surrounding aether, by induced stress or otherwise, 

 acting simply as the medium of transmission. I cannot 

 see that any of these theories accounts for the enercv 

 acquired by a falling body. 



My own view of gravitation differs from these radiciil 

 I believe that kinetic energy of the aether is the funda- 

 mental cause of gravitation, and that a gravitating body 

 plays a secondary role only in disturbing the normr"- 

 uniform distribution of the aether's energ}', in a mann^ 

 shall endeavour to explain later. 



Let us assume, then, that the aether is endowed with 

 very great kinetic energy normally uniform in distribution. 



Kinetic energy implies motion of something possessed 

 inertia. Now, inertia is a fundamental attribute of 

 aether. Sir J. J. Thomson holds that all inertia is in< : 

 of the aether. The .-ether is highly elastic also, wh; 

 with its inertia, enables it to possess kinetic energy 

 wave form, as exemplified in radiation. By the t 

 wave, I mean progressive motion locally periodic; doub:! 

 the aether as a whole is stationary. Hence we may i 

 sider the kinetic energy of the aether as consisting 

 ather waves of some kind. 



These waves, vast in aggregate energ}-, eternal in r 

 sistence, without finite source or destination, are imag: 

 as being propagated in straight lines in ever}- conceiv; 

 direction. This isotropic distribution of kinetic ener. 

 essential to my theory of gravitation, was, for me. 

 difficult conception until I reflected that isotropic radi 

 energy is approximately realised in the interior of any 

 furnace with uniformly heated walls. 



Any kind of waves capable of exerting motive action on 

 the atoms or molecules of matter will fulfil the require- 

 ments ; but I shall first consider the transverse, electro- 

 magnetic waves of radiation, because these are the kind of 

 jether waves we are familiar with. 



Of course, intrinsic .T?ther waves, if of the radiation 

 kind, cannot be of any frequency at present known to us 

 as radiation, because then all bodies would become heat-^H. 



