Gravitational Matter through Infinite Space. 167 



•Suppose that ether is given uniformly spread through space 

 to infinite distances in all directions. Any spherical portion 

 of it, if held with its surface absolutely fixed, would by the 

 mutual gravitation of its parts become heterogeneous ; and 

 this tendency could certainly not be counteracted by doing 

 away with the supposed rigidity of its boundary and by the 

 attraction of ether extending to infinity outside it. The 

 pressure at the centre of a spherical portion of homogeneous 

 gravitational matter is proportional to the square of the radius, 

 and therefore, by taking the globe large enough, may be 

 made as large as we please, whatever be the density. In fact, 

 if there were mutual gravitation between its parts, homo- 

 geneous ether extending through all space would be essentially 

 unstable, unless infinitely resistant against compressing or 

 dilating forces. If we admit that ether is to some degree 

 condensable and extensible, and believe that it extends through 

 all space, then we must conclude that there is no mutual 

 gravitation between its parts, and cannot believe that it is gra- 

 vitationally attracted by the sun or the earth or any ponderable 

 matter ; that is to say,, we must believe ether to be a substance 

 outside the law of universal gravitation.] 



§ 10. In the meantime, it is an interesting and definite 

 question to think of what the weight of a column of lumini- 

 ferous ether of-infinite height resting on the sun, would be, 

 supposing the sun cold and quiet, and supposing for the 

 moment ether to be gravitationally attracted by the sun as if 

 it were ponderable matter of density 5 X 10~ 18 . You all know 

 the theorem for mean gravity due to attraction inversely as 

 the square of the distance from a point. It shows that the 

 heaviness of a uniform vertical column AB, of mass w per 

 unit length, and having its length in a line through the centre 

 of force C, is 



raw m\r mw .? ri-n 



0A-<JB ;0r ('A lfCB = CO ' 



where m denotes the attraction on unit of mass at unit distance. 

 Hence writing for mw/GA, mwOA/G A 2 , we see that the attrac- 

 tion on an infinite column under the influence of a force 

 decreasing according to inverse square of distance is equal to 

 the attraction on a column equal in length to the distance of 

 its near end from the centre, and attracted by a uniform force 

 equal to that of gravity on the near end. The sun's radius is 

 6^7 x 10 8 cms., and gravity at his surface is 27 times* 



* This is founded on the following- values for the sun's mass and radius 

 and the earth's radius : — Sun's mass = 324000 earth's mass ; sun's radius 

 = C97C00 kilometres: earth's radius ^0-371 kilometres. 



