394 COSVIC PHILOSOPST [pt. ii. 



added, in accordance with a pro raid principle of distribu- 

 tion, to the tangential momenta of the various planets 

 concerned in the theft. There can be little doubt, there- 

 fore, that all the planetary orbits, both primary and second- 

 ary, are steadily enlarging, and that this process must go on 

 until that synchrony between revolution and rotation now 

 witnessed in our moon becomes universal, unless it is pre- 

 viously checked by the cessation of tidal phenomena. As 

 between the earth and moon, for example, the ultimate 

 result of the whole process must be the lengthening of the 

 terrestrial day until it corresponds with a lunar month, so 

 that the earth and moon will move in relation to each other 

 just as if joined together by a rigid rod. This result will 

 actually be realized unless forestalled by the completed 

 refrigeration of the earth, which will put an end to the 

 tidal friction. In like manner the sun's rotation must 

 diminish until equilibrated with the motions of the planets, 

 unless this result is forestalled by the completed refrigera- 

 tion of the sun. And in all cases, so long as the process 

 goes on, there must be a tendency, however slight, for the 

 planets to recede from the sun. 



The action of this set of circumstances, however, though 

 hitherto no doubt predominant, is strictly limited in duration. 

 Sooner or later an equilibration of motions will be reached, 

 and this receding tendency will cease to be manifested. It 

 is quite otherwise with the opposing set of circumstances 

 which we have now to consider. We have now to contem- 

 plate a cause which operating from the very outset, and still 

 insidiously operating, will continue to operate long after the 

 process just described has come to an end. Each year's dis- 

 coveries show more and more conclusively that the inter- 

 planetary spaces are filled with matter. The existence of 

 some interplanetary and interstellar matter is indeed a 

 necessary condition for the transmission of light and othei 

 forms of radiance. Now wherever a body moves through 3 



