336 S. Ilaughton—G. H. Darwin's Theory of the 



light enough to form them.* If the outer planets consist of 

 discrete meteoric stones moving around a solid or liquid mi 

 cleus, the difficulty respecting their specific gravity would dis- 

 appear. 



3. The recent researches connecting the November, the 

 August, and other periodic swarms of shooting stars with com- 

 ets tend in the direction of showing that comets in cooling, 

 break up into discrete solid particles (each no doubt having 

 passed through the liquid condition) ; and that probably the 

 solar nebula cooled in like manner into separate fiery tears, 

 which soon solidified by radiation into the cold of space. 



4. Mr. Huggins's recent comparisons of the spectroscopic 

 appearances of comets and incandescent portions of meteoric 

 stones, showing the presence in both of hydrocarbon and nitro- 

 gen compounds, confirm the conclusions drawn from the iden- 



paths of comets and meteoric periodic shooting stars. 



5. Mr. H. A. Newton, in a remarkable paper read before the 

 Sheffield Meeting of the British Association (1879), showed the 

 possibility (if not probability) of the asteroids being extinct 

 comets, captured and brought into the solar system by the 

 attraction of some one or other of the outer large planet-, and 

 permanently confined in the space between Mars and Jupiter, 

 which is the only prison cell in the solar system large enough 

 to hold permanently such disorderly wanderers. 



In the same paper, Professor Newton threw out the idea 

 that some of the satellites of the large planets might also be of 

 cometary origin. 



From all these and other considerations it is therefore allow- 

 able to suppose that the earth and moon when they separated 

 from the solar nebula, did so as a swarm of solid meteoric 

 stones, each of them having the temperature of inl 

 space ; i. e. something not much warmer than 460° F. below 

 the freezing point of water. 



Mr. George H. Darwin has shown, admirably, how the earth- 

 moon system may have been developed from the time when the 

 earth-moon formed one planet revolving on its axis in a few 

 hours, to the present time, when the earth and moon (in conse- 

 quence of tidal friction) have pushed each other asunder to a 

 distance of sixty times the radius of the earth.f 



In his paper on the tidal friction of a planet^: (supposed 

 viscous and under the influence of bodily tides caused in 

 it by an external body such as the moon), Mr. Darwin has 

 found a remarkable equation of condition, which may be 

 thus expressed : 



