2O THOUGHTS ON NATURAL PHILOSOPHY. 



draw into a single mass the materials of the ring, 

 and thus we would have a planet formed, while the 

 satellites of that planet would be developed from the 

 still nascent planet in the same way as the planet 

 itself originated from the sun. In this way we account 

 most simply for the uniformity in the direction in 

 which the planets revolve and for the mutual 

 proximity of the planes in which their orbits are 

 contained. The rotation of the planets on their 

 axes is also explained, for at the time of the first 

 formation of the planet it must have participated in the 

 rotation of the whole nebula, and by the subsequent 

 contraction of the planet the speed with which the 

 rotation was performed must have been accelerated. 



There is quite a different method of approaching 

 the subject, which leads in a very striking manner to 

 conclusions practically identical with those we have 

 just sketched!. We may commence by dealing with 

 the sun as we find it at the present moment, and then 

 reasoning back to what must have been the case in 

 the earlier epochs of the history of our system. The 

 stupendous daily outpour of heat from the sun at 

 the present time is really, when properly studied, a 

 profound argument in support of the nebular theory. 

 The amount of the sun's heat has been estimated. 

 We receive on the earth less than one two-thousand- 

 millionth part of the whole radiation. It would seem 

 that the greater part of the rest of that torrent flows 

 away to be lost in space. Now what supplies this 

 heat ? We might at first suppose that the sun was 

 really a mightily heated body radiating out its heat as 

 white hot iron does, but this explanation cannot be 



