56 



the discovery of the two satellites of Mars, one of which revolves very much 

 faster than Mars rotates on its axis. 



A theory that perfectly explains all the known facts may gel a bearing 

 and acceptance without any great amount of demonstration, but when many 

 important facts appear at variance with a theory it becomes necessary to 

 show how these facts may be accounted for by the theory, or to look with 

 suspicion on the theory as a whole. 



There are many other facts than those just mentioned which cause 



distrust. Take for example the probable density of the ring that is supposed 



to have formed Neptune. If all the matter now in the Solar system were 



expanded till it formed a sphere the size of the orbit of this planet its average 



l 



density would be about 216,000,000,000 the present density of the sun. The 



density at the center would probably be many times that at the equator. 



which would make the densitj 7 of the abandoned ring much less than 



1 



210,000,000,000 th of the present density of the sun. This would be many 



times as rare as the best vacuum yet obtained. To suppose that any such 

 mass of matter, spread out in a ring whose diameter must have been at least 

 thirty times the diameter of the earth's orbit, ever collected in one place to 

 form Neptune is a very great tax on the imagination. As a matter of fact 

 it can be shown that this is physically impossible. This process involves 

 long intervals of time and would make the outer planets much older than 

 the earth, and other nearer planets. There is no observational data to 

 support this idea; all that there are directly contradict it. On the supposi- 

 tion that the sun has radiated heat in the past as it does now, and that the 

 shrinkage of the sun is responsible for the development of its energy, it is 

 possible to tell how many years ago the sun was large enough to fill the orhit 

 of the earth. The earth must therefore be younger than this. All evi- 

 dences in the earth itself point to an age of a least sixty million years, and 

 on the above assumptions upon which the theory of La Place rests, the sun, 

 sixty million years ago, was much larger than the earth's orbit. The prob- 

 ability is then that the assumptions are wrong. Other more technical ob- 

 jections, some of which are even more conclusive, I must pass over. 



Another theory of Evolution based on tidal relations among sun, planets 

 and satellites has been elaborated in more recent years, and either by itself 

 or in connection with the foregoing has been used to explain our present 

 system. The application of this theory to the Earth — Moon system has 

 been elaborated by Professor George Darwin. He supposes thai the earth 



