198 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



nebula will give rise to a cluster of a few hundred stars, or whether 

 it will turn into a stellar universe on the same scale as the great 

 system of some thousands of millions of stars which forms our 

 galactic system. There is now no suggestion that it has anything 

 to do with the formation of so insignificant a system as the solar 

 system. But in preferring the nebular hypothesis to that of Cham- 

 berlin and Moulton, it is necessary to make a certain reservation. 

 We have hitherto taken it for granted that the formation of a sys- 

 tem of planets is a normal feature of the evolution of a star. Most 

 of my arguments have referred to the development of stars in gen- 

 eral, and would become irrelevant if it could be admitted that the solar 

 system were an exceptional formation violating ordinary expectation. 



We know that at least a third of the stars are double stars, and 

 I do not think there is any reason to think that planetary systems 

 would be formed when the evolution takes that course; but until 

 recently it was taken for granted that the remaining single stars 

 would generally (or at least frequently) be the rulers of systems 

 of planets. Jeans has recently pitched a bombshell into the camp, 

 suggesting that the solar system is a freak system — the result of a 

 rare accident, which could only happen to one star out of a very 

 large number. He found no way of accounting for it as a normal 

 process. I have not the specialist knowledge necessary to criticize 

 the details of the working of the nebular or of the planetismal theory 

 of development, but before regarding Jeans' argument as conclusive 

 (he himself makes reservations) I should be more satisfied if the 

 effect of radiation pressure had been taken into account. It is fairly 

 clear that radiation pressure plays a great part in the separation of 

 nebulous matter into stars, and although I have no definite reason 

 to think that it can account for the separation of planets from the 

 sun, I do not feel satisfied that we have got at the whole truth until 

 that point has been duly examined. 



Supposing, however, that we are forced* to accept Jeans' sugges- 

 tion that the solar system is a freak system, some of my objections 

 to the Chamberlin-Moulton hypothesis are removed. I can not ad- 

 mit that the conditions of collision which that hypothesis requires 

 are normal features in the formation of stars; but they might have 

 happened occasionally in the history of the universe, and produced 

 the solar system, the sun being thus an exceptional star born out 

 of due time. But if my arguments against Chamberlin's hypothesis 

 fall to the ground, there are probably other astronomers prepared 

 to attack it in other directions. 



The new views as to the age of the earth are now pretty well 

 known to geologists. I may sum them up briefly in the statement 

 that Lord Kelvin's estimate of the extent of geological time need not 

 now be taken any more seriously than Archbishop Ussher's, and 



