126 RECORD OF SCIENCE FOR 1887 AND 1888. 



stitutiiig' the nebula uioie tluiii they would the inner, and the result is 

 a central condensation surrounded by an ellipsoidal atmosphere revolv- 

 injj^ with increasing- velocity as we proceed outwards from the center. 

 Instead of su[)|>osing that the planets are formed froui rings detached 

 from this nebula by over spinning, Brauu assumes that their formation 

 is determined simply by centers of condensation which existed in the 

 nebula itself. These formations have commenced at distances from the 

 central nucleus much greater than the present distances of the planets, 

 and the immediate result is an approach to the sun within a distance 

 at which its attraction is balanced by the outward gaseous pressure and 

 the centrifugal force. The incipient planet, animated with the greater 

 angular velocity of the stratum to which it originally belonged, revolves 

 now at a rate slightly superior to that of the new medium in which it 

 finds itself. Hence the possibility of its sweeping up and annexing 

 fresh matter as it proceeds along the coils of its narrowing orbit, until 

 a point is reached when the planet, or eveu*sun, has drawn in all the 

 matter which stood in its way, and gravity aloue exactly balances cen- 

 trifugal force. The planet then revolves in an orbit sensibly the same 

 as at present. 



As the planet in approaching the sun has encountered strata of in- 

 creasing density, so that the tangential resistance on the side towards 

 the sun has always been greater than on the oi)posite side, it is easy to 

 see that a planetary or " direct" rotation must be set up in the direction 

 of revolution. In order to explain the rotation of Neptune, which is 

 probably, like its satellite, retrograde, it is necessary to suppose that 

 the birth of the planet was preceded by the formation of a ring, while 

 Uranus is regarded as a "limiting instance" between the annular and 

 the nuclear methods of generation. 



Ingenious explanations are derived for the rapid motion of Pliobos, 

 the inner satellite of Mars, for the swifter rotation of the larger planets, 

 and for the increase in their density and decrease in their mass as we 

 approach the sun. Mars is an exception; but there are reasons for 

 thinking it of later formation, so that when it came into existence the 

 space allotted for its growth was already greatly exhausted, owing 

 chiefly to the powerful attraction of Jupiter. 



While nniny of Father Eraun's conclusions will not be accepted by 

 cosmogonists, his work forms an able contribution to the subject. Its 

 api)earance may be said to mark the definite abandonment by sound 

 thinkers of the annular method of planet and satellite formation, and 

 gradually it is becoming clear that " while the various members of the 

 solar family owned unrpiestionably a common origin, they can scarcely 

 be said to have had a common history." 



Janssen delivered an interesting discourse bearing upon the same sub- 

 ject at Paris, October 25, 1887— "The age of the stars," in which he 

 reviewed the steps leading us to the belief that each star must have a 

 beginning, a period of activity, a decline, and an end ; and he points 



