i 4 2 THE EVOLUTION OF 



rotation and extended round the Equator. As the nebula 

 parted with its heat by radiation into space, it contracted, 

 and so, by the operation of a well-known mechanical law, 

 the motion of all its parts became accelerated. The 

 increased rate of rotation developed a greater centrifugal 

 force, and this, in its turn, caused a further emphasis of 

 the spheroidal form. After a time a stage was reached 

 at which centrifugal force, always greatest at the Equator, 

 had there acquired a value at which it balanced the gravi- 

 tation of the mass. Consequently, upon further shrinkage, 

 the equatorial parts were left behind in the form of a 

 ring, still retaining the motion of rotation that they pos- 

 sessed while forming a part of the parent mass. By the 

 repetition of the process a number of revolving rings were 

 shed in succession. The central nucleus of the shrunken 

 nebula now forms the Sun, and the rings, breaking and con- 

 densing, have formed the planets and their satellites. To 

 account for the small mass of the whole planetary system 

 in comparison with the Sun, and also for the very moderate 

 speed of rotation of the Sun itself, Laplace found it neces- 

 sary to assume that the nebula was very strongly condensed 

 towards its centre. It was regarded, indeed, as little 

 more than a tenuous atmosphere of the Sun pervading the 

 space now occupied by the planetary system. 



The Nebular Hypothesis has always been regarded, as 

 it was regarded by Laplace himself, as a speculation rather 

 than as a scientific theory. Whether the nebula would 

 develop in the manner imagined, whether it would shed 

 rings, and whether, if they were shed, they would condense 

 into planetary globes, are problems that might conceivably 

 be matters of mechanical investigation, but they have so 

 far proved to be beyond the powers of modern mathematics. 

 The fact, however, that the process does appear to be, in 

 its essential character, a probable one, while its results 



