THE NEBULAR HYPOTHESIS 179 



Why they revolve so nearly in the same plane. 



What comets are. 



Why these revolve in elongated orbits so dissimilar 

 from planetary orbits. 



Whether the sun moves in space, and why, and how. 

 x What effect the stellar attractions exert upon the 

 solar system, internally and externally. 



The source of the sun's heat. 



The nature of the stars and their mutual relations. 



The cause of the moon's secular acceleration. 



The origin of the earth's equatorial ring and her 

 continental configurations. 



It was to supply some of the deficiencies above 

 enumerated that Laplace invented his celebrated Nebular 

 Hypothesis, which he first announced in 1796, and twelve 

 years afterwards republished, with certain corrections 

 and additions. In brief, his scheme was this: 



The solar system originally existed in the form of a 

 gaseous nebula that extended out from the center of the 

 sun (which he regarded as stationary) to somewhat be- 

 yond the orbit of the outermost planet (at the time of 

 his death Neptune was yet undiscovered). This nebula 

 had an inherent motion of rotation around an axis at 

 right angles to its plane and passing through the center 

 of the sun. At first the nebula was probably spheroidal 

 in shape, but by virtue of its axial rotation it was gradu- 

 ally flattened out into a discal form. In the course of 

 time the rotary motion, by reason of the contraction of 

 the mass, became accelerated, so that finally the nebula 

 spun around so swiftly that it cast off an outside ring. 

 Relieved of the weight, the residuum revolved all the 

 faster, and a second ring was cast off. This process con- 

 tinued then until as many rings had been thus shed as 

 there are planets, and there finally remained only the cen- 

 tral residual mass which now constitutes our sun. This 

 latter, being eternally hot and bright, naturally suggested 

 to Laplace that the nebula was incandescent to begin 

 with and retained its temperature throughout the pro- 

 cess, an inference all the more plausible because the in- 



