THE ORIGIN OF THE EARTH. 5 



this must have led to accelerated rotation. "When the rotation reached 

 a certain speed, the centrifugal acceleration at the equator of the 

 spheroid would come to equal the centripetal acceleration of gravity. 

 The equatorial portion would then no longer contract, while the re- 

 mainder, having less rotational speed and continuing to shrink from 

 further loss of heat, would draw away from it. The equatorial portion 

 is commonly said inaccurately to be " thrown off." The hypothesis 

 assumes that the equatorial matter so left behind would constitute a 

 ring resembling the rings of Saturn; indeed, there is little doubt that 

 the Saturnian rings suggested the theory. It further assumes that, as 

 the cooling and contraction of the spheroid continued, additional rings 

 to the number of the present planets were left behind. 



It has been objected that the matter left behind in this way would 

 separate particle by particle, forming a kind of disk, and not a series 

 of definite rings, since the centrifugal acceleration would come into 

 equality with the centripetal gradually and at different times for 

 each successive distance from the center. There could have been 

 no appreciable cohesion to restrain the separation; on the contrary, 

 the individual velocities of the molecules should have aided individual 

 separation. But this is not the view taken by the hypothesis. 



Formation of gaseous spheroids. — It is further assumed that as the 

 rings cooled they parted at their weakest points and collected into 

 spheroids which were still hot and gaseous like the original spheroid. 

 These afterwards followed a similar course of evolution, detaching 

 rings in most cases, which in turn parted and gathered into smaller 

 spheroids. 



The spheroids into which the first set of rings contracted are assumed 

 to have at length condensed into the several planets, those of the 

 secondary rings into the satellites, while the great central spheroid 

 formed the sun. 



The earth-moon ring. — In the case of our planet, it is held that 

 the matter of the earth and moon together was originally separated 

 from the solar spheroid as a common ring at a rather late stage in 

 the evolution of rings, and that this ring coalesced into a hot gaseous 

 spheroid which, in the course of its contraction, in turn detached a 

 ring from its equator, and that this ring condensed into the moon. 



air at the earth's surface (Tests of the Nebular Hypothesis, Astrophys. Jour., Vol. 

 II, 1890, p. 114). 



