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known of these and the one which has had the most influence on philosophic 

 thought is known as the Nebular Hypothesis of La Place. It was first 

 announced about a hundred years ago and has been accepted as probably 

 representing planetary evolution until recent years although based largely 

 on assumptions. La Place was one of the greatest of astronomers and 

 mathematicians since the time of Newton and doubtless his name alone car- 

 ried conviction where a little independent investigation and reasoning would 

 have been more profitable. It is quite evident that La Place never regarded 

 this theory as seriously as it was regarded by others after his death. 



You are all familiar with the main outlines of the theory. It assumes 

 that the matter now composing the sun, the planets and their satellites was 

 once diffused though a sphere perhaps as large as the present orbit of Nep- 

 tune, that in some way (unknown) the mass started to revolve and therefore 

 to flatten at the poles and extent at the equator, and that with the radiation 

 of heat and consequent shrinkage in volume, the revolution had been has- 

 tened and soon a point had been reached where the gravitational force at the 

 equator was balanced by the centrifugal force due to the revolution. At 

 this point, according to the theory, a more or less broad ring was abandoned 

 by the revolving mass. It went on shrinking, and increasing its velocity of 

 motion till the same process was repeated. Each ring was then supposed to 

 collect into a sphere and go through the same process in a small way, thus 

 accounting for satellite systems of the various planets, although there was 

 no investigation to establish the way in which this was done, or even to show 

 that it was -possible. No doubt this whole scheme was suggested by the 

 planet Saturn which shows a ring system very much as La Place supposed 

 existed around the sun, but which we now know differs very materially from 

 any of his hypothetical rings. 



As stated above, this theory implies that the planets should all be very 

 nearly, if not exactly, in one plane, that they should travel in the same 

 direction around the sun, that the satellites of each planet should all go in 

 the same direction and in one plane, and that the periods of revolution of the 

 satellites should be longer than the rotation periods of their primaries. 

 These conditions seemed nearly fulfilled at the time of La Place, but since 

 then we have had the discovery of Neptune with its satellite very much 

 inclined to the orbit of the planet, and revolving backward at that, we have 

 had the discovery of the satellites of Uranus also revolving retrograde and 

 very much out of the planet's plane of revolution. We have had, moreover, 



