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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, 
