Theories of the Earth's Origin — Upham. 203 
traces the beginning and development of the solar system 
from an original gaseous nebula, an exceedingly tenuous 
and intensely heated cloud of matter, extending in a 
spheroidal form beyond the orbit of Neptune, the outer- 
most planet. By its gravitation and resulting contraction, 
the nebula is supposed to have acquired a movement of 
rotation, with polar flattening. Whenever the outer equa- 
torial belt of the revolving nebula attained a centrifugal 
force exceeding the attraction toward the central mass, a 
part would be left behind, either as a relatively small re- 
volving nebulous body, or as a ring of such matter, some- 
what like the rings of Saturn. Later the ring, if it was at 
first of that form, would be broken; and finally the de- 
tached mass would be gathered into a globe, which, in its 
condensation, would form satellites in the same manner 
as outer parts of the great central mass formed the suc- 
cessive planets. 
Under this theory the principal features of our plane- 
tary system, implying unity of origin and development, 
find a consistent general explanation. Prof. Charles A. 
Young has enumerated these features, which could only 
have originated by some process of orderly evolution, as 
follows :* 
1. The orbits of the planets are all nearly circular. 
2. They are all nearly in one plane excepting con- 
siderable divergence of some of the little asteroids. 
3. The revolution of all is in the same direction. 
4. There is a curiously regular progression of dis- 
tances between the planetary orbits. 
5. There is a roughly regular progression 0/ density, 
increasing both ways from Saturn. 
6. The plane of the planets' rotation nearly coincides 
with that of the orbits. 
7. The direction of the rotation is the same as that 
of the orbital reiH^lution excepting probably the two outer- 
most planets. 
8. The plane of orbital revolution of the satellites 
is nearly coincident with that of the planet's . rotation. 
* Text-Book of General Astronomy, 1893, p. 515. 
