Nebular Theory. — Misfockles. 229 
2. The nebular theory, as elaborated by Laplace, and 
before him presented in several of its main features by Im- 
manuel Kant is, in brief, as follows : 
All the heavenly bodies, large and small, which compose 
the entire present solar system, have orig^inated from a com- 
mon, vast nebula. This nebula was spherical in form and filled 
space as far as to the orbit of the most distant planet. The mass 
composing' this nebula contracted towards the center ; by 
this contraction, rotation and heat were generated. As the 
contraction and consequent condensation went on, the rotation 
increased, which again caused an expansion in the equatorial 
region and depressions at the poles. On account of the rapid 
rotation the centrifugal force finally increased to such a degree 
that it overcame the force of contraction. This, again, caused 
the separation of a portion of the nebula at the equator, which 
portion continued to revolve around the nebular mother in 
the form of a ring. This operation reoccurred as often as the 
rotation increased to such a degree, that the centrifugal force 
overcame the centripetal or inward tending force. Thus one 
ring after another was formed ; and in time these rings broke, 
contracted, and formed themselves into individual nebulae, all 
rotating and revolving around the common center of gravity. 
These new bodies, in their turn, threw ofif rings, which, like 
the former, became spherical nebulae. The former became the 
planets ; the latter the moons of the planets, while the original 
and constantly contracting nebula — finally forming into a mass 
of glowing lava — became the Sun and center of the system. 
The cause which led to the construction of and subsequent 
adherence to the nebular theory, was the wonderful symmetry 
of the solar system. The advocates of this theory noticed that 
the planets revolve in analogous orbits around the equator of 
the Sun whose plane of rotation thus becomes identical with 
the orbits of the planets. They found also, that the same rela- 
tions exist between the planets and their moons, so that the 
moons revolve around the equator of the planets, and that the 
plane of the planet's rotation thus coincides with the orbits of 
the moons, or nearly so. The object of the nebular theory, 
thus, is to explain how the whole solar system was originally 
formed and put in motion, and how the order and motion of 
its various parts have since been retained and preserved. The 
