THE ORIGIN. AND DEVELOPMENT OF THE EARTH 109 
that the planetesimal hypothesis, as the proposed explanation 
of Chamberlin and Moulton is called, does not rely upon any 
set origin of the spiral nebulae; it merely proposes an origin 
for them. Its true basis is the existence of the spirals, which 
is unquestioned. 
Our present sun shoots out great protruberances to the 
heights of many thousands of miles at velocities which, were 
it not for the great weight of the sun’s atmosphere, would 
carry them to the outer limits of the solar system, or perhaps 
even beyond ft. Let us now suppose that another sun were 
to approach ours. The attraction between the two, due to 
eravity, would greatly increase the tension upon the sun, and 
would thus cause great tidal protuberances to arise. These 
protruberances, were the forces causing them great enough, 
might well leave the sun, never to return. Of such material, 
arising much as in the manner briefly outlined above, the 
planets and satellites are supposed to be composed. 
We have said that the forms of the spiral nebulae seem to 
imply that they originated through two types of movement— 
outward, and rotatory. The outward movement we have just 
accounted for in the projection of the protruberances from 
the parent sun through the attraction of another sun passing 
relatively near it. It now remains for us to account for the 
rotatory motion. 
The protuberances would, according to this hypothesis, be 
thrust out as the ancestral sun and the passing star were 
swinging about their common center of gravity. The protu- 
berance shot from the sun in the direction of the star would 
be drawn into a curved path by the.attraction of the star, and 
the same would be true of the opposite projection, but to a 
lesser extent. The accompanying diagram, taken from Moul- 
ton shows how this would develop a_ spiral from the 
partially disrupted sun. Since in the course of rotation the 
inner parts of the spiral moved more rapidly than the outer, 
just as the small hand of a clock rotates more rapidly than the 
large, the arms became more closely coiled, finally developing 
a closely coiled spiral probably somewhat similar to the 
accompanying restoration. Since the parent sun was gase- 
ous, as it is today, the particles composing the arms must have 
originally been in a free molecular state. Their enormous 
