Nebular Theory. — Mistuckles. 231 
How the rotation of a nebula is caused by the contraction 
and condensation of its mass, can, accordin<Tf to Herbert Spen- 
cer, be explained in the following- manner : 
"Each portion of such vapor-like matter must begin to 
move toward the common center of gravity. The tractive 
forces which would of themselves carry it in a straight line to 
the center of gravity are opposed by the resistant forces of the 
medium through which it is drawn. The direction of move- 
ment must be the resultant of these — a resultant which, in 
consequence of the unsymmetrical form of the flocculus, must 
be a curve directed, not to the center of gravity, but toward one 
side of it. And it may be readily shown that in an aggrega- 
tion of such flocculi, severally thus moving, there must, by com- 
position of forces, eventually result a rotation of the whole 
nebula in one direction." (First Principles, p. 198, §76.) 
To a merely casual observer this may, perhaps, seem rea- 
sonable and many may consider it tolerably satisfactory. Let 
us not, however, be satisfied with this superficial probability, 
hut analyze the statement and thus determine to what an ex- 
tent it is reasonable. 
It assumes that it is the force of gravity which first set 
the matter in motion and which afterwards continued to cause 
this motion or rotation. As the nebulous mass contracts and 
condenses, the attracted parts meet resistant forces in a sup- 
posed medium through which they must pass, the result of 
which is a curved motion to one side instead of a motion in a 
straight line towards the center. 
Here we notice at once a most remarkable circumstance, 
which has been left unexplained, but which nevertheless, is of 
great importance, namely, that the medium through which the 
attracted parts must pass does not move in consequence of the 
vapor-like or gaseous condition of the nebula. Why does not 
this medium move together zmtli the vapor, sinking towards the 
center? 
This medium cannot be supposed to offer a resistance simi- 
lar to the resistance offered by atmospheric air to objects pass- 
ing through it, for the nebula itself, according to Laplace, was 
thin as air long after the rotation commenced. To suppose 
such a resisting medium in the nebular matter in that stage 
of evolution, would be the same as to suppose that air in 
