10 FROM NEBULA TO NEBULA 



From time to time a few men have ventured to doubt 

 the likelihood of so ethereal a thing as a nebula (a 

 thousand times rarer than the air we breath) to rotate in 

 this way, and have sought to assign mechanical causes 

 by way of furthering the main purpose. Thus, in his 

 First Principles (76) Herbert Spencer says: 



If we assume the first stage in nebular condensation to be 

 the precipitation into flocculi of denser matter previously dif- 

 fused through a rarer medium (a supposition both physically 

 justified and in harmony with certain astronomical observations), 

 we shall find that nebular motion is interpretable in pursuance 

 of the above general laws. Each portion of such vaporlike 

 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 movement must be 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 aggregation of 

 such flocculi, severally thus moving, there must, by composition 

 of forces, eventually result a rotation of the whole nebula in one 

 direction. 



Spencer's explanation, however, failed to satisfy 

 his interpreter, John Fiske, who takes issue with him 

 and in his magnum opus (Cosmic Philosophy, p. 360), 

 adduces a version of his own, as follows : 



Note first that we are obliged to accredit the various parts 

 of this genetic nebula with motions bearing some reference to a 

 common center of gravity; for the rotation of the resulting sys- 

 tem must have had an equivalent amount of motion for its ante- 

 cedent, and it is a well known theorem of mechanics that no sys- 

 tem of bodies can acquire a primordial rotation merely from the 

 interaction of its own parts. In making this assumption, how- 

 ever, we are simply carrying out the principle of the continuity 

 of motion. It is not necessary to suppose, in addition, that all 

 these motions primordially constituted a rotation of the whole 

 mass in one direction. Such a hypothesis seems to me not only 

 gratuitous, but highly improbable. It is more likely that these 

 primeval motions took the shape of currents, now aiding and now 

 opposing one another, and determined hither and thither accord- 

 ing to local circumstances. In any case, such indefiniteness of 

 movement must finally end in a definite rotation in one direction. 

 For unless the currents tending eastward are exactly balanced 



