THE MILKY WAY. 265 



upon the Milky Way itself as a spiral nebula, and this 

 is the third hypothesis of which I wished to speak. 



How are we to explain the very singular appear- 

 ances presented by the spiral nebula;, which are too 

 regular and too constant to be due to chance? To 

 begin with, it is sufficient to cast one's eyes upon one 

 of these figures to see that the mass is in rotation, and 

 we can even see the direction of the rotation : all the 

 spiral radii are curved in the same direction, and it is 

 evident that it is the advancing wing hanging back 

 upon the pivot, and that determines the direction of 

 the rotation. But that is not all. It is clear that 

 these nebulae cannot be likened to a gas in repose, 

 nor even to a gas in relative equilibrium under the 

 domination of a uniform rotation ; they must be 

 compared to a gas in permanent motion in which 

 internal currents rule. 



Suppose, for example, that the rotation of the central 

 nucleus is rapid (you know what I mean by this word), 

 too rapid for stable equilibrium. Then at the equator 

 the centrifugal force will prevail over the attraction, 

 and the stars will tend to escape from the equator, 

 and will form divergent currents. But as they recede, 

 since their momentum ot rotation remains constant 

 and the radius vector increases, their angular velocity 

 will diminish, and it is for this reason that the advan- 

 cing wing appears to hang back. 



Under this aspect of the case there would not be 

 a true permanent motion, for the central nucleus 

 would constantly lose matter which would go out 

 never to return, and would be gradually exhausted. 

 But we may modify the hypothesis. As it recedes, 

 the star loses its velocity and finally stops. At that 



