THE MILKY WAY. 263 



other hand, such an equiHbrium could only be pro- 

 visional, for, in consequence of collisions, the molecules 

 — I mean the stars — will acquire considerable velocities 

 in a direction perpendicular to the Milky Way, and 

 will end by emerging from its plane, so that the 

 system will tend towards the spherical form, the only 

 figure of equilibrium of an isolated gaseous mass. 



Or else the whole system is animated with a common 

 rotation, and it is for this reason that it is flattened, 

 like the Earth, like Jupiter, and like all rotating 

 bodies. Only, as the flattening is considerable, the 

 rotation must be rapid. Rapid, no doubt, but we 

 must understand the meaning of the word. The 

 density of the Milky Way is 10^^ times as low as the 

 Sun's ; a velocity of revolution ^Jio'-'' times smaller 

 than the Sun's would therefore be equivalent in its 

 case from the point of view of the flattening. A 

 velocity 10^^ times as slow as the Earth's, or the 

 thirtieth of a second of arc in a century, will be a 

 very rapid revolution, almost too rapid for stable 

 equilibrium to be possible. 



In this hypothesis, the observable individual motions 

 will appear to us uniformly distributed, and there will 

 be no more preponderance of the components parallel 

 with the Galactic plane. They will teach us nothing 

 with respect to the rotation itself, since we form part 

 of the rotating system. If the spiral nebula? are other 

 Milky Ways foreign to ours, they are not involved 

 in this njtation, and we might study their individual 

 motions. It is true that they are very remote, for if 

 a nebula has the dimensions of the Milky Way, and 

 if its apparent radius is, for instance, 20", its distance 

 is 10,000 times the radius of the Milky Way. 



