256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



There is, perhaps, a difficulty in explaining how this special distribu- 

 tion of motion could have been set up originally, but at any rate it 

 is the only distribution which could survive for any length of time. 

 Since we find that the stars in the mean have the same motion as 

 the cloud it follows that they also have the velocity corresponding to 

 a circular orbit. This had been assumed by investigators of galactic 

 rotation without much attempt at justification. By reference to the 

 cloud we are now able to establish it firmly. 



I think it is not going too far to say that the mere existence of 

 the cosmic cloud is in itself a proof of galactic rotation. What has 

 kept this gas distended through the galaxy instead of collapsing long 

 ago into a dense nebula at the center ? Some of the ways of evading 

 collapse possible for particles with long free paths (the stars) are 

 not open to a regular gas ; so that the possible answers are very lim- 

 ited. The distension could be maintained by temperature; but the 

 temperature of the cosmic gas (about 15,000°) is far too low. The 

 only alternative is rapid rotation sufficient to counteract the pull of 

 gravity. The very existence of the cosmic cloud, therefore, depends 

 on rotation ; and although there is not the same theoretical necessity 

 for rotation of the galaxy of stars, the observed agreement of the 

 motions shows that the latter also must be rotating. 



The conception of the stellar system as a gas with stars for mole- 

 cules, and its comj^arison with the genuine cosmical gas filling the 

 same region and rotating in the same way, helps us to see more clearly 

 the difficulty in the way of constructing a strictly permanent stellar 

 system. With the cosmic cloud we have no difficulty in fulfilling 

 the strict conditions of equilibrium, but that is because we neglect 

 viscosity. Viscosity — the rubbing of one zone of gas on another zone 

 rotating at a slightly different speed — must slowly change the dis- 

 tribution of rotation. Viscosity objects to the existing differential 

 rotation, and tries to set up a condition of its own. But it can never 

 succeed; it can only act as spoil-sport. As soon as it effects any 

 serious change the density gradients already mentioned must be set 

 up — which means that the cloud falls to the center of the system or 

 departs into outer space. I daresay that in the end viscosity tri- 

 umphantly establishes the law of motion it is striving for — only there 

 is nothing left to obey it. 



Similarly in the system of the stars we have a tug-of-war between 

 the viscosity conditions and the simple pressure conditions which 

 must inevitably end in the collapse or disruption of the system. The 

 only question is how to arrange some kind of balance which will 

 stave off this fate for a reasonable time. I have already referred to 

 certain current solutions which seem to me to insist too strongly on 

 the fulfilment of what we here identify as viscosity conditions, per- 



