240 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



a region containing perhaps 10 to 100 million stars, this is but a small 

 part of the whole. We have to take a risk in inferring the nature of 

 the complete system from the small sample within reach. 



Throughout the nineteenth century astronomers working on stellar 

 motions concentrated their attention on one main theme — the solar 

 motion, or velocity of our sun as an individual star with respect to 

 the system as a whole. For our present discussion of the system of 

 the stars this has no particular interest, being merely a distorting 

 factor in our outlook which is sometimes troublesome to eliminate. 

 We are concerned with the stellar motions remaining after our own 

 translational velocity has been allowed for; they are by no means 

 those of an unorganized crowd. By later researches four leading 

 peculiarities have been discovered. I give them in historical order : 



(1) Star streaming, i. e., a tendency of the stars to move to and fro 

 along one particular axis in space rather than in directions at right 

 angles to it. 



(2) A strong correlation between the velocity and the physical 

 characteristics of the stars. For example, stars classed as of " late " 

 spectral type have a higher average speed than those of " early " 

 type. 



(3) Stars of exceptionally high velocity (greater than 80 km per 

 sec.) are found to be moving exclusively toward one hemisphere of 

 the sky. 



(4) An effect rather complicated to describe which we interpret 

 as evidence of rotation of the whole system. This is the main theme 

 of my lecture. 



In conjunction with these results we have to consider a matter of 

 common knowledge inferred from the apparent distribution (not the 

 motions) of the stars. Our stellar system has a very oblate form. 

 It is believed to be almost a disk — resembling the spiral nebulae seen 

 abundantly in the vast universe be3'^ond the confines of our galaxy. 



NATURE OF THE ROTATION 



The discovery of the fourth effect and the interpretation placed on 

 it are due to J. H. Oort of Leiden. Among other investigators 

 should be mentioned especially B. Lindblad, who had been develop- 

 ing the hypothesis of galactic rotation for other reasons, and J. S. 

 Plaskett, to whom we owe the most convincing evidence. 



It will help us to understand what kind of indication of rotation 

 we might look for in a system of stars, if we transfer our attention 

 for a moment to a phenomenon nearer home, namely Saturn's rings. 

 These have a rough resemblance to the disklike form attributed to 

 our galaxy. At one time there was a division of opinion as to whether 

 the rings were solid structures or whether they consisted of swarms 



