6 SCIENCE PROGRESS 



are to be regarded as preceding the B stars in the order of 

 evolution. Of the ninety-one known Wolf-Rayet stars, seventy 

 are in the Milky Way and all the remainder are in the Magellanic 

 Clouds. A similar concentration is shown by the gaseous nebulae : 

 Hertzsprung found for the pole of their plane of concentration 

 the position R. A. iQ2°7, December + 28 ,- i, in close agreement 

 with the position of the galactic pole derived by Pickering- — R. A. 

 i90°o, December + 28°-o. 



These facts must be considered in conjunction with a remark- 

 able result first announced by Prof. W. W. Campbell, from an 

 analysis of the line-of-sight velocities of stars measured at the 

 Lick Observatory — viz , that the average velocity of the stars of 

 given spectral type increases with progression of the type. This 

 is illustrated by the following table, summarising Campbell's 

 results : 



Table III 



The same result has been found from a discussion of the 

 proper motions by Boss and by the present author. The inter- 

 pretation of this phenomenon is by no means eas}'. Although 

 all stars are not of equal age, some having condensed from 

 nebulous matter at a much later date than others, } T et it is natural 

 to suppose that gravitational forces have acted upon the matter 

 composing every star for the same length of time. On the other 

 hand, the fact that the average velocity of a star increases with 

 its age, and that the velocities of the early-type stars are very 

 small, seems to suggest that these gravitational forces due to the 

 stellar system as a whole do not become effective until the 

 nebulous matter has condensed into a star. It may possibly be 

 that the resultant velocity of the materials which go to form the 

 star is practically zero. However, the facts which we have 

 gathered together would seem to indicate that the birth of stars 

 occurs mainly in the galactic plane. The stars then acquire 



