SLIPHER— NEBULA. 



407 



Referring to the table of velocities again : the average velocity 

 570 km. is about thirty times the average velocity of the stars. And 

 it is so much greater than that known of any other class of celestial 

 bodies as to set the spiral nebulae aside in a class to themselves. 

 Their distribution over the sky likewise shows them to be unique — 

 they shun the Milky Way and cluster about its poles. 



The mean of the velocities with regard to sign is positive, imply- 

 ing the nebulas are receding with a velocity of nearly 500 km. 

 This might suggest that the spiral nebulae are scattering but their 

 distribution on the sky is not in accord with this since they are in- 

 clined to cluster. A little later a tentative explanation of the 

 preponderance of positive velocities will be suggested. 



Grouping the nebulae as in Table IIL, there appears to be some 

 evidence that spiral nebulas move edge forward. 



TABLE III. 



Velocities of Spiral Nebula Grouped. 



The form of the spiral nebulae strongly suggests rotational mo- 

 tion. In the spring of 1913 I obtained spectrograms of the spiral 

 nebulas N.G.C. 4594 the lines of which were inclined after the 

 manner of those in the spectrum of Jupiter, and, later, spectro- 

 ' grams which showed rotation or internal motion in the Great Andro- 

 meda Nebula and in the two in Leo N.G.C. 3623 and 3627 and in 

 nebulas N.G.C. 5005 and 2683 — 'less well in the last three. The mo- 

 tion in the Andromeda nebula and in 3623 is possibly more like 

 that in the system of Saturn. It is greatest in nebula N.G.C. 4594. 

 The rotation in this nebula has been verified at the Mt. Wilson 

 Observatory. 



