some Stars towards or from the Earth. 145 



it seems probable that y Cassiopeise has a small motion of reces- 

 sion. 



In the calculation of the estimated velocities the wave-lengths em- 

 ployed are those given by Angstrom in his ' Recherches sur le spectre 

 solaire' (Upsal, 1868). The velocity of light was taken at 185,000 

 miles per second. 



The velocities of approach and of recession which have been 

 assigned to the stars in this paper represent the whole of the motion 

 in the line of sight which exists between them and the sun. As 

 we know that the sun is moving in space, a certain part of these 

 observed velocities must be due to the solar motion. I have not at- 

 tempted to make this correction, because, though the direction of 

 the sun's motion seems to be satisfactorily ascertained, any estimate 

 that can be made at present of the actual velocity with which he is 

 advancing must rest upon suppositions, more or less arbitrary, of 

 the average distance of stars of different magnitudes. It seems not 

 improbable that this part of the stars' motions may be larger than 

 would result from Otto Struve's calculations, which give, on the sup- 

 position that the average parallax of a star of the first magnitude 

 is equal to 0"*209, a velocity but little greater than one fourth of 

 the earth's annual motion in its orbit. 



It will be observed that, speaking generally, the stars which the 

 spectroscope shows to be moving from the earth (Sirius, Betelgeux, 

 Rigel, Procyon) are situated in a part of the heavens opposite to Her- 

 cules, towards which the sun is advancing, while the stars in the 

 neighbourhood of this region, as Arcturus, Vega, a Cygni, show a 

 motion of approach. There are in the stars already observed excep- 

 tions to this general statement ; and there are some other considera- 

 tions which appear to show that the sun's motion in space is not 

 the only, or even in all cases, as it may be found, the chief cause 

 of the observed proper motions of the stars*. 



There can be little doubt that in the observed stellar move- 

 ments we have to do with two other independent motions — ramely, 

 a movement common to certain groups of stars, and also a motion 

 peculiar to each star. 



Mr. Proctor has brought to light strong evidence in favour of the 

 drift of stars in groups having a community of motion, by his gra- 

 phical investigation of the proper motions of all the stars in the 

 catalogues of Mr. Main and Mr. Stonef . The probability of the 

 stars being collected into systems was early suggested by Michell 

 and the elder HerschelJ. One of the most remarkable instances 



* As the velocities assigned to the stars are, for reasons already stated, pro- 

 visional only, I feel some hesitation in drawing from them the obvious conclu- 

 sions which they would suggest. The velocities given in the Tables for those 

 stars which are moving in direction in accordance with the sun's motion towards 

 Hercules do not bear to each other the relation which they should have if they 

 were mainly produced by the sun's motion. Even for these stars, therefore, we 

 must look elsewhere for the cause to which they are chiefly due. 



t See " Preliminary Paper on certain Drifting Motions of the Stars," Proc. 

 Roy. Soc. vol. xviii. p. 169. 



\ Sir William Herschel writes : — " Mr. Michell's admirable idea of the stars 



Phil. Mag. S. 4. Vol. 45. No. 298. Feb. 1873. L 



