382 Royal Society. 



stars are arranged, this tendency is masked ; but when the proper 

 motions are indicated in maps, by affixing to each star a small arrow 

 whose length and direction indicate the magnitude and direction of 

 the star's proper motion, the star-drift (as the phenomenon may be 

 termed) becomes very evident. 



It is worthy of notice that Madler, having been led by certain con- 

 siderations to examine the neighbourhood of the Pleiades for traces 

 of a community of proper motion, founded on the drift he actually 

 found in Taurus his well-known theory that Alcyone (the lucida of 

 the Pleiades) is the common centre around which the sidereal system 

 is moving. But in reality the community of motion in Taurus is only 

 a single instance, and not the most striking that might be pointed 

 out, of a characteristic which may be recognized in many regions of 

 the heavens. In Gemini and Cancer there is a much more striking 

 drift towards the south-east, the drift in Taurus being towards the 

 south-west. In the constellation Leo there is also a well-marked 

 drift, in this case towards Cancer. 



These particular instances of star-drift are not the less remarkable 

 that they (the stars) are drifting almost exactly in the direction due to 

 the proper motion which has been assigned to the sun, because the 

 recent researches of the Astronomer Royal have abundantly proved 

 that the apparent proper motions of the stars are not to be recognized 

 as principally due to the sun's motion. Mr. Stone has shown even 

 that we must assign to the stars a larger proper motion, on the ave- 

 rage, than that which the sun possesses. Looking, therefore, on the 

 stars as severally in motion, with velocities exceeding the sun's on 

 the average, it cannot but be looked upon as highly significant that 

 in any large region of the heavens there should be a community of 

 motion such as I have described. We seem compelled to look upon 

 the stars which exhibit such community of motion as forming a 

 distinct system, the members of which are associated indeed with the 

 galactic system, but are much more intimately related to each other. 



In other parts of the heavens, however, there are instances of a 

 star-drift opposed to the direction due to the solar motion. A re- 

 markable instance may be recognized among the seven bright stars 

 of Ursa Major. Of these, the stars /3, y, S, e, and £ are all drifting 

 in the same direction, and almost exactly at the same rate, towards 

 the " apex of the solar motion," that is, the point from which all the 

 motions due to the sun's translation in space should be directed. If 

 these five stars, indeed, form a system (and I can see no other reason- 

 able explanation of so singular a community of motion), the mind is 

 lost in contemplating the immensity of the periods which the revolu- 

 tions of the components of the system must occupy. Madler had 

 already assigned to the revolution of Alcor around Mizar (£ Ursse) 

 a period of more than 7000 years. But if these stars, which appear 

 so clear to the naked eye, have a period of such length, what must 

 be the cyclic periods of stars which cover a range of several degrees 

 upon the heavens ? 



In like manner the stars a, /3, and y of Arietis appear to form a 

 single system, though the motion of a is not absolutely coincident 



