SIR DAVID GILL, ON THE SIDEREAL UNIVERSE. 



179 



swarm would not retain its globular form. Hence if we suppose 

 the individual bees to constitute the stars of our sidereal 

 universe, and if the motions of the stars resemble in some 

 degree the motions of a swarm of bees about a central queen, 

 we may assume that if we select a field of stars in any particu- 

 lar direction their motion will in the mean be zero. 



If then the velocities of approach to the stars in any particular 

 area are measured spectroscopically the average velocity of the 

 sun's approach to these stars should be nearly the same as if 

 the stars were at rest. 



Fig. 4 (p. 191), i.e., the diagram of spots, shows you in a simple 

 graphic way the results of all published determinations of the 

 velocity of stars in the line of sight. The spots represent the 

 centres of areas of the sky in which there are two, three, or four 

 stars of which the apparent velocities of their motions with 

 respect to the sun have been determined. The positions of these 

 areas are plotted in Eight Ascension and Declination. A black 

 spot indicates that the sun is moving towards the stars in that 

 particular area and a ring the reverse. The size of each spot or 

 ring is made proportional to the average velocity of the star- 

 motion with respect to the sun in each area. 



Nearly all the black spots are seen to be together, and nearly 

 all the rings together, and this indicates the motion of the sun to 

 be away from the rings and towards the black spots, and the exact 

 apex of each group can be readily estimated and can be calcu- 

 lated mathematically with a very considerable precision. You 

 will note how the calculated apices of motion fall in the middle 

 of the largest spots, and you will also note that there are a few 

 rings amongst the black spots and vice versa, but this merely 

 shows that tliese exceptional stars have exceptional motions of 

 their own. 



The general result of this calculation is to show that the 

 sun's motion through space is at the rate of about 13 miles a 

 second. 



Now thus far I have assumed that the motions are accidental, 

 in other words, going back upon our old analogy, that the queen- 

 bee is at rest. The great discovery was made by Professor 

 Kapteyn and announced at the meeting of the British Associa- 

 tion in South Africa, 1905, that we cannot assume that this 

 motion is accidental. He proved that there were at least two 

 great streams, or — to use our former analogy — that instead of 

 there being one queen-bee accompanied by its surrounding bees 

 there are two queen-bees, each with her own swarm. Imagine 

 two queen-bees approaching each other, and that each bee 



