444 SCIENCE PROGRESS 



to determine the distance of the cluster to be 120 light-years 

 away from us (that is, light from the cluster takes 120 years 

 to reach us ; the distance in miles, if that be preferred, is 800 

 million million); and it is receding in an oblique direction. It 

 passed us closest about 8,000 centuries ago, at about half its 

 present distance : and he gathered further particulars of its 

 history and shape, which we can scarcely stop to notice here. 

 One further point, however, is of importance. The individual 

 stars seem to keep their places in the procession without in- 

 ternal rearrangement — they move in almost strictly parallel 

 lines and at the same pace. 



Now the streams of stars to which Kapteyn called attention 

 are of a different kind ; the internal movements are considerable ; 

 it is only the average movement which is steadily in one 

 direction. But when we take such average movements in 

 different parts of the sky they tend to converge to a point 

 like the actual motions of the individual stars of the Taurus 

 cluster. It is this convergence of average movements which 

 Mr, Eddington has represented so beautifully in his diagrams. 

 He divided the whole sky into thirty-four areas, and found 

 (from the great catalogue of movements just published by 

 Prof. Boss) the average movements in each area. It would 

 take too long to explain how he identified the average move- 

 ment for each of the two drifts : it must suffice that the process 

 was ingenious and effective.^ He was able to draw the two 

 lines for each of the thirty-four regions, or rather for each of 

 seventeen pairs which he preferred to use. The test of the 

 validity of the hypothesis is that these lines should converge 

 to two points in the sky representing the goals towards which 

 the two clouds of stars are drifting. The reader can judge 

 for himself. In one case the convergence is very striking. It 

 is not, of course, perfect : we could scarcely expect perfection 

 when dealing with averages of imperfect observations, but the 

 approximation is clearly a very close one. In the other case the 

 convergence is less marked, but the reality is brought home 

 to us by an analogy. " If from seventeen points," writes Mr. 

 Eddington, " distributed uniformly all over the earth, tracks 

 (great circles) were drawn, every one of which passed across 



' Those who care to read more will find Mr. Eddington's paper in the 

 November number of the Monthly Notices of the Royal Astronomical Society. 

 It gives references to previous work. 



