May 24, 18S8] 



NA TURE 



81 



double star in posse) will be greatest of all, because (4) 

 condensation may ultimately bring the central swarm 

 almost entirely within the orbit of the secondary (cometic) 

 body, in which case no collision could happen. 



In the light of what has gone before it is as easy to 

 test these points as the former ones. 



I will take them in order. 



The Frequent Occurrence of Variability in Group II. 



The total number of stars included in Argelander's 

 Catalogue, which deals generally with stars down to the 

 ninth magnitude, but in which, however, are many stars 

 between the ninth and tenth, is 324,118. The most com- 

 plete catalogue of variables (without distinction) that we 

 have has been compiled by Mr. Gore, and published in 

 the Proceedings of the Royal Irish Academy (series ii. 

 vol. iv. No. 2, July 1884, pp. 150-63). I find 191 known 



variables are given, of which m are in the northern 

 hemisphere and 80 in the southern hemisphere. 



In the catalogue of suspected variable stars given in 

 No. 3 of the same volume (January 1885, pp. 271-310), 

 I find 736 stars, of which 381 are in the northern and 

 355 in the southern hemisphere. 



Taking, then, those in the northern hemisphere, both 

 known and suspected, we have the number 492. 



We have then as a rough estimate for the northern 

 heavens one variable to 659 stars taken generally. 



The number of objects of Group II. observed by DuneV, 

 and recorded in his admirable memoir, is 297 of these, 

 forty-four are variable. 



So that here we pass from 1 in 657 to 1 in 7. 



Of the great development of variability-conditions in 

 this group then there can be no question. 



To apply the other tests above referred to, I have 

 made a special study of the observations of each variable 



Fig. 13.— Explanation of the variability of the bodies of Group II. (3) Mini uum variation. Under the conditions repreiented, the smaller swarm will 

 never be entirely oat of the larger one, and at penastron the number of collisions will not be very greatly increased ; consequently the variation in 

 the amount of light given out wdl be small. 



recorded by Duner. I find they may be grouped 

 follows : — 



I. All ba ids visible bid narrow. 



