VOL. LXXIX.] PHILOSOPHICAL TRANSACTIONS. 5^3 



still farther, and extend his surmises by imperceptible steps to other nebulae, 

 that still preserve the same characteristics, with the only variations of vanishing 

 brightness, and reduction of size. 



Other clusters there are that, when they come to be compared with some of 

 the former, seem to contain stars of an equal magnitude, while their compres- 

 sion appears to be considerably different. Here the supposition of their being at 

 different distances will either not explain the apparently greater compression, or, 

 if admitted to do this, will convey to us a very instructive consequence : which 

 is, that the stars which are thus supposed not to be more compressed than those 

 in the former cluster, but only to appear so on account of their greater distance, 

 must needs be proportionally larger, since they do not appear of less magnitude 

 than the former. As therefore one or other of these hypotheses must be true, 

 it is not at all improbable but that, in some instances, the stars may be more 

 compressed ; and in others, of a greater magnitude. This variety of size, in 

 different spherical clusters, I am however inclined to believe may not go further 

 than the difference in size, found among the individuals belonging to the same 

 species of plants, or animals, in their different states of age, or vegetation, after 

 they are come to a certain degree of growth. A further inquiry into the circum- 

 stance of the extent, both of condensation and variety of size, that may take 

 place with the stars of different clusters, we shall postpone till other things have 

 been previously discussed. 



Let us then continue to turn our view to the power which is moulding the 

 different assortments of stars into spherical clusters. Any force, that acts unin- 

 terruptedly, must produce effects proportional to the time of its action. Now, 

 as it has been shown that the spherical figure of a cluster of stars is owing to 

 central powers, it follows that those clusters which, ceteris paribus, are the most 

 complete in this figure, must have been the longest exposed to the action of 

 these causes. This will admit of various points of views. Suppose for instance 

 that 5000 stars had been once in a certain scattered situation, and that other 

 5000 equal stars had been in the same situation; then that of the two clusters 

 which had been longest exposed to the action of the modelling power, we sup- 

 pose, would be most condensed, and more advanced to the maturity of its 

 figure. An obvious consequence that may be drawn from this consideration is, 

 that we are enabled to judge of the relative age, maturity, or climax of a side- 

 real system, from the disposition of its component parts ; and, making the de- 

 grees of brightness in nebulae stand for the different accumulation of stars in 

 clusters, the same conclusions will extend equally to them all. But we are not 

 to conclude, from what has been said, that every spherical cluster is of an equal 

 standing in regard to absolute duration, since one that is composed of a thousand 

 stars only, must certainly arrive to the perfection of its form sooner than 



VOL. XVI. 4 G 



