I9I2.] OF THE GLOBULAR CLUSTERS. 155 



for the motions of the planets; and what has been found true of our 

 sun, as respects a growth of mass, from the records of ancient 

 ecHpses, will naturally be adopted for other solar stars, while a secu- 

 lar decrease in mass may be assumed for some of the Sirian stars, 

 owing to the intensity of their radiation. 



XV. The Building of Clusters and Nebula Condensed 



Towards the Center, as Illustrated by the Very 



Elongated Elliptic Orbits of our System of 



Comets. 



If we seek to inquire how clusters and nebulae much condensed 

 towards the center are built up by the process of capture, we shall 

 find the general mathematical treatment by Green's theorem already 

 given very satisfactory, for large bodies of the type of stars. It is 

 equally convincing mathematically as applied to small bodies of the 

 type of comets, but it is perhaps well to notice how the comets de- 

 scending to our sun in very elongated ellipses have served to supply 

 material for building up the planets and sun. This remarkable 

 system of comets, with elliptic orbits equally diffused in all directions 

 about our sun, is a sure sign that the nebulosity now condensed into 

 our comets came originally from the fixed stars. 



But if on the one hand, this equality of distribution of the 

 aphelia in every direction points to the original entrance of the mate- 

 rial into our nebula from without, the other equally remarkable prop- 

 erty of high eccentricity, on the other, points to a similar conclusion. 

 At the same time this coming in of matter from a distance makes 

 possible the growth of the planets near the center of the system, 

 because near perihelion the comets often pass so close to the planets 

 as to have their orbits transformed, and their masses disintegrated 

 and their dust absorbed by the planets. It is by moving against the 

 resistance due to comets, and meteor swarms that the planetary 

 orbits have been rendered so perfectly circular that the Greeks be- 

 lieved that the Deity had chosen the circle for the paths of the 

 planets, because the circle was held by the ancient geometers to be 

 a perfect figure. 



Now what takes place about our sun, in the solar cluster, may 



