PROPER MOTION OF THE STARS. 249 



fr)m each other, and the configuration of the constellations 

 themselves, cannot in long periods remain the same. The 

 Southern Cross will not always shine in the heavens exactly 

 in its present form ; for the four stars of which it consists 

 move with unequal velocity in different paths. How many 

 thousand years will elapse before its total dissolution, cannot 

 be calculated. In the relations of space and the duration of 

 time, no absolute idea can be attached to the terms great and 

 small. 



In order to comprehend under one general point of view the 

 changes that take place in the heavens, and all the modifications 

 which in the course of centuries occur in the physiognomic 

 character of the vault of heaven, or in the aspect of the firma- 

 ment from any particular spot, we must reckon as the active 

 causes of this change: (1), the precession of the equinoxes 

 and the nutation of the earth's axis, by the combined opera- 

 tion of which new stars appear above the horizon, and others 

 become invisible; (2), the periodical and non-periodical varia- 

 tions in the brightness of many of the fixed stars ; (3), the 

 sudden appearance of new stars, of which a few have 

 continued to shine in the heavens; (4), the revolution of 

 telescopic double stars round a common centre of gravity. 

 Among these so-called fixed stars which change slowly and 

 unequally both in the intensity of their light and in their 

 position, twenty principal planets move in a more rapid 

 course, five of them being accompanied by twenty satellites. 

 Besides the innumerable, but undoubtedly rotatory fixed 

 stars, forty moving planetary bodies have up to this time 

 (October, 1850) been discovered. In the time of Copernicus 

 and of Tycho Brahe, the great improver of the science of 

 observation, only seven were known. Nearly two hundred 

 comets, five of which have short periods of revolution and are 

 interior, or, in other words, are inclosed within those of the 



