122 THE EVOLUTION OF SATELLITES. 



Mr. See explains the high, degree of eccentricity in these binary orbits 

 by the influence of tidal friction. The tide undoubtedly operates under 

 conditions which give it a wide scope, when two large masses are 

 revolving about one another; and tidal friction is the only known cause 

 capable of converting a nearly circular orbit into a very eccentric one. 

 But this does not afford quite sufficient reason for the acceptance of 

 the theory, for the assumption is involved that orbits now very eccen- 

 tric were formerly nearly circular. Mr. See accordingly also puts for- 

 ward a theory of the method by which double stars originated, and to 

 this I shall return later. 



At first it may not be easy to see how the truth of this theory of the 

 origin of the eccentricity is to be tested: it may be worth while, there- 

 fore, to point out the direction which to me, at least, seems the most 

 XDromising in the search for confirmation or refutation. 



It is thought by some spectroscopists that the ages of the stars are 

 already determinable by the nature of their spectra, and although the 

 theories which have been advanced do not meet with universal accept- 

 ance, yet they foreshadow views which may some day be universally 

 accepted. It has been plausibly contended that stars which are young 

 in their evolution must consist of incandescent gas, and must there- 

 fore have 8]3ectra furrowed by bright lines; later in their histories they 

 are supposed to become more condensed and to give continuous spectra. 

 Now, if from theories of this kind we could ascertain the stage of evo- 

 lution of a binary system, we should be able to form a judgment of the 

 truth of the tidal theory; for the younger systems should present 

 smaller eccentricity or orbit than the older ones, and the i)erio(lic times 

 in the young systems should be shorter on the whole than those in the 

 old ones. Delicate spectroscopic measurements make it theoretically 

 possible to determine the relative masses of a binary pair, but hitherto 

 the measurements have been carried to a successful issue in only a very 

 few cases. It is to be expected, however, that the number of known 

 masses will be largely multiplied in the future. A small star must cool 

 more rapidly than a large one, and should present the appearance of 

 greater age. We may hope, then, in time, not only to attain to crucial 

 tests of spectroscopic theories of age, but also to be furnished with the 

 materials for judging of the truth of the tidal theory of evolution of 

 stellar systems. 



The second and yet more speculative branch of Mr. See's theory is 

 that which concerns the mode of origin of binary systems. Man must 

 ultimately be brought face to face with the incomprehensibility of the 

 origi^ of matter and motion, but this consideration will never prevent 

 him from peering into the past to the utmost of his powers. It is 

 certain that the stars are continually undergoing cliange, and it seems 

 impossible to accept their existence as an ultimate fact not susceptible 

 of explanation. Thus we feel bound to trace their histories back 

 to a past so remote that their preceding course of evolution becomes 

 inscrutable. 



