144 POPULAR SCIENCE MONTHLY. 



moves very perceptibly every second, while he must watch the next 

 wheel for several seconds to see any motion. If the time at his disposal 

 is limited, he will not be able to see any motion at all in the weight. 

 But an examination of the machinery will show him that the weight 

 must be falling at a certain rate, and he can compute that, at the end 

 of a certain time, the weight will reach the bottom, and the clock 

 will stop. He can also see that there must have been a point from 

 above which the weight could never have fallen. Knowing the rate of 

 fall, he can compute how long the weight occupied in falling from this 

 point. His final conclusion will be that the clock must in some way 

 have been wound up and set in motion a certain number of hours or 

 days before his inspection. 



If the theory that the heat of the stars is kept up by their slow 

 contraction is accepted, we can, by a similar process to this, compute 

 that these bodies must have been larger in former times, and that there 

 must have been some finite and computable period when they were all 

 nebulas. Not even a nebula can give light without a progressive change 

 of some sort. Hence, within a certain finite period the nebulae them- 

 selves must have begun to shine. How did they begin? This is the 

 unsolvable question. 



The process of stellar evolution may be discussed without consider- 

 ing this question. Accepting as a fact, or at least as a working hypoth- 

 esis, that the stars are contracting, we find a remarkable consistency 

 in the results. Year by year laws are established and more definite con- 

 clusions reached. It is now possible to speak of the respective ages of 

 stars as they go through their progressive course of changes. This 

 subject has been so profoundly studied and so fully developed by Sir 

 William and Lady Huggins that I shall depend largely on their work 

 in briefly developing the subject.* 



At the same time, in an attempt to condense the substance of many 

 folio pages into so short a space, one can hardly hope to be entirely 

 successful in giving merely the views of the original author. The fol- 

 lowing may, therefore, be regarded as the views of Sir William Hug- 

 gins, condensed and arranged in the order in which they present them- 

 selves to the writer's mind. 



There is an infinite diversity among the spectra of the stars; scarcely 

 two are exactly alike in all their details. But the larger number of these 

 spectra, when carefully compared, may be made to fall in line, thus 

 forming a series in which the passage of one spectrum into the next in 

 order is so gradual as to indicate that the actual differences represent, 

 in the main, successive epochs of star life rather than so many funda- 

 mental differences of chemical constitution. Each star may be con- 

 sidered to go through a series of changes analogous to those of a human 



* Publications of Sir William Huggins's Observatory, Vol. I; Lcnion, 1899. 



