198 THE POPULAR SCIENCE MONTHLY. 



of the solar atmosphere. Lockyer considers that his researches lend 

 great probability to this view. The whole subject is one of intense 

 interest, and we may rejoice that it is occupying the attention, not 

 only of such men as Abney, Dewar, Hartley, Liveing, Roscoe, and 

 Schuster in our own country, but also of many foreign observers. 



When geology so greatly extended our ideas of past time, the con- 

 tinued heat of the sun became a question of greater interest than ever. 

 Helmholtz has shown that, while adopting the nebular hypothesis, we 

 need not assume that the nebulous matter was originally incandescent ; 

 but that its present high temperature may be, and probably is, mainly 

 due to gravitation between its parts. It follows that the potential 

 energy of the sun is far from exhausted, and that with continued 

 shrinking it will continue to give out light and beat, with little, if 

 any, diminution for several million years. 



Like the sand of the sea, the stars of heaven have ever been used 

 as effective symbols of number, and the improvements in our methods 

 of observation have added fresh force to our original impressions. 

 We now know that our earth is but a fraction of one out of at least 

 75,000,000 worlds. But this is not all. In addition to the luminous 

 heavenly bodies, we can not doubt that there are countless others, in- 

 visible to us from their greater distance, smaller size, or feebler light ; 

 indeed, we know that there are many dark bodies which now emit no 

 light, or comparatively little. Thus, in the case of Procyon, the exist- 

 ence of an invisible body is proved by the movement of the visible 

 star. Again, I may refer to the curious phenomena presented by 

 Algol, a bright star in the head of Medusa. This star shines without 

 change for two days and thirteen hours ; then, in three hours and a 

 half, dwindles from a star of the second to one of the fourth magni- 

 tude ; and then, in another three and a half hours, reassumes its origi- 

 nal brilliancy. These changes seem certainly to indicate the presence 

 of an opaque body, which intercepts at regular intervals a part of the 

 light emitted by Algol. 



Thus the floor of heaven is not only "thick inlaid with patines of 

 bright gold," but studded also with extinct stars ; once, probably, as 

 brilliant as our own sun, but now dead and cold, as Helmholtz tells us 

 that our sun itself will be, some seventeen million years hence. 



The connection of astronomy with the history of our planet has 

 been a subject of speculation and research during a great part of the 

 half-century of our existence. Sir Charles Lyell devoted some of the 

 opening chapters of his groat work to the subject. Haughton has 

 brought his very original powers to bear on the subject of secular 

 changes in climate, and CrolPs contributions to the same subject are 

 of great interest. Last, but not least, I must not omit to make men- 

 tion of the series of massive memoirs (I am happy to say, not yet 

 nearly terminated) by George Darwin on tidal friction, and the influ- 

 ence of tidal action on the evolution of the solar system. I may, per- 



