8 PHYSICAL SCIENCE 



circumstance directly in favor of, the triumphant advances which 

 have distinguished astronomical science from the epoch of Galileo 

 down to the present day. 



Not less noteworthy than his high estimate of the position of 

 astronomy in his time are Laplace's anticipations of the course of 

 future progress. Our admiration is kindled by the clearness of his 

 vision with respect to ways and means, and by the penetration of 

 his predictions of future discoveries. Advances in sidereal astro- 

 nomy, he rightly thought, would depend chiefly on improvements 

 in telescopes; while advances in dynamical astronomy were to 

 come along with increased precision in the observed places of the 

 members of the solar system and along with the growing perfection 

 of analysis. It is almost needless to say that Laplace's brilliant anti- 

 cipations have been quite surpassed by the actual developments. 

 Observational astronomy has become one of the most delicately 

 perfect of all the sciences; dynamical astronomy easily outstrips all 

 competitors in the perfection of its theories and in the certainty of 

 its predictions; while the newly developed branch of astrophysics 

 supplies the last link in the chain of evidence of the essential unity 

 of the material universe. 



The order of the dimensions and the order of the mass contents 

 of the visible universe, at any rate, have been pretty clearly made 

 out. In addition to the vast aggregate of direct observational evi- 

 dence collected and recorded during the past century, numerous 

 theoretical researches have gone far, also, to interpret the laws which 

 reign in the apparent chaos of the stars. The solar system, with its 

 magnificent subsystems, has been proved to exhibit the type of 

 stellar systems in general. 



In a profound investigation recently published, Lord Kelvin 

 has sought to correlate under the law of gravitation the principal 

 observed data of the visible universe. Assuming this universe to 

 lie within a sphere of radius equal to the distance of a star whose 

 parallax is one thousandth of a second of arc, he concludes that 

 there must be something like a thousand million masses of the mag- 

 nitude of our sun within that sphere. Light traveling at the rate of 

 300,000 kilometers per second would require about six thousand 

 years to traverse the diameter of this universe, and while the aver- 

 age distance asunder of the visible stars is considerably less, it is 

 still of the same order. It is only essential, therefore, to imagine 

 our luminary surrounded by a thousand million such suns, most 

 of which are, in all probability, attended by groups of planets, to 

 get some idea of the quantity of matter within visual range of our 

 relatively insignificant terrestrial abode. And the imposing range of 

 the astronomer's time-scale is perhaps impressively brought home to 

 us when we reflect that a million years is the smallest convenient 



