September 30, 1904.] 



SCIENCE. 



421 



other physical sciences; and that the im- 

 mense distances which separate the celestial 

 bodies, instead of being an obstacle to, are 

 a fortunate 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 esti- 

 mate of the position of astronomy in his 

 time are Laplace's anticipations of the 

 course of future progress. Our admira- 

 tion 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 astron- 

 omy, 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 need- 

 less to say that Laplace's brilliant antici- 

 pations have been quite surpassed by the 

 actual developments. Observational as- 

 tronomy 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 astro- 

 physics 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 evidence 

 collected and recorded during the past cen- 

 tury, 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 sub- 

 systems, has been proved to exhibit the 

 type of stellar systems in general. 



In a profound investigation recently 

 published, Lord Kelvin has sought to corre- 

 late under the law of gravitation the prin- 

 cipal 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 are, he concludes that there must 

 be something like a thousand million masses 

 of the magnitude of our sun within that 

 sphere. Light traveling at the rate of 

 300,000 kilometers per second would re- 

 quire about six thousand yeai-s to traverse 

 the diameter of this universe, and while 

 the average 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 rela- 

 tively 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 unit for 

 recording the life history of a star, while 

 the current events in that history are trans- 

 mitted across the interstellar medium by 

 vibrations which occur at the rate of about 

 six hundred million million times per sec- 

 ond. Measured by its accumulation of 

 achievements, then, the astronomy of to-day 

 fulfils the requirements of a highly devel- 

 oped science. It is characterized by a vast 

 aggregate of accurately determined facts 

 related by theories founded on a small 

 number of hypotheses. In the past it has 

 called forth the two greatest of all sys- 

 tematic treatises, the 'Principia' of Newton 

 and the 'Mecanique Celeste' of Laplace. 

 It has probably done more also than any 

 other science, up to the present time, to 

 illuminate the dark periods during which 

 man has floundered in his struggle for 



