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SCIENCE. 



[N. S. Vol. XX. No. 509. 



advancement; and the indications are that 

 its prestige will long continue. 



But there are spots on every sun; ajid 

 lest some may infer, even hiimorously, as 

 Carlyle did seventy-odd years ago, that 

 our system of the world is ' as good as per- 

 fect,' attention should be called to some 

 noteworthy defects in astronomical data 

 and to some singular obscurities in astro- 

 nomical theory. Here, however, gi'eat 

 caution and brevity are essential to avoid 

 poaching on the preserves of our colleagues 

 of the sections. It may suffice, therefore, 

 to merely mention, under the head of de- 

 fective data, the low precision of the solar 

 parallax, the aberration constant, the 

 masses of the members of the solar system, 

 and the uncertainty of our time unit, 

 already referred to. Two instances, like- 

 wise, which belong to the general field of 

 physics as well, may suffice as illustrations - 

 of obscurities in astronomical theory. 

 Stated in the order of their apparent com- 

 plexity, these obscurities refer to the law of 

 gravitation and to the phenomenon of 

 stellar aberration. Probably both are re- 

 lated, and one may hope that any explana- 

 tion of either Avill throw light on the other. 



So long as no attempt is made to recon- 

 cile the law of gravitation with other 

 branches of physics; progress, up to a cer- 

 tain point, is easy; and probably great ad- 

 vantage has resulted from the fact that 

 dynamical astronomers have not been 

 seriously disturbed by a desire to harmon- 

 ize this law with the more elementary laAvs 

 of mechanics. Perhaps they have uncon- 

 sciously rested on the platform that gravi- 

 tation is one of the 'primordial causes' 

 which are impenetrable to us. There are 

 some indications that even Laplace and 

 Fourier did so rest. But, however this 

 may be, it has grown steadily more and 

 more imperative during the past century 

 to explain gravitation, or to discover the 

 mechanism which provides that the force 



between two widely separated masses is 

 proportional to their product directly and 

 to the square of the distance between them 

 inversely. All evidence seems to indicate 

 that the ether must provide this mechan- 

 ism ; but, strangely enough, so far, the ether 

 has baffled all attempts to reveal the secret. 

 The problem has been attacked also on the 

 purely observational side of the numerical 

 value of the gravitation constant. But the 

 splendid experimental researches for this 

 purpose throw no light on the mechanism 

 in qviestion, and, unfortunately, they bring 

 out values for the constant of a low order 

 of precision. 



With regard to stellar aberration, it 

 must be at once admitted that we have 

 neither an adequate theory nor a pre- 

 cisely determined fact. The astronomer 

 has generally contented himself with the 

 elementary view that aberration is a purely 

 Idnematical phenomenon; that the earth 

 not only slips through the ether without 

 sensible retardation, but that the ether slips 

 through the earth without sensible effects. 

 This difficulty was recognized, in a way, 

 by Young and Fresnel, and, although the 

 subject of elaborate investigation in recent 

 decades, it has proved equally baffling with 

 Newtonian gravitation. As in the case of 

 the latter also, the numerous attempts made 

 to determine the constant of aberration by 

 observational methods have been rewarded 

 by results of only meager precision. Pos- 

 sibly the time has arrived when one may 

 raise the question. Within what limits is 

 it proper to speak of a gravitation constant 

 or of an aberration constant? 



If Ave agree with Laplace that astronomy 

 is entitled to the highest rank among the 

 physical sciences, we can accord nothing 

 short of second place to the sciences of the 

 earth. Most of them are, indeed, inti- 

 mately related to astronomy; and some of 

 them are scarcely less ancient in their 

 origins, less dignified in their objects, or 



