THE UNITY OF PHYSICAL SCIENCE 9 



unit for recording the life-history of a star, while the current events 

 in that history are transmitted across the interstellar medium by 

 vibrations which occur at the rate of about six hundred million 

 million times per second. Measured by its accumulation of achieve- 

 ments, then, the astronomy of to-day fulfills the requirements of a 

 highly developed 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 systematic 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 advance- 

 ment; and the indications are that its prestige will long continue. 



But there are spots on every sun; and lest some may infer, even 

 humorously, as Carlyle did seventy-odd years ago, that our system 

 of the world is "as good as perfect," attention should be called to 

 some noteworthy defects in astronomical data and to some singular 

 obscurities in astronomical theory. Here, however, great caution 

 and brevity are essential to avoid poaching on the preserves of our 

 colleagues of the Sections. It may suffice, therefore, merely to 

 mention, under the head of defective 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, likewise, which belong to the general field 

 of physics as well, may suffice as illustrations of obscurities in astro- 

 nomical theory. Stated in the order of their apparent complexity, 

 these obscurities refer to the law of gravitation and to the phenome- 

 non of stellar aberration. Probably both are related, and one may 

 hope that any explanation of either will throw light on the other. 



So long as no attempt is made to reconcile the law of gravitation 

 with other branches of physics, progress, up to a certain point, is easy; 

 and probably great advantage has resulted from the fact that dynam- 

 ical astronomers have not been seriously disturbed by a desire to 

 harmonize this law with the more elementary laws of mechanics. 

 Perhaps they have unconsciously 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. However this may be, it has grown steadily more and more 

 imperative during the past century to explain gravitation, or to dis- 

 cover 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 mechanism; 

 but, strangely enough, so far, the ether has baffled all attempts to 

 reveal the secret. The problem has been attacked also on the purely 



