10 PHYSICAL SCIENCE 



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 question, 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 precisely determined fact. 

 The astronomer has generally contented himself with the elementary 

 view that aberration is a purely kinematical 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 re- 

 warded by results of only meagre precision. Possibly 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 con- 

 stant? 



If we 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, 

 intimately related to astronomy; and some of them are scarcely 

 less ancient in their origins, less dignified in their objects, or less 

 perfect in their theories. Primarily, also, it should be observed, geo- 

 physics is not simply a part of, but is the very foundation of, astro- 

 nomy; for the earth furnishes the orientation, the base-line, and the 

 timepiece by means of which the astronomer explores the heavens. 

 Geology, likewise, in the broader sense of the term, as we are now 

 coming to see, is a fundamental science not only by reason of its 

 interpretations of terrestrial phenomena, but also by reason of its 

 parallel interpretations of celestial phenomena; for there is little 

 doubt that in the evolution of the earth we may read a history which 

 is in large degree typical of the history of celestial bodies. In any 

 revised estimate, therefore, of the relative rank of the physical 

 sciences, while it would be impossible to lower the science of the 

 heavens, it would appear essential to raise the sciences of the earth 

 to a much higher plane of importance than was thought appropriate 

 by our predecessors of a hundred years ago. 



As with physics, chemistry, and astronomy, the wonderful progress 

 of the nineteenth century in geophysical science has been along 

 lines converging towards the more recondite properties of matter. 

 All parts of the earth, through observation, experiment, induction, 

 and deduction, have yielded increasing evidence of limited unities 

 amid endless diversities. Adopting the convenient terminology of 



