182 PROGRESS OF SCIENCE IN THE CENTURY. 



tion; but activity continues on each of these lines, 

 and it may be more accurate to say that the suc- 

 cession of astronomical systems Ptolemaic, Co- 

 pernican, Kepplerian, Newtonian, etc., implies 

 mainly a progress in the lucidity, validity, brevity, 

 and universality of descriptive formulce. 



APPLICATIONS OF THE GKAVITATION-FOKMULA. 



A great part of astronomy is concerned with appli- 

 cations of the gravitation-formula to the phenomena 

 of the heavens; another department has to do with 

 topographical relations, with mapping out positions 

 and orbits ; while a third kind of enquiry deals with 

 the physical and chemical nature of the celestial 

 bodies. Laplace, Bradley, and Herschel may be 

 named as representative great masters in these 

 three departments, which have been not very hap- 

 pily distinguished as " gravitational," " observa- 

 tional," and " descriptive." Adopting this classifica- 

 tion, Mr. Berry notes in his Short History of As- 

 tronomy * that " gravitational astronomy and exact 

 observational astronomy have made steady progress 

 during the nineteenth century, but neither has been 

 revolutionised, and the advances made have been 

 to a great extent of such a nature as to be barely 

 intelligible, still less interesting, to those who are 

 not experts. . . . Descriptive astronomy, on the 

 other hand, which can be regarded as being almost 

 as much the creation of Herschel as gravitational as- 

 tronomy is of Newton, has not only been greatly de- 

 veloped on the lines laid down by its founder, but 

 has received chiefly through the invention of spec- 

 trum analysis extensions into regions not only un- 

 thought of, but barely imaginable a century ago." 

 * P. 355. 



