CELESTIAL MECHANICS 395 



extended as the present. I limit myself, therefore, to reminding you 

 that the lunar theory offers yet unsolved problems to the theoretical 

 astronomer, in spite of the splendid results of the savants named, 

 and in spite of the fact that Newcomb has succeeded in improving 

 Hansen's Lunar Tables, and that at present these represent the 

 observations well. 



I have purposely omitted the names of Jacobi and Hamilton, so 

 well known in celestial mechanics, whose theories have received 

 further development and application from Delaunay, Tisserand, and 

 Hill, and have served Poincare* as a starting-point for his remarkable 

 theories, beginning with his beautiful Prize Memoir and continuing 

 through the M&hodes Nouvelles de la M&anique Ctleste, etc. I do 

 not feel justified in expressing myself with reference to the value 

 and meaning of this last work, for the simple reason that I am not 

 mathematically competent to do so. That Hill's and Poincar6's 

 theories introduce a new epoch, whose fruits the twentieth century 

 will harvest, there seems to exist no doubt. 



The nineteenth century has added a new chapter in celestial mechan- 

 ics, the theory of meteors and comets in their relation to one another. 

 We owe to the clever researches of Schiaparelli, Newcomb, Bredichin, 

 and others the remarkable insight obtained into the motion of these 

 small bodies, which remain individually invisible, except when they 

 penetrate our atmosphere and blaze up, or become visible by being 

 crowded together in the form of comets. 



