290292] 



Orbits of Comets 



373 



two of these have been seen at two returns, that known as 

 Olbers's comet in 1815 and 1887, and the Pons-Brooks 

 comet in 1 8 1 2 and 1 884. Fourteen other comets with periods 

 varying between 3^ years (Encke's) and 14 years (Tuttle's), 

 have been seen at more than one return ; about a dozen 

 more have periods estimated at less than a century ; and 

 20 or 30 others move in orbits that are decidedly elliptic, 

 though their periods are longer and consequently not known 



1898 



FIG. 87. The path of Halley's comet. 



with much certainty. Altogether the paths of about 230 

 or 240 comets have been computed, though many are 

 highly uncertain. 



292. In the theory of the tides the first important advance 

 made after the publication of the Mecanique Celeste was 

 the collection of actual tidal observations on a large scale, 

 their interpretation, and their comparison with the results 

 of theory. The pioneers in this direction were Lubbock 

 ( 286), who presented a series of papers on the subject 



