XXIV.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 537 



this occuneiice, Baily assigning the year 6io b.c, but 

 Airy has calculated that the exact day was the 28th of 

 I\Iay, 584 B.C. There can be no doubt that this and other 

 predictions of eclipses attributed to ancient philosophers 

 were due to a knowledge of the IMetonic Cycle, a period of 

 6.585 days, or 223 lunar months, or about ig years, after 

 which a nearly perfect recurrence of the phases and 

 eclipses of the moon takes place ; but if so, Thales must 

 have had access to long series of astronomical records of 

 the Egyptians or the Chaldeans. There is a well-known 

 story as to the happy use wliich Columbus made of the 

 power of predicting eclipses in overawing the islanders of 

 Jamaica who refused him necessary supplies of food for his 

 fleet. He threatened to deprive them of the moon's light. 

 " His threat was treated at first with indifference, but 

 when the eclipse actually commenced, the barbarians vied 

 with each other in the production of the necessary supplies 

 for the Spanish fleet." 



Exactly the same kind of awe which the ancients ex- 

 perienced at the prediction of eclipses, has been felt in 

 modern times concerning the return of comets. Seneca 

 asserted in distinct terms that comets would be found to 

 revolve in periodic orbits and return to sight. The ancient 

 Chaldeans and the Pythagoreans are also said to have 

 entertained a like opinion. But it was not until the age 

 of Newton and Halley that it became possible to calculate 

 the path of a comet in future years, A great comet 

 appeared in 1682, a few years before the first publication of 

 the Prindpia, and Halley showed that its orbit corresponded 

 with that of remarkable comets recorded to have appeared 

 in the years 1531 and 1607. The intervals of time were 

 not quite equal, but Halley conceived the bold idea that 

 this difference might be due to the disturbing power of 

 Jupiter, near which the comet had passed in the interviil 

 1 607- 1 682. He predicted that the comet would return 

 about the end of 1758 or the beginning of 1759, and 

 though Halley did not live to enjoy the sight, it was 

 actually detected on the night of Christmas-day, 1758. 

 A second return of the. comet was witnessed in 1835 

 nearly at the anticipated time. 



In recent times the discovery of N'e])tune has been the 

 most remarkable instance of previ.'iion in astronomical 



