1 82 LIGHT SCIENCE FOR LEISURE HOURS. 



to predict the return of comets. How far it may be 

 safe to accept the statements of Apollonius is uncer- 

 tain. He ascribed other powers to the Chaldeans, of 

 which we may fairly doubt their possession for in- 

 stance, the power of predicting earthquakes and floods. 

 In fact, there is so marked a disposition among ancient 

 writers to exaggerate the acquisitions of Chaldean 

 astronomers, that it becomes extremely difficult to dis- 

 tinguish truth from falsehood. Still, there is sufficient 

 evidence of their skill and patience as observers, to 

 render it fully possible that they may have discovered 

 the periodicity of one or two comets. 



But until the rise of modern astronomy, the opinion 

 which was almost universally held respecting comets 

 was that of Aristotle, that they are of the same nature 

 as meteors or shooting-stars, existing either in the air 

 not far above the clouds, or certainly far below the 

 moon. 



The discovery of the periodicity of Halley's comet 

 following quickly on Newton's announcement of the 

 law of gravitation, led astronomers to examine the 

 orbits of all the comets which became visible, with the 

 hope of finding that some of these bodies may be tra- 

 velling in re-entering paths. But inasmuch as none 

 of the brilliant comets of whose appearance records had 

 been preserved seemed to have ever revisited the earth 

 save Halley's alone, while even Halley's travelled in an 

 orbit of enormous extent, an orbit which reached out 

 in space more than three times as far as the orbit of 

 the most distant known planet, astronomers held that 



