174 DISCOVERY CH. 



that three comets, which appeared in 1531, 1607, and 

 1682, had practically the same path or orbit, and he 

 concluded that they were really one and the same body 

 travelling around the sun in a period of about seventy- 

 five years. He predicted, therefore, that the comet 

 would appear again in the year 1758, or thereabouts ; 

 that is, seventy-six years after 1682. He knew he would 

 not be alive to see if the prediction was fulfilled, but he 

 expressed the hope that, when the comet was seen, 

 posterity would remember the prediction had been 

 made by an Englishman. This was the first prediction 

 of the periodic return of a comet, and Halley's boldness 

 in making it was justified completely. The comet, 

 which was anxiously awaited, appeared in 1758 and 

 again in 1835 and 1910. By examining old records, 

 Halley's comet, as it is properly named, has been traced 

 back in steps of about seventy-five years to the year 

 240 B.C. 



Newton showed that the law of gravitation was suffi- 

 cient to account for the motion of the comet of 1680 

 to which he applied it ; and since then the paths of 

 hundreds of comets have been calculated on the same 

 principle. Most comets are unannounced visitors to 

 the solar system, called from the depths of space by 

 the attractive influence of the sun, but following paths 

 which carry them away again into the outer darkness. 

 Some, however, like Halley's comet, tra verse orbits which 

 are closed curves, and these return after an interval 

 which may be reckoned in years or hundreds of years. 

 But the path of every comet of which sufficient observa- 

 tions are available is calculated upon the basis of 

 the law of gravitation. The apparently adventitious 

 motions of these bodies are thus reducible to perfect 

 law and order ; and the decline of the superstitious 



