THE STORY OF NINETEENTH-CENTURY SCIENCE 



so contracted that it must complete its circuit in about 

 three and a half years. Shortly afterwards another 

 comet, revolving in a period of about six years, was dis- 

 covered by Biela, and given his name. Only two more 

 of these short-period comets were discovered during our 

 first half -century, but latterly they have been shown to 

 be a numerous family. Nearly twenty are known which 

 the giant Jupiter holds so close that the utmost reach of 

 their elliptical tether does not let them go beyond the 

 orbit of Saturn. These aforetime wanderers have adapt- 

 ed themselves wonderfully to planetary customs, for all 

 of them revolve in the same direction with the planets, 

 and in planes not wide of the ecliptic. 



Checked in their proud hyperbolic sweep, made cap- 

 tive in a planetary net, deprived of their trains, these 

 quondam free lances of the heavens are now mere 

 shadows of their former selves. Considered as to mere 

 bulk, they are very substantial shadows, their extent be- 

 ing measured in hundreds of thousands of miles ; but 

 their actual mass is so slight that they are quite at the 

 mercy of the gravitation pulls of their captors. And 

 worse is in store for them. So persistently do sun and 

 planets tug at them that they are doomed presently to 

 be torn into shreds. 



Such a fate has already overtaken one of them, under 

 the very eyes of the astronomers, within the relatively 

 short period during which these ill-fated comets have 

 been observed. In 1832 Biela's comet passed quite near 

 the earth, as astronomers measure distance, and in doing 

 so created a panic on our planet. It did no greater harm 

 than that, of course, and passed on its way as usual. 

 The very next time it came within telescopic hail it was 

 seen to have broken into two fragments. Six years later 



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