A HISTORY OF SCIENCE 



the true meaning of their condition. It was not even 

 known how closely some of them are enchained until 

 the German astronomer Encke, in 1822, showed that 

 one which he had rediscovered, and which has since 

 borne his name, was moving in an orbit so contracted 

 that it must complete its circuit in about three and 

 a half years. Shortly afterwards another comet, re- 

 volving in a period of about six years, was discovered 

 by Biela, and given his name. Only two more of these 

 short-period comets were discovered during the first half 

 of last 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 

 adapted 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 

 being 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 



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