COMETS. 215 



this order if, as our comet-tracker Hind suspected, 

 its path in our neighbourhood is parabolic, so that 

 either it has an enormously long period of revolution, 

 or came to us across the 'interstellar spaces them- 

 selves how useless is it to set down the array of 

 numbers representing the extension of its path, or the 

 years during which the comet has been voyaging 

 through desert space ! The comets indeed which 

 come from the star-depths and observation renders it 

 all but certain that some have done so cannot in any 

 case have pursued a voyage less than twenty billions 

 of miles in length, and cannot have been less than eight 

 million years upon the road. That, too, was but their 

 latest journey. From the last sun they visited to our 

 own sun, such was their voyage ; but who shall say 

 how many such voyages they had pursued, or how 

 many they will complete after leaving our sun's 

 neighbourhood, before the time comes when some 

 chance brings them near enough to a disturbing planet 

 to cause their path to become a closed one ? And even 

 those comets which are now known to follow a closed 

 path, returning again and again to the neighbourhood 

 of the sun, need only be studied thoughtfully to 

 present similarly startling conceptions. No matter 

 what theory of their origin we adopt, we are brought 

 face to face with the thought of time-intervals so 

 enormous that practically they must be viewed as 

 infinite. If we take the assumption that a comet of this 

 order, which had been travelling on a path of parabolic 

 or hyperbolic nature towards our sun, had been 



