100 CELESTIAL PHENOMENA. 



may therefore suggest the possibility of a catastrophe, 

 if we may apply that term to the extraordinary phse- 

 nomenon of a collision, which has not occurred within 

 historical times, and of which we cannot with certainty 

 predict the consequences. Granting that small masses 

 possessed of enormous velocity may exert a notable force, 

 Laplace, after showing that the mass of the comet of 1770 

 is probably less than >, ' of that of the earth, has assigned 

 with a certain degree of probability, for the average mass 

 of comets, a quantity far below even To-oVo-o f the 

 earth's mass, or only about l a ! - of that of the moon ( 54 ). 

 The passage of Biela's comet across the earth's orbit must 

 not of course be confounded with its proximity to, or 

 encounter with, our globe itself. "When this passage took 

 place on the 29th of October, 1832, the earth was a full 

 month in time from the point of intersection of the two 

 paths. The orbits of Encke's and Biela's comets also inter- 

 sect each other ; and it has been justly remarked, that in the 

 course of the many perturbations which such small bodies 

 suffer from the greater planetary masses, there is a possibility 

 of their meeting ( 55 ) ; and that should this take place about 

 the middle of the month of October, the inhabitants of the 

 Earth might behold the extraordinary spectacle of an encoun- 

 ter between two cosmical bodies, and possibly of their mutual 

 penetration and amalgamation, or of their destruction by 

 exhausting emanations. Such events, the consequences either 

 of deflection produced by disturbing masses, or of originally 

 intersecting orbits, may have taken place frequently in 

 the course of millions of years, and in the vast extent of 

 immeasurable space ; they would, however, be isolated occur- 

 rences, having as little general influence on other cosmical 



