104 CELESTIAL PHENOMENA. 



one-sixth of the sun's diameter from the surface of that 

 body; an approach eight or nine times nearer than Lsxell's 

 comet made to the Earth, and equal to only seven-tenths of 

 the Moon's distance from the Earth. Owing to the feeble- 

 ness of the light of distant comets, perihelia which take 

 place beyond the orbit of Mars can very seldom be observed 

 by the inhabitants of the earth ; and of all the comets wliich 

 have been computed hitherto, that of 1729 is the only one 

 which has its perihelion between the orbits of Pallas and 

 Jupiter ; it was even observed beyond the latter planet. 



Since a degree of scientific knowledge, sometimes 

 sound, but oftener vague and partial, has extended into 

 wider circles of social life, the fears of possible evils 

 threatened by comets have perhaps rather increased in 

 weight as their direction has become more definite. The 

 certainty that within the known planetary orbits there are 

 comets which visit our regions at short intervals, the con- 

 siderable perturbations which their paths undergo by the 

 attractions of Jupiter and Saturn, whereby apparently harm- 

 less bodies might be converted into dangerous ones, the 

 intersection of the Earth's orbit by that of Biela's comet, 

 the recognition of the existence of a cosmical ether, wliich 

 as a retarding medium tends to contract all orbits, the 

 differences between individual comets, which allow of the sup- 

 position of considerable diversity in the mass of the nucleus, 

 are motives of alarm which by their number and variety 

 are fully equivalent to the vague fears which prevailed in 

 former centuries of "fiery swords," and "long haired 

 blazing stars," threatening universal conflagration. The 

 tranquillising considerations which, on the other hand, 

 have been derived from the calculus of probabilities, being 



