BIELA'S COMET. 149 



very little cohesion, and in which, the attractive power of 

 the nucleus is exceedingly small. 



Professor Santini, by employing Plantamour's elements, 

 had computed that this comet would arrive at its peri- 

 helion in 1852, Sept. 28.72, Berlin mean time. An error, 

 however, has been detected in Plantamour's computations, 

 which being corrected, reduces the time of perihelion 

 passage to Sept. 25.26. The mean time of passage of the 

 two nuclei was found to be Sept. 23.36 ; that is, the comet 

 reached its perihelion nearly two days earlier than the 

 time computed from observations at the preceding return. 

 Encke's comet exhibits a similar peculiarity, and this fact 

 is accounted for by supposing that a thin ethereal me- 

 dium pervades the planetary spaces, sufficiently dense to 

 produce some impression upon a comet, but incapable of 

 exercising any sensible influence on the movements of 

 the planets. 



The Imperial Academy of Sciences of St. Petersburg, 

 has proposed the theory of Biela's comet as a prize prob- 

 lem, the memoirs to be presented on the 1st of August, 

 1857. The Academy demands a rigorous investigation 

 of the elements of the orbit described by the center of 

 gravity of the comet, founded on a discussion of all the 

 observations from 1772 to 1852. 



