ASTRONOMY OF THE NINETEENTH CENTURY. 421 



sun (perihelion) 2 J hours earlier at each revolution. The orbit of the 

 comet lies entirely within the orbit of Jupiter, and nearly in the same 

 plane. The perturbations to which the motion of the comet is liable 

 are, from the enormous mass of Jupiter, excessive. Between 1819 and 

 1822 the perihelion passage was thus retarded for nine days. When 

 all allowance had been made for the perturbations produced by the 

 planets, Encke found that the diminution of the periodic time from 

 i, 213 in 17 89 to i, 211 days in 1838 remained unaccounted for, and 

 he conjectured that it might be due to the resistance of some medium 

 pervading space, and too attentuated to affect the movements of a 

 planet, but capable of retarding the motion of a body composed of 

 material so thin as that of which there is reason to believe a comet is 

 constituted. The announcement of a comet revolving regularly in a 

 short period was a discovery which excited very great interest among 

 astronomers, and each return of Encke's comet has been regularly 

 observed. 



Another periodical comet of much interest is named after BIELA, a 

 Bohemian military officer, who first observed it on February the 2yth, 

 1826. After eight weeks' observation it was found that the orbit was 

 elliptical, and that the comet was identical with one observed in 1772 

 and again in 1805. Its period was calculated to be 2,455 days. In 

 the year 1828 Dr. Olbers announced that in 1832 this comet would 

 be within 20,000 miles of the orbit of the earth, and this announce- 

 ment was the cause of no small degree of popular alarm ; for people 

 in general were not sufficiently scientific to consider that the earth's 

 orbit was a different thing from the earth itself, and it was feared that 

 between our planet and the filmy comet there might occur a collision 

 which would be disastrous to the former. Some persons, who thought 

 themselves more foreseeing than their neighbours, sold their lands and 

 houses, perceiving that property of this kind would certainly become 

 much depreciated in value as the day approached on which the whole 

 planet was expected to be shattered into ruin. They evidently con- 

 sidered that on that last day portable possessions would be more advan- 

 tageous. In order to allay in some degree the alarm of the multitude, 

 some astronomers endeavoured to explain to them that though the comet 

 would on a certain day be not more than 20,000 miles distant from a 

 particular part of the earth's orbit, the earth itself would not reach that 

 point of her orbit until a month after the dreaded comet had passed, 

 and that the two bodies would at no time be nearer to each other than 

 the orbit of Mercury is to that of the earth. Biela's comet escaped 

 observation at its next return to perihelion, on account of its then 

 occupying a part of the heavens near the sun. At the end of 1845 it 

 was seen again nearly at the calculated time, but with a strange difference 

 in its configuration, for it had divided into two distinct comets which 

 pursued their course together. The space by which the two parts of 

 the comet were separated was calculated to be 157,000 miles; but it 



