194 COSMICAL ARRANGEMENTS. 



revolution about the sun is much less than that of 

 the comets which have excited most notice ; for 

 while they appear only at long intervals of years, 

 the body of which we are now speaking returns 

 to its perihelion every 1208 days, or in about 

 three years and one-third. Another observable 

 circumstance in this singular body, is its extreme 

 apparent tenuity : it appears as a loose indefinitely 

 formed speck of vapour, through which the stars 

 are visible with no perceptible diminution of their 

 brightness. This body was first seen by Mechain 

 and Messier, in 1786,* but they obtained only 

 two observations, whereas three, at least, are 

 requisite to determine the path of a heavenly 

 body. Miss Herschel discovered it again in 

 1795, and it was observed by several European 

 astronomers. In 1805 it was again seen, and 

 again in 1819. Hitherto it was supposed that 

 the four comets thus observed were all different ; 

 Encke, however, showed that the observations 

 could only be explained by considering them as 

 returns of the same revolving body ; and by 

 doing this, well merited that his name should be 

 associated with the subject of his discovery. The 

 return of this body in 1822, was calculated 

 beforehand, and observed in New South Wales, 

 the comet being then in the southern part of the 

 heavens ; but on comparing the calculated and 



* Airy on Encke's Comet, p. 1. note. 



