184 COSMOS. 



remain invisible, indicates the existence of many thousands. 

 We except the aerolites or meteoric asteroids, as their nature 

 is still enveloped in great obscurity. Among the comets, 

 those must be distinguished whose orbits have been calcula- 

 ted by astronomers, and such of which there are only incom- 

 plete observations, or mere indications recorded. As, accord- 

 ing to Galle's last accurate enumeration, 178 had been cal- 

 culated up to the year 1847. so it may be admissible to adopt 

 as the total number, with those which have been merely in 

 dicated,the assumption of six or seven hundred observed com- 

 ets. "When the Comet of 1682, predicted by Halley, appeared 

 again in 1759, it was considered very remarkable that three 

 comets should be visible in the same year. At the present 

 time, the investigation of the heavens is carried on simultane- 

 ously at several parts of the globe, and with such energy, 

 that in each of the years 1819, 1825, and 1840, four were 

 discovered and calculated; in 1826, five; and in 1846. even 

 eight. 



Of comets visible with tfoe naked eye, more have been ob- 

 served recently than during the latter part of the previous 

 century ; but among them, those which have a great brill- 

 iancy in the head and tail still remain, on account of their 

 unfrequency, remarkable phenomena. It will not be with- 

 out interest to enumerate how many comets, visible in Europe 

 to the naked eye, have appeared during the last centuries.* 

 The epoch in which they were most numerous was the six- 

 teenth century, during which twenty-three such comets were 

 seen. The seventeenth numbered twelve, and of these only 

 two during its first half. In the eighteenth century only eight 

 appeared, but nine during the first fifty years of the nineteenth 

 century. Among these, the most beautiful were those of 

 1807, 1811, 1819, 1835, and 1843. In earlier ages ; thirty 

 or forty years have frequently passed without such a spec- 

 tacle presenting itself in a single instance. In the years, 

 however, during which comets seldom appear, there may be 

 a number of large comets whose perihelia are situated be- 

 yond the orbits of Jupiter and Saturn. Of the telescopic 

 comets, there are at the present time, upon an average, at 

 least two or three discovered annually. In three successive 

 months (1840) Galle discovered three new comets ; from 1764 

 to 1798, Messier discovered twelve ; from 1801 to 1827, Pons 

 discovered twenty-seven. Thus Kepler's expression as to the 



* Iu the seven half centuries from 1500 to 1850, altogether 52 comets 

 have appeared which were visible to the naked eye ; in'separate succes- 



