COMETS. 537 



?. those must be distinguished whose orbits have been 

 calculated by astronomers ; and such of which there are only 

 incomplete observations, or mere indications recorded. As, 

 according to Galle's last accurate enumeration, 178 had been 

 calculated up to the year 1847, so it may be admissible to 

 adopt as the total number, with those which have been 

 merely indicated, the assumption of six or seven hundred 

 observed comets. When the Comet of 1682, predicted by 

 Ilalley, appeared again in 1759, it was considered very re- 

 markable that three comets should be visible in the same 

 year. At the present time, the investigation of the heavens 

 is carried on simultaneously 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 the naked eye, more have been 

 observed recently than during the latter part of the previous 

 century ; but among them those which have a great brilliancy 

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

 quency, remarkable phenomena. It will not be without 

 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 

 sixteenth century, during which 23 such comets were seen. 

 The seventeenth numbered 12, and of these only 2 during its 

 first half. In the eighteenth century only 8 appeared, but 9 

 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 spectacle presenting itself in a 

 single instance. In the years, however, during which comets 

 seldom appear, there may be a number of large comets whose 



6 In the seven half centuries from 1500 to 1850, altogether 

 52 comets have appeared which were visible to tlie naked eye; 



