352 MASSES OF COMETS. SECT. XXXVI. 



diameter, like that of the comet of 1811, which ap- 

 peared to Sir William Herschel like a luminous point 

 in the middle of the nebulous matter. The nuclei, 

 which seemed to be formed of the denser strata of that 

 nebulous matter in successive coatings, are sometimes 

 of great magnitude. Those comets which came to the 

 sun in the years 1799 and 1807, had nuclei whose di- 

 ameters measured 180 and 275 leagues respectively, 

 and the second comet of 1811 had a nucleus of 1350 

 leagues in diameter. 



It must however be stated, that as comets are gene- 

 rally at prodigious distances from the earth, the solid 

 parts of the nuclei appear like mere points of light, so 

 minute that it impossible to measure them with any 

 kind of accuracy, so that the best astronomers often 

 differ in the estimation of their size, by one-half of the 

 whole diameter. The transit of a comet across the sun 

 would afford the best information with regard to the 

 nature of the nuclei. It was computed that such an 

 event was to take place in the year 1827 ; unfortunately 

 the sun was hid by clouds from the British astronomers, 

 but it was examined at Viviers and at Marseilles at the 

 time the comet must have been projected on its disc, 

 but no spot or cloud was to be seen, so that it must 

 have had no solid part whatever. The nuclei of many 

 comets which seemed solid and brilliant to the naked 

 eye have been resolved into mere vapor by telescopes 

 of high powers ; in Halley's comet there was no solid 

 part at all. 



The nebulosity immediately round the nucleus is so 

 diaphanous that it gives little light ; but at a small dis- 

 tance the nebulous matter becomes suddenly brilliant, 

 so as to look like a bright ring round the body. 

 Sometimes there are two or three of these luminous 

 concentric rings separated by dark intervals, but they 

 are generally incomplete on the part next the tail. 



These annular appearances are an optical effect, 

 arising from a succession of envelops of the nebulous 

 matter with intervals between them, of which the first 

 is sometimes in contact with the nucleus and sometimes 

 not. The thickness of these bright diaphanous coatings 

 in the comets of 1799 and 1807 were about 7000 and 



