N E 15 



principles, found the density of the earth to be to that of water as 5.48 

 : 1. The internal parts of the earth are .*. much denser than those at 

 the surface ; though in what manner the dense parts are disposed of must 

 he uncertain. 



MOUNTAIN, correction for height of. See Refraction. 



MOUNTAINS, visibility of See Refraction. 



N 



NEBULA, and Clusters of Stars. (Herschel.) 



On Herschel's Catalogue of new Nebuke, and clusters of stars. 



The telescope used was a Newtonian reflector of 20 feet focal length, 



and 18 inches aperture. The sweeping power was 157. The field of 



view 15'. 4". 



The Nebulae are divided into classes like the double stars. (See Stars 

 double. ) Thus in the 1st class, the degree of brightness of the Nebula 

 has been the leading feature, as most likely to point out those which or- 

 dinary instruments may be expected to reach. The 1 st class .'. contains 

 the brightest of them j the 2d those which shine but with a feeble light ; 

 and in the 3d are placed all the very faint ones. It should be observed, 

 that what Herschel calls bright, or very bright among those of the first 

 class, are commonly less distinguishable than what Messier, in his Cata- 

 logue des Nebules (given in Wollaston) calls faint ; on account of the su- 

 periority in the instruments of the former observer. 



Besides this general division, there are added a 4th and 5th class, which 

 contain Nebulae deserving a more particular description. The 4th class 

 contains Planetary Nebulae, i.e. stars with burs, with milky chevelure, 

 with short rays, remarkable shapes, &c. The 5th class very large Ne- 

 bulae. 



The 6th, 7th, and 8th classes contain clusters of stars sorted according 

 to their apparent compression, like the Catalogue of double stars, so that 

 the closest and richest clusters take up the first or 6th class ; the bright- 

 est, largest, and pretty much compressed ones, the second or 7th class ; 

 and those which consist only of scattered and less collected large stars, 

 are put into the last. 



Note. When a superior power and telescope increase the brightness 

 of a nebula, but at the same time only make the tinge of it more nU 

 192 



