N AV 



NEC 



prismatic zeolite or mesotype, occurring 

 in chalk-stone porphyry in Wurtemberg 

 and Bohemia, &c. 



NATRON. Native carbonate of soda. 

 There are two kinds, the common and 

 the radiated. See Natrium. 



NATURAL. A character in music, 



marked □, employed to make a sharp 

 note a semitone lower, and a flat note a 

 semitone higher ; or, in other words, to 

 restore to the scale of the natural key of 

 C any note which had been made flat or 

 sharp. 



NATURAL ORDERS OF PLANTS. 

 That arrangement of plants in which 

 groups are formed by the association 

 together of those genera, which have the 

 greatest resemblance one to another in 

 all their characters taken together. Na- 

 ture has pointed out this plan in several 

 well-marked groups, and it is the object 

 of botanists to extend the principle to all 

 the individuals of the vegetable kingdom. 



NATURAL PHILOSOPHY. The 

 science which teaches the nature and 

 properties of matter, the motions of 

 bodies, their connexions with, and their 

 influence upon, one another. The sub- 

 ject is distinguished into the two branches 

 of Mechanical Philosophy and Chemistry. 



NAUCUM. An old Latin term ap- 

 plied by botanists to the exterior coat of 

 the drupe ; it is soft and fleshy, and 

 separable from the interior, hard, and 

 bony coat, which is called the endocar- 

 pium, or stone. Gsertner applied the 

 term nauca to seeds which have a very 

 large hilum, as that of the horse-chest- 

 nut. 



NAUTICAL DISTANCE. In navi- 

 gation, this term denotes the line which 

 makes with the meridian lines the angle 

 called the course. In other words, the 

 rhumb line intercepted between any two 

 places through which the rhumb line 

 passes, is their nautical distance. 



NAUTI'LID^. The sixth family of 

 Lamarck's polythalamous cephalopods, 

 named from the genus nautilus. It forms 

 the fifth family of the Polythalamaceae 

 of De Blainville. 



NAU'TILITES. Fossil shells, analo- 

 gous to the nautilus, found in various 

 strata, from the oldest limestones and 

 sandstones of the Silurian system, to 

 those covering the chalk. 



NAVIGATION {navigo, to manage a 



ship). The science which teaches the 



mariner how to conduct his ship from 



one port or place to any other. It con- 



231 



sists in the practical application of a 

 branch of mathematics, and of a branch 

 of astronomy, which owes its name to 

 this application. 



NEACOMIAN SYSTEM. A geolo- 

 gical formation comprising the green 

 sand system, and divided into the lower 

 green sand — gait, or Cambridge clay — and 

 upper green sand. The last two are con- 

 sidered by some writers as a part of the 

 Cretaceous system. 



NEBULA (dim. of nubes, a cloud). A 

 dusky speck in the heavens, exhibiting 

 a dim and cloudy light, scarcely visible 

 to the naked eye, and supposed to con- 

 sist of a cluster of stars closely connected 

 in the most beautiful arrangement, and 

 at such an inconceivable distance from 

 our system, that each of them is con- 

 jectured to be composed of several thou- 

 sand distinct luminous bodies. Nebulae 

 have been distinguished by Sir W. 

 Herschel into — 



1. Clusters of stars, in which the stars 

 are clearly distinguishable ; and these, 

 again, into globular and irregular clusters. 



2. Resolvihle nebulce, or such as excite 

 a suspicion that they consist of stars, and 

 which any increase of the optical powers 

 of the telescope may be expected to re- 

 solve into distinct stars. 



3. Nebula, properly so called, in which 

 there is no appearance whatever of stars ; 

 which, again, have been subdivided into 

 subordinate classes, according to their 

 brightness and size. 



4. Planetary nebulce, having, as their 

 name imports, exactly the appearance of 

 planets, and, whatever be their nature, 

 of enormous magnitude. 



5. Stellar nebulis, of a round or oval 

 figure, increasing more or less in density 

 towards the central point : they differ 

 extremely, however, in this respect, the 

 condensation in some being slight and 

 gradual, in others great and sudden, so 

 as to represent a star with a slight burr 

 round it. 



6. Nebulous stars, presenting the ap- 

 pearance of sharp and brilliant stars sur- 

 rounded by a perfectly circular disc, or 

 atmosphere of faint light, in some cases 

 dying away on all sides by insensible 

 gradations ; in others, almost suddenly 

 terminated. 



NECESSARY. " This word is used 

 as the contrary to 'impossible' in all its 

 senses, and is, of course, liable to a cor- 

 responding ambiguity. Thus, it is 

 * mathematically necessary ' that two 

 sides of a triangle should be greater than 



