PAR 



PAR 



uppermost is dilated, and called vexillum, 

 or the standard ; the two lateral are con- 

 tracted and parallel, and called al(£, or 

 the wings ; the two lower are contracted, 

 parallel, generally coherent by their an- 

 terior margin, and termed carina, or the 

 keel. 



PAPPUS (TrdTTTroc, the down of the 

 cheek). A botanical term for the feathery 

 appendage which crowns the fruit of 

 many composite plants, being, in fact, a 

 reduced calyx. A familiar instance oc- 

 curs in the down of the dandelion. 



PARA- (Trapd). A Greek preposition 

 "With various meanings, as through, near, 

 about, &c. In some chemical compounds, 

 it denotes near to, and expresses a close 

 alliance between two compounds. 



PARA'BOLA {napafioXri, the Conic 

 Section parabola, so named because its 

 axis is parallel to the side of the cone). 

 A section of a cone by a plane parallel 

 to one of its sides ; it is therefore an open 

 curve, which would spread out wider and 

 wider, if the cone were extended. A 

 simple illustration of the parabola is the 

 curved line described by a body thrown 

 forward, as of a bullet from a gun, a jet 

 of water from a fire-engine, &c. 



Paraboloid. A geometrical solid, formed 

 by the revolution of a parabola round its 

 axis (See Conoid). The term is also 

 applied to two surfaces of the second de- 

 gree, viz. the elliptic paraboloid and the 

 hyperbolic paraboloid, which answer to 

 the curve parabola, 



PARACE'NTRIC VELOCITY. An 

 expression denoting the rate at which a 

 moving body approaches a certain centre, 

 without reference to the rate at which it 

 moves in space. 



PARACEPHALO'PHORA (Trapd, be- 

 side, Ke<pa\t], the head, 0epa), to carry). 

 The name given by De B lain villa to a 

 class of molluscous animals, in which 

 the head is but little distinct from the 

 body, but is always provided with some 

 of the organs of sense. They are divided 

 into the sub-classes dioica, monoica, and 

 hermaphrodita. 



PARADI'SIDiE. The Birds of Para- 

 dise ; a family of the Insessores, or 

 Perchers, characterized by the extraor- 

 dinary development of their feathers. 

 They are confined to New Guinea and 

 the neighbouring islands. See Tenui- 

 rostres. 



PA'RADOX {irapaho^ia, marvellous- 

 ness). A statement which is opposed 

 to general belief, being, at its first enun- 

 ciation, apparently contradictory of some 



established maxim or truth. Thus, " the 

 wise man alone is rich," is a paradox, 

 for the statement would be ridiculed by 

 all, except the wise. 



PA'RAFFIN. Petroline. A particular 

 hydrocarbon produced in the distillation 

 of wood. Its name is derived from pa- 

 rum affinis, denoting its remarkable in- 

 difference to other bodies in a chemical 

 point of view. 



PARALLACTIC INSTRUMENT. 

 Ptolemy's Rules. An astronomical in- 

 strument invented by Ptolemy for deter- 

 mining the moon's parallax, and described 

 in his Almagest. 



PA'RALLAX (TrapdWaf^f, altema- 

 ation). Parallax may be generally defined 

 to be the change of the apparent situ- 

 ation of an object arising from a change 

 of the real situation of the observer. In 

 astronomical language, the word has a 

 more technical meaning, and is restricted 

 to the difference of the apparent positions 

 of any celestial object when viewed from 

 a station on the surface of the earth, 

 and from its centre. The centre of the 

 earth is the general station to which all 

 astronomical observations are referred : 

 but, as we observe from the surface, a 

 reduction to the centre is needed; and 

 the amount of this reduction is called 

 parallax. 



1. Parallax, diurnal and annual. By 

 the preceding paragraph it is plain that 

 the parallax of a body signifies the differ- 

 ence between its apparent place and its 

 true place, or that in which it would be 

 seen, if the observer were situated at the 

 centre to which the motion is referred. 

 When the point of reference is the centre 

 of the earth itself, the change of aspect 

 is called the diurnal parallax ; when it 

 is the centre of the earth's orbit, the 

 change is called the annual parallax, or 

 parallax of the great orb. The latter is 

 the angle under which the semidiameter 

 of the earth's orbit would be seen from a 

 superior planet, or from a fixed star. 



2. Parallax, horizontal. The parallax 

 changes with the planet's distance from 

 the earth, and also with its altitude above 

 the horizon. It is nothing when the 

 planet is in the zenith ; and greatest, the 

 distance .from the earth remaining the 

 same, when the planet is in the horizon. 

 In the latter case, it is called horizontal 

 parallax. Thus, by saying that the 

 moon's horizontal parallax is 58' on a 

 certain day, we mean that the moon's 

 distance is such, that if she were in the 

 horizon of any place, the spectator's de- 



