OST 



OVA 



prising the locusts, grasshoppers, crickets, 

 &c. ; having their anterior wings coria- 

 ceous and overlapping each other; the 

 posterior partly coriaceous, partly mem- 

 branous, and folded longitudinally when 

 at rest. 



O'RTIVE AMPLITUDE (ortus, a 

 rising). An astronomical term, denoting 

 the arc of the horizon intercepted be- 

 tween the point where a star rises and 

 the east point. Hence it is also called 

 eastern amplitude. 



ORY'CTOGNO'SY {hpvKToi, dug up, 

 7vt5o-tf, knowledge). Oryctology. That 

 branch of science which relates to fossil 

 organic remains. The term is sometimes 

 applied to that branch of mineralogy 

 which relates to the classification of 

 minerals. 



OSCILLA'TION {oscillum, an image 

 hung on ropes, and swung up and 

 down in the air). The swinging or vi- 

 bratory motion by which a body, when 

 made to change its place, returns in a 

 direction opposite to that in which it 

 first moved. The motion of a pendulum, 

 or that of a string of any musical instru- 

 ment strained tight by the finger, belongs 

 to this class of motion. 



Oscillation, Centre of. That point in 

 an oscillating pendulum, in which the 

 whole moving force is concentrated, and 

 at which, if it meet with resistance, it 

 ■will instantly stop, without vibration or 

 strain of its other parts. This point diflTers 

 from the centre of gyration, because the 

 motion of the body is produced by the 

 gravity of its own particles ; whereas, in 

 gyration, the body is put in motion by 

 some other force, acting at one place 

 only. See Percussion. 



OS'MELITE (o'to), to smell). A mi- 

 neral occurring on calcareous spar, mixed 

 with datholite, and emitting at the ordi- 

 nary temperature of a room a distinct 

 clayey smell, whence its name is derived. 



O'SMIUM (6o-M»/, odour). A metal 

 discovered in 1803, in the grains of na- 

 tive platinum, together with iridium, 

 and named from the pungent and pecu- 

 liar smell of its oxide. 



OSSEOUS BRECCIA {osseus, bony). 

 The cemented mass of fragments of bones 

 of extinct animals found in caverns and 

 fissures. 



OS'TIOLUM (dim. of ostium, a door). 

 A little door ; the orifice of the perithe- 

 cium of some fungaceous plants. 



OSTRACEiE. The Oyster tribe; an 

 order of the Conchiferous Mollusca, 

 named from the genus ostrea, in which 

 244 



the two halves of the mantle are sepa- 

 rated the whole way round, or the foot 

 is absent or very small; they are usually 

 fixed by the shell to solid bodies. 



OSTRA'CIDiE. Oysters; a family of 

 the atrachian bivalves, named from the 

 genus ostrea; the animal is sedentary, 

 generally afiixed by the under valve ; the 

 shell is foliaceous, rarely pearly. 



OSTRACO'DA {SarpaKov, a shell, 

 ei3or, form). A family of the Entomo- 

 straca, comprising those which have the 

 shell folded so as to resemble those of 

 bivalves. 



OSTROTODA. An order of the en- 

 tomostracous Crustacea, in which the 

 body is entirely enclosed under a large 

 shield having the form of a bivalve shell. 

 The animals of this order are commonly 

 termed monoculous, from the two eyes 

 being united into one mass. 



OTO'LOPHI (oif, diTOi, the ear, X6- 

 (po^, a crest). A group of Batrachians, 

 which have the muzzle angular, and the 

 head furnished on each side with a crest 

 which extends over the parotid portion. 



OUTLIER. A term applied by some 

 mineralogists to a portion of a stratum 

 which occurs at some distance, detached 

 from the general mass of the formation 

 to which it belongs. 



OVAL {ovum, an egg). A curve drawn 

 by taking a string of any certain length, 

 and fixing, not one end as in drawing 

 the circle, but both ends to different 

 points, and then carrying a point outside 

 the string, always keeping it stretched 

 as far as possible. The name of this 

 figure is derived from its resemblance to 

 the transverse section of an egg. Ellipses 

 are ovals which are formed by the above 

 fixed law, but the latter is a popular terra 

 for any curved figure, approaching to 

 that shape. The carpenter's oval, for 

 example, is made up of circular arcs, 

 which unite without leaving any angular 

 appearances at their junctures. 



Ovals of Descartes. These, though not 

 Ellipses, are governed by a determinate 

 law, which constitutes them as varieties 

 of that curve. As in the Ellipse the two 

 lines drawn from the foci to any point of 

 the circumference vary, so that the in- 

 crement of one shall always be equal to 

 the simultaneous decrement of the 

 others ; so, in the Cartesian Ovals, these 

 increments are in an invariable ratio. 

 " These curves may therefore be defined 

 the locus (place) of the vertex of a tri- 

 angle, on a given base, one of whose 

 sides bears a given ratio to the sum or 



