ORS ! 



of the Latins. It occurs in lamellar 

 masses, composed of indistinct crystalline 

 particles, for the most part in kidney and 

 other imitative forms ; it volatilizes be- 

 fore the blow-pipe with a white smoke, 

 and with the odour of both sulphur and 

 arsenic. Sp. gr. 3'4 to 3'6. It is found in 

 veins, in various metalliferous formations, 

 in various parts of the world, but the 

 finest comes from Persia, in brilliant yel- 

 low masses ; this is commonly termed 

 golden orpiment. Orpiment is used as a 

 pigment, and is prepared also artificially 

 for this purpose. 



OR'RERY. A machine constructed for 

 the purpose of representing and illustrat- 

 ing the movements and phases of the 

 planetary system. This machine was in- 

 vented by George Graham, but it takes 

 its name from the circumstance of the 

 Earl of Orrery having procured a copy of 

 Graham's machine, by means of a work- 

 man of the name or Rowley; it was 

 named after the Earl by Sir R. Stele, 

 on the supposition that his machine was 

 the first of the sort constructed. Orreries 

 convey only very rnde and contracted no- 

 tions of the planetary movements and 

 proportions. 



OR'RIS. The plant iris, of which orris 

 seems to be a corruption: fleur-de-lis or 

 flag-flower. 



OR'RIS-ROOT. The root of a white flow- 

 ering species of iris, the Irisflorentina, a 

 native of the south of Europe. It is used 

 in the manufacture of hair powder and 

 some other articles, on account of its 

 grateful odour. See ORRIS. 



ORSE'DEW, ) Manheim or Dutch Gold , an 



ORSE'DCE. I inferior sort of gold-leaf 

 (rather brass-leaf} prepared principally at 

 Manheim from a sort of flue brasa. 



5 CRT 



ORTHOCER'ATA, \ An extinct genus of 



ORTHOCER'ATITE. ) cephalopoda, inha- 

 biting polythalmous straight shells : 

 hence the name from o%6o?, straight, and 

 ziq/x.;, a horn. The orthoceratite (says 

 Mr. Kemble) resembles an ammonite un- 

 rolled, having its chambers separated by 

 transverse septa, concave externally and 

 convex internally , the septa being pierced 

 by a siphuncle. There are many varie- 

 ties, some of which are upwards of two 

 feet in length. Part of the pavement of 

 the palace of Hampton Court, and that of 

 the hall of University College, Oxford, 

 are composed of marble containing re- 

 mains of orthoceratites. Some species, 

 found in the carboniferous limestone of 

 Dumfries-shire, are nearly the size of a 

 man's thigh. 



ORTHOD'KOMICS, from o%8o<;, straight, 

 and fyuof, course. The art of sailing in 

 the arc of a great circle, which is the 

 shortest distance between two points on 

 the surface of the globe. 



OR'THOGON, from egflo?, straight, right, 

 and -yiuvioi, an angle. A rectangular figure". 



ORTHOGRAPH'IC. 1. Relating to ortho- 

 graphy. 2. An orthographic projection 



of the sphere is one in which the eye is 

 supposed to be at an infinite distance : so 

 called because being made on a plane, 

 passing through the middle of the sphere, 

 and the eye placed vertically at an in 

 finite distance, all the perpendiculars fall 

 in the common intersection of the sphere, 

 with the plane of the projection. 



ORTHOG'RAPHY, from o^8og, right, and 

 yftttfti, writing, description. In gram- 

 mar, that branch which teaches how 

 words should be spelled. In geometry, the 

 art of delineating the foreright plan, of 

 any object, and of expressing the heights 

 or elevations of each part, so called be- 

 cause it determines things by perpendi- 

 cular lines falling on the geometrical 

 plane. In perspective, the foreright side 

 of any plane, that is, the side or plane 

 that lies parallel to a straight line, that 

 may be imagined to pass through the 

 outward convex points of the eyes, con- 

 tinued to a convenient length. In archi- 

 tecture, the elevation of a building, show- 

 ing all the parts in their true proportions 

 \n fortification, the profile or representa 

 tion of a fortress, or other work, in all its- 

 parts, as they would appear if perpendi- 

 cularly cut from top to bottom. 



ORTHOPNOS'A, from o$Qo;, erect, and 

 jrej, breathing. 1. A disease of the 

 lungs, in which respiration can only be 

 performed ia an upright posture. 2. In- 

 ability of breathing in a recumbent pos- 

 ture. 



OKTBOP'TERA. An order of insects, the- 



