RH Y 



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nor its angles right angles. It is, in 

 fact, a compressed parallelogram. 



In Crystallography, the rhomboid^ or 

 rhombohedron, as it is sometimes called, 

 is a figure contained within six equal 

 rhombs ; two only of its solid angles are 

 formed by the union of three equal plane 

 angles, and these are called the terminal 

 solid angles ; the other six, which are 

 similar to one another, are lateral solid 

 angles. This form would result from 

 elongating or compressing the cube in 

 the direction of a line passing through 

 two of its opposite solid angles ; in the 

 former case an acute, in the latter an 

 obtuse rhomboid w^ould result. 



RHOMBUS (p6M/3ur, a rhomb). A 

 four-sided figure, which has all its sides 

 equal ; but its angles are not right 

 angles ; it is an equilateral ijblique paral- 

 lelogram. '^i^>tr 



RHUMB-LINE [rumbo, or rumo, Por- 

 tuguese, a course). AThe shortest line 

 which can join two pomts on the globe, 

 cutting all the meridians which it crosses 

 at the same angle ; when delineated on 

 , the globe, it forms a curve termed the 

 i loxodromic curve. 



RHUMBS. The thirty-two points of 

 the horizon, as marked on the circle of 

 the mariner's compass; they serve to 

 calculate the angle which a ship's course 

 makes with the magnetic needle. 



RHUTE'NIUM. This, and Plura- 

 nium, are names of two supposed 

 metals, contained in the insoluble re- 

 sidue left after the action of nitro- 

 muriatic acid on the Uralian ore of 

 platinum. 



RHY'NCHOLITHES (pd-Yxof, a beak, 

 \iOos, a stone). Beak-shaped fossils ; 

 the extremities of the mandibles of Ce- 

 phalopods, allied to the nautilus. 



RHYNCHO'PHORiE {pvyxof, a muz- 

 zle, (pepoi, to bear). The Weevil tribe ; 

 a family of the tetramerous Coleoptera, 

 characterized by the prolongation of the 

 anterior part of the head into a kind of 

 muzzle. 



RHYNCHOSA'URUS (py^xo?, a beak 

 travpa, a lizard). A fossil animal pre- 

 senting certain characters, which con- 

 nect th^ great class of reptiles, on one 

 hand with the birds, and on the other 

 with quadrupeds. The skull and several 

 bones of the extremities of this animal 

 were found some years ago in the Grin- 

 sill quarries, near Warwick. 



RHYTIDO'MA {p(ni^, 2l wrinkle). 

 The name given by Mohl to the deciduous 

 scales of the barj^ of trees produced by 



the formation of epiphloeum inside the 

 liber or mesophloeum. 



RIGEL. A star of the first magni- 

 tude, on the left foot of Orion. 



RIGHT (in Mathematics). A term 

 generally used as opposed to oblique. Its 

 use may, however, have arisen from an 

 idea of simplicity: a right line is a 

 straight line, and the latter has been de- 

 fined to be " the shortest way between 

 two points." A right angle is formed by 

 two right lines perpendicular to each 

 other, and contains 90° ; it is the most 

 simple of angles. A right cone, cylinder, 

 prism, pyramid, &c., are those whose 

 sides are perpendicular, or at right 

 angles, to the plane of the base. 



RINGENT {ringo, to grin). A term 

 applied in Botany to certain corollas, the 

 petals of which cohere into the form of 

 a mouth, which gapes on pressing the 

 sides, as in Antirrhinum. 



RINGS, FAIRY. The fanciful name 

 given to patches or to circles of luxuriant 

 grass, observed in fields ; they are sup- 

 posed to owe their luxuriance to the 

 nutritive influence of decaying fungi. 



RIPPLE-MARK. A term applied by 

 Geologists to the undulations which oc- 

 cur on the surface of many rocks, re- 

 sembling the ridges and indentations 

 left on mud and sand by small waves of 

 water. They are most distinct on sur- 

 faces where a change of deposit has taken 

 place, as where sandstones alternate with 

 thin clay partings, 



ROASTING. A chemical process, by 

 which mineral substances are divided, 

 some of their principles being volatilized, 

 and others changed, so as to prepare 

 them for further operations. 



ROBUR CAROLINUM. Royal Oak ; 

 a modern southern constellation, consist- 

 ing of twelve stars. 



ROCK. A geological term, denoting 

 the solid parts of the crust of the earth, 

 formed of a single mineral species, or 

 composed of two or more species. One 

 or several rocks united by certain com- 

 mon characters, constitute a formation, 

 or connected series ; and several forma- 

 tions constitute a system of rocks. Rocks 

 have been distinguished, according to 

 peculiarities in their constitution, and in 

 their form and arrangement, into the 

 Plutonic, the Aqueous, and the Vol- 

 canic. 



1. The Plutonic Rocks are primary or 

 primitive in arrangement, igneous in 

 constitution, and unst ratified in form. 



2. The Aqueous Rocks are secondary 



O 



