SH A 



S ID 



comprehending those species which are 

 not suspended by a pedicle. 



SET OF CURRENT. The direction 

 of a current is called its set ; a current 

 which flows towards the nnw. quarter, 

 is said to set nnw. The velocity of a 

 current is called its drift. 



SETA. Literally, a bristle ; hence 

 applied to the bristle-like stalk which 

 supports the theca of mosses ; and, 

 hence, the epithet setose denotes a sur- 

 face covered with short, stiff hairs, as the 

 leaf of bugloss, the pappus of some com- 

 posite plants, &c. 



SEVENTH. In Music, a dissonant 

 interval, of which there are three kinds, 

 the minor or ordinary seventh, from G 

 to F; the diminished seventh, from C 

 sharp to B flat ; and the major or sharp 

 seventh, fro n C to B. 



SE'VERITE. A hydrated silicate of 

 alumina, found near St. Sever in France, 

 in a gravelly soil, in pieces from two to 

 five inches in diameter. 



SEXTANS. The Sextant; a modern 

 southern constellation, consisting of 

 forty-one stars, situated hetween Re- 

 gulus and Cor Hydra?. 



SEXTANT. An astronomical and 

 nautical instrument, commonly called 

 Hadley'sy by means of which the direct 

 angular distance of any two objects may 

 be measured, or the altitude of a single 

 one determined, either by measuring its 

 distance from the visible horizon (such 

 as the sea-offing, allowing for its dip), or 

 from its own reflection on the surface of 

 mercury. The principle of this instru- 

 ment is the optical property of reflected 

 rays, thus announced: — "The angle be- 

 tween the first and the last directions of a 

 ray which has suffered two reflections in 

 one plane, is equal to twice the inclina- 

 tion of the reflecting surfaces to each 

 other." 



SEXTILE ASPECT. An astrological 

 term, denoting the aspect of two planets, 

 when they are distant from each other 

 the sixth part of a circle, or sixty de- 

 grees. 



SEXUAL SYSTEM. A mode of ar- 

 ranging plants, invented by Linnaeus, 

 and founded upon the number and pecu- 

 liarities of the sexual organs. See 

 Botany. 



SHADOW. If an opaque body be 

 enlightened on one side only, the side 

 which is more remote from the source of 

 light will continue dark, and beyond it 

 there will be a space not affected by the 

 light ; this constitutes the shadow of the 

 302 



opaque body. If any object intercept 

 this space, that side of it which is next 

 to the opaque body will be darkened by 

 the projection of the shadow upon it. 



SHALE {schalen, German, to peel, to 

 split). A provincial term adopted by 

 geologists to denote an indurated slaty 

 clay, or argillaceous matter, indurated, 

 with a slaty structure, and a dull grey 

 streak, and naturally divided into laminae 

 parallel to the plane of deposition. When 

 much intermixed with carbonaceous mat- 

 ter, and impregnated with bitumen, it 

 is usually named bituminous shale. 

 When highly impregnated with silica, 

 it passes into flinty slate. 



SHARP. A character in Music, 

 marked J, employed to raise any note 

 of the natural scale, a semitone higher. 

 The double sharp, marked x , is used in 

 chromatic music for raising a note two 

 semitones above its natural state. Thus 

 C double sharp is D natural. 



SHEAR-STEEL. This substance, so 

 called because fitted for making clothiers' 

 shears, scythes, &c., is prepared by lay- 

 ing several bars of common steel toge- 

 ther, and heating them in a furnace until 

 they acquire the welding temperature. 

 The bars are then beaten together with 

 forge-hammers, after which they are 

 drawn anew into bars for sale. 



SHELL LIMESTONE. Muschelkalk. 

 A compact limestone, of a smoke-grey 

 colour, and in certain localities contain- 

 ing a great variety of fossils. The most 

 esteemed variety for ornamental pur- 

 poses is that from Carinthia, called 

 lumachella, or fire marble. Shell lime- 

 stone is distinguished from Magnesian 

 limestone by its never presenting the 

 shells of the genus Producta, which 

 occur in this deposit; and from the Lias, 

 by the absence of the Ammonites and 

 Gryphaeae which characterize the latter. 



SHELL MARL. A deposit of clay, 

 peat, and other substances mixed with 

 shells, which collects at the bottom of 

 lakes. 



SHIELDS OF LICHENS. Little 

 coloured cups or lines, also called sou- 

 tella and apothecia, appearing on the 

 upper surface of lichens; they are sur- 

 rounded by a rim, and contain the asci, 

 or sporuliferous tubes. 



SHINGLE. The loose and completely 

 water-worn gravel on the sea-shore. 



SIDEREAL DAY (sidus, a star). The 

 space of time elapsing between two con- 

 secutive returns of a star to the same 

 meridian. This is equal to the time 



