T AL 



TAN 



genus tachydromus is the type ; also to a 

 family of saurian reptiles, and to a family 

 of dipterous insects. 



TA'CHYLITE. A mineral which re- 

 sembles obsidian, and has also been sup- 

 posed to be similar to isopyre. It occurs 

 in small masses in basalt and wacke. 



TACKING. A nautical term for an 

 operation with the rudder and sails, by 

 which, when a ship is proceeding in the 

 course of an acute angle with the direc- 

 tion of the wind on one of her bows, her 

 head may be turned towards the wind, so 

 as to enable her to sail on a course 

 making nearly the same angle with its 

 direction on the other bow. 



TA'GLIA. A name, borrowed from 

 the Italian, for a particular combination 

 of pulleys. The term is applied either 

 to a system of fixed pulleys collected in 

 one common block ; or to a system of 

 moveable pulleys in a separate block, to 

 which the weight is attached. Several 

 taglias may be so combined that one acts 

 upon another; this arrangement con- 

 stitutes a compound taglia. 



TALC. A simple mineral, laminated 

 like mica, and often similar in colour. 

 It is easily distinguished from this mine- 

 ral by being much softer, and, although 

 flexible, not elastic. It occurs as a con- 

 stituent of certain rocks. 



TALC-SLATE. A talcose rock, con- 

 sisting of talc and quartz arranged in 

 laminae; it is of various colours, glisten- 

 ing, and has an unctuous feel. It is 

 generally associated with steatite, asbes- 

 tus, actinolite, and serpentine. 



TALCITE. A rare mineral, called 

 nacrite by Jameson, and earthy talc by 

 Werner. It occurs in veins, with sparry 

 limestone and galena, in the mining dis- 

 trict of Freyberg. 



TALCOSE GRANITE. Protogene. 

 A mixture of felspar, quartz, and talc or 

 chlorite. It occurs in Cornwall, where, 

 on decomposing, it yields the China-clay 

 or Porcelain-earth, which is annually ex- 

 ported in large quantities. 



TALCOSE ROCKS. These resemble 

 the micaceous rocks, and comprise chlo- 

 rite-slate, talc-slate, and serpentine. 



TA'LPID^ {talpa, a mole). The 

 Mole tribe ; a family of the insectivorous 

 vertebrata, characterized by their subter- 

 ranean habits. 



TALUS. A term applied (o a sloping 

 heap, formed of fragments broken off by 

 the action of the weather from the face 

 of a steep rock, and accumulated at its 

 base. The word talus is borrowed from 

 328 



the language of fortification, in which it 

 denotes the outside of a wall of which 

 the thickness is diminished by degrees, 

 as it rises in height, to make it the 

 firmer. 



TA'NGENT [tango, to touch). A 

 straight line which touches, but does not 

 cut, a curve. A straight line is said to 

 be a tangent to a circle, when it touches 

 the circumference only on one point, 

 •making a right angle with the radius at 

 the point of contact. In Mechanics, a 

 force, which acts upon a wheel in the 

 direction of such a line, is said to be tan- 

 gential; and it is in this direction that 

 motion is communicated between wheels 

 and pinions, or from one wheel to an- 

 other. 



1 Tangent of an angle or arc. Aline 

 drawn from that extremity of the arc 

 through which the diameter of the 

 circle passes (which extremity is called 

 the beginning of the arc) perpendi- 

 cular to that diameter, touching the cir- 

 cle, but not cutting it, and terminated 

 by the radius of the circle produced, 

 and passing through the other extre- 

 mity of the arc. 



2. Tangential plane. A plane which 

 touches a curvilineal solid. It is from 

 this plane that we measure the angles of 

 incidence on the tangential point, whe- 

 ther the impinging ray be reflected or 

 refracted. 



3. Tangential force. Another term for 

 centrifugal force. Thus, when two forces 

 act upon a body, — the centripetal attract- 

 ing the body towards the centre of gra- 

 vity, the tangential driving it in another 

 direction, there will result a curvilinear 

 motion around the central point. 



TANNIC ACID. A tribasic acid oc- 

 curring in the bark of all the varieties of 

 Quercus and many other trees, and in 

 gall-nuts, from which it is procured in 

 greatest purity. What is commonly 

 called tannin is tannic acid mixed with 

 some foreign matters. 



TA'NTALUM. Columbium. A metal 

 found in the two Swedish minerals, tan- 

 talite and yttrotantalite. Its name is 

 derived from the insolubility of its oxide 

 in acids, in allusion to the fable of Tan- 

 talus. 



TANTALUM ORE. Columhite of 

 Hatchett. A prismatic ore of tantalum, 

 occurring as a coarse red granite in Fin- 

 land, 



TA'NTALUS'S CUP. A philosophical 

 toy, for exhibiting the principle of the 

 siphon. The siphon is placed in a cup, 



