TR A 



TRA 



TRACHE'LIPODS (rpax^i^op, the 

 neck, nov^, irodot, the foot). A designa- 

 tion of those moUusca which have the 

 locomotive disc or foot attached to the 

 head. Under this title, which represents 

 the third order of Lamarck, all the uni- 

 valved shells are arranged. 



TRACHE'NCHYMA (rpaxela, the 

 trachea, Iyx^Mci* ^"7 thing poured in). 

 A designation of the vascular tissue of 

 plants, consisting of spiral vessels, which 

 resemble the trachea of insects. 



TRA'CHYLITE. The name of a 

 mineral substance resembling obsidian. 



TRA'CHYTE (rpaxy?, rough). A 

 variety of lava essentially composed of 

 glassy felspar, and frequently having 

 detached crystals of felspar in the hase 

 or body of the stone, giving it the struc- 

 ture of porphyry. It sometimes contains 

 hornblende and augite ; and when these 

 predominate, the trachyte passes into the 

 varieties of trap called greenstone, ba- 

 salt, dolorite, &c. The term is derived 

 from the peculiar rough feel of the 

 rock. 



TRACTION {tracHo, drawing). In 

 Mechanics, the act of drawing a body 

 along a plane, by the power of animals 

 or of steam. The angle of traction is the 

 angle which the direction of the power 

 makes with a given plane. The power 

 excited to produce the effect is called 

 the force of traction. See Horse-power. 



TRA'CTRIX or TRACTORY. A 

 curve described by a heavy point attached 

 to a string, the other end of which is 

 moved along a given straight line or 

 curve. It is characterized by this pro- 

 perty, that the tangent is always equal to 

 a given line. 



TRADE WINDS. In a belt extend- 

 ing about 30° on each side of the equator, 

 the wind is observed to blow all the year 

 round from nearly the same quarter of 

 the heavens : to the north of the equator 

 it blows nearly from the n.e. quarter, 

 and to the south of the equator from the 

 s.E. quarter. These winds, from the great 

 assistance which they afford to commerce, 

 are called the n.e. and the s.e. trade 

 winds. ' When ships are bound from 

 Europe to the West Indies, or to any part 

 of North America, south of the parallel 

 of about 38°, they seek the aid of these 

 winds; but when they return, they keep 

 away to the northward for the purpose 

 of avoiding them. 



TRAJE'CTORY. A technical name 

 formerly given to a curve required to be 

 found by means of certain conditions ; 

 341 



generally used for the required path of a 

 projectile acted on by given forces. 



TRAMMELS. Elliptic compasses 

 used for making figures of an elliptii 

 form. In this instrument, a bar carry- 

 ing a pencil is guided by two pins which 

 move in grooves. 



TRANSCENDE'NTAL (<raw5cenrfo, to 

 go beyond a certain limit). In the phi- 

 losophy of Kant, the transcendental ele- 

 ments of knowledge are those which are 

 original or primary, or those which are 

 determined & priori in reference not only 

 to human cognition, but also to man's 

 collective activity; and which, conse- 

 quently, are the basis of empirical know- 

 ledge, or that which is determined d 

 posteriori. In short, all pure knowledge \ 

 makes up the transcendental philosophy, 

 and on it rest the authority and possi- 

 bility of cognition. The terra is thus 

 synonymous with metaphysical. 



A transcendental problem, according to 

 the earliest meaning of the term, is one 

 the equation of which is infinitely high, 

 or contains an infinite series of powers of 

 an unknown quantity, so that its highest 

 degree transcends every degree. But 

 the term transcendental has undergone 

 many changes of meaning. The writers 

 in the Penny Cyclopaedia predict that 

 it will settle into the following : " a tran- 

 scendental result will be one which is 

 incapable of expression, except by a 

 definite integral, or by an infinite series 

 which cannot be otherwise expressed 

 than by a definite integral." 



TRANSFORMATION. A term in 

 Mathematics denoting a change made in 

 the object of a problem or in the shape 

 of a formula, in order to facilitate the 

 solution, calculation, or use of the ori- 

 ginal problem or formula ; as when an 

 equation is transformed into another 

 equation, having roots which bear simply 

 a relation to the roots of the former. All 

 the operation of Algebra consists in trans- 

 formation, from and after the point at 

 which the problem to be solved is re- 

 duced to an equation. 



TRANSIT {transeo, to pass over). In 

 Astronomy, this term denotes the pas- 

 sage of one heavenly body over the disc 

 of a larger one, as that of Mercury, when 

 he transverses the sun's disc under the 

 form of a black spot. When the nearer 

 body has the greater apparent diameter, 

 so as to hide the other, the passage is 

 termed an occultation of the latter. 



TRANSIT INSTRUMENT. Instru- 

 ment des Passages, An instrument by 

 Q3 



