TRO 



TRO 



are iron, manganese, and phosphoric 

 acid. 



TRITOLI. A powder used for polish- 

 ing metals and stones, first imported 

 from Tripoli, which, as well as a certain 

 kind of siliceous stone of the same name, 

 has been lately found to be composed of 

 the flinty cases of infusoria. It is found 

 at Bakewell, in Derbyshire, where it is 

 called rotten-stone. 



TRITO'XIDE. An oxide containing 

 one atom of base combined with three 

 atoms of oxygen. 



TRO'CHlDiE. A family of phyto- 

 phagous Gasteropods, which have a tur- 

 binate, mostly trochiform, or top-like 

 shell, the substance of which is almost 

 always perlaceous; the outer lip never 

 thickened ; the aperture entire, closed 

 by a shelly or horny operculum. 



TROCHI'LIDiE {trochilus, a hum- 

 ming-bird). The Humming-bird tribe ; 

 a family of the Insessores, or Perching- 

 birds, characterized by the brilliancy of 

 their plumage, and by the humming 

 sound which accompanies their flight. 

 See Tenuirostres. 



TROCHI'N^. Trochuses, or Top- 

 Bhells ; a sub-family of the Trochidcs, 

 named from the genus trochus ; the 

 shape of these shells is pyramidal, the 

 body-whorl flattened, and the aperture 

 closed by a horny operculum. 



TRO'CHOID {rpoxoi, a wheel, ei3o?, 

 likeness). A curve described by any 

 point in a wheel as it rolls forward on a 

 plane. The term is synonymous with 

 cycloid. 



TROCHOI'DAL CURVES (rpoxor, a 

 wheel or hoop, €t3o9, likeness). " Under 

 this term is included a large number of 

 lines which are produced by the com- 

 position of two circular motions, includ- 

 ing the straight line, the circle, the el- 

 lipse, a class of curves called epitro- 

 choids, of which one particular case is 

 called the epicycloid, and a class called 

 hypotrochoids, of which one particular 

 case is the hypocloid. Among these 

 must also be included the extreme case 

 in which one of the motions is rectilinear, 

 which gives the common trochoid, the 

 cycloid, and a class of spirals which in- 

 cludes the involute of the circle, the 

 spiral of Archimedes, and others." — Pen. 

 Cycl. 



Trochoidal and Planetary Motions. In 

 the trochoidal motion, which is univer- 

 sally adopted, one circle is made to roll 

 like a hoop, either upon a straight line, 

 or upon the circumference of another 

 347 



circle. The planetary motion resembles 

 that in which a planet and its satellite 

 move round the sun. Here a circle, 

 without any rolling, has its centre carried 

 round the circumference of another. 



TROGO'NIDiE. A family of the In- 

 sessores, or Perching-birds, remarkable 

 for the beauty of their plumage, com- 

 prising the woodpecker, the cuckoos, the 

 trogons, the toucans, &c. 



TRONA. The name given in Africa 

 to the sesqui-carbonate of soda, imported 

 from the coast of Barbary, where it is 

 collected by the natives. 



TRO'PHI (Tp6(/)or, a nourisher). In- 

 strumenta cibaria. In insects, the organs 

 which form the mouth, consisting of an 

 upper and an under lip, and comprising 

 the mandibles, maxillse, and palpi, or 

 the parts employed in acquiring and pre- 

 paring food. 



TROPHOPOLLEN. The name given 

 by Turpin to the septum of the anther of 

 plants, from which the pollen has been 

 incorrectly supposed to separate. By 

 others, this part has been termed the 

 receptacle of the pollen, and by Link the 

 raphe. 



TRO'PHOSPERM {rpecjxa, to nourish, 

 (jirepiMa, seed). The name given by 

 Richard to the placenta in plants, the 

 part from which the ovules arise. It 

 generally occupies the whole or a portion 

 of one angle of each cell. 



TROPICS (rpoTTiKov, from TpeTrw, to 

 turn). Those two circles on the earth, 

 over which the sun seems directly to 

 pass, when he is at the greatest distance 

 from the equator northward and south- 

 ward (viz. 23^ degrees) ; hence, the one 

 is called the Northern, the other the 

 Southern Tropic. They correspond to 

 the sun's position at the solstices, and 

 are the limits of the torrid zone. 



1. Tropic of Cancer. A designation of 

 the Northern Tropic, because, when the 

 sun appears to move vertically over this 

 tropic, he appears to be in the beginning 

 of Cancer. 



2. Tropic of Capricorn. A designation 

 of the Southern Tropic, because, when 

 the sun appears to move vertically over 

 this tropic, he appears to be in the begin- 

 ning of Capricorn. 



3. The term Tropic denotes a point 

 where a turn is made ; for the line of the 

 ecliptic quitting the equator in the first 

 point of Aries, continues to rise higher 

 northward from it till it reaches the first 

 point of Cancer, when it turns to the 

 southward ; and, after again cutting the 



