I T R I 



I - The trigons of Mars and Saturn are by aflrologers held 



t malific afpctls. 



' Trioon', Trigonum, was alfo a mufical inftrument, ufed 

 among the ancients. 



The trigon was a kind of triangular lyre, or harp, and 

 was invented by Ibycus. 



It was ufed at feafts, and played on by women, who 



'. ftruck it either with a quill, or beat it with fmall rods of 

 different lengtlis and weights, to occafion a diveriity in the 

 founds. See Trigonum. 



j Trigon or Triangle of Signs, is the name of an inft.ru- 



j ment ufed for drawing the arcs of the figns upon dials : it 

 may be made of brafs or any other fohd matter, and of 



I any fize at pleafure. This inilrument is conftrufted after 



i the following manner: firft draw the line a b {Plate XXI. 

 AJlronomy, fg. 5. ) reprefenting the axis of the world, and 

 a c perpendicular to it, reprefenting the radius of the equi- 

 noftial ; and about the point a defcribe the circular arc 

 </i: i" at pleafure. Then reckon 235° both ways from the 

 point c upon the faid arc for the fun's greateft declination, 

 and draw the two lines ad., a e, for the fummer and winter 

 tropics : likewife draw the line de, which will be bifefted 

 by the radius of the equinoftial in ; about which point, 

 as a centre, draw a circle, whofe circumference paffes 

 through the points d and e of the tropics, and divide the 

 circumference into 12 equal parts, beginning from d: 

 through each point of divifion equally diitant from d and e, 

 draw occult lines parallel to the radius of the equinoftial circle : 

 thefe lines will interfeft the arc d c m the points, through 

 which and the centre a, lines being drawn, will reprefent 

 the beginnings of the figns of the zodiac at 30" diftance 

 from each other. But to divide the figns into every loth 

 or 5th degree, the circumference muft; be divided into 

 thirty-fix or feventy-two equal parts. The characters of 

 the figns are annexed as in the figure ; and when the trigon 

 is divided into every loth or 5th degree, the letter of the 

 month is placed to the firll 10° of each fign agreeing with 

 it. However, this inftrument may more readily be made by 

 means of a table of the fun's declination ; for having drawn 

 the two lines a b and a c at right angles, lay the centre of a 

 protraftor on the point a, with its limb towards the point 

 c; and keeping it fixed, count 23^° on both fides of the 

 radius a c for the tropics of 25 and VJ, 20° 12' for the be- 

 ginnings of the figns Si, n, f , and x:^, and 11° 30' for 

 y» i'.E) I'U and K. And thus the fpaces for each fign 

 may be graduated in every loth and 5th degree by means 

 of a table. The equinoctial points of v and === are placed 

 at the end of the radius of the equinoftial a c. 



Trigon of Diurnal and NoHurnal Arcs. Thefe are 

 drawn upon fun-dials by curve-lines, like the arcs of the 

 figns, and by means of them the ihadow of the ftyle (hews 

 iiow many hours the fun is above the horizon, in any given 

 day, &c. 



The trigon of figns is tlie fame for all latitudes, the fun's 

 declination being the fame for the whole earth ; but the 

 diurnal arcs are different for every particular latitude, and 

 as many of thefe arcs are drawn upon a dial, as there are 

 hours of difference between the longeft and fhorteft days of 

 the year. For the conftrudion of this fort of trigon, draw 

 the right line R Z {Jg. 6. ) for the radius of the hour-line 

 of 1 2, or of the equinoftial : and about the point R, with 

 any opening of the compafl'es at pleafure, defcribe the cir- 

 cular arc T S V, and lay off both ways on It from the point 

 S two arcs, S V, ST, each equal to the complement of the 

 latitude. Then draw the right line T X V, and about the 

 point X, as a centre, defcribe the circumference of a circle 

 T Z V Y, whicli divide into forty-eight equal parts by dotted 



T R I 



lines, drawn parallel to the radius of the equinoaial R Z : 

 then thefe lines will intcrfetl the diameter T X V in points", 

 through which and the point R you may draw the radii of 

 the hours. The angles which all the radii make at the point 

 R may be found trigonometrically by the following propor- 

 tion : as radius is to the co-tangent of the latitude, fo is 

 tlie tangent of the difference between the femidiurnal arc at 

 the time of the equinox, and the arc propofed to the tangent 

 of the fun's required declination. 



The trigon of figns may be annexed to a rule or index 

 •^ (J'S- 7- ) in order to draw the arcs of the figns upon great 

 dials. The diurnal arcs may be drawn hkewife upon this 

 trigon, but the arcs of the figns and diurnal arcs too muft. 

 not be drawn upon one and the fame dial, for avoiding con- 

 fufion. In the centre of the index there is a fmall hole, 

 through which is put a pin, that the inftrument may turn 

 about the centre of a dial. The trigon Aides along the in- 

 dex, and may be fixed in any part of it by means of the 

 fcrew B. The arcs of the figns with their ciiarafters are 

 round about the circumference, and there is a fine thread 

 fixed in the centre, in order to extend over the radii quite 



to the hour-lines of a dial For a farther account of thefe 



inftrumcnts and their ufe, fee Bion's Conftr. and Ufe of 

 Matii. Iiiilr. by Stone, p. 231, &c. 



TRIGONAL Leaf, among Botanifls. See Leaf. 

 Trigonal Numbers. See Triangular Numbers. 

 TRIGONELLA, in Botany, the diminutive of trigona, 

 three-fided, alluding to its little triangular flower. — Linii. 

 Gen. 388. Schreb. 510. Willd. Sp. PI. v. 3. 1398- 

 Mart. "Mill. Diet. v. 4. Sm. Prodr. Fl. Graec. Sibth. v. 2. 

 107. Ait. Hort. Kew. v. 4. 397. JufT. 356. Lamaixk 

 lUuilr. t. 611. Gsrtn. t. 152. (Foenum graec um ; 

 Tourn. t. 230. ) — Clafs and order, Diadelphia Decandria, 

 Nat. Ord. P apilionaceit, Linn, Leguminof^, Juff. 



Gen. Ch. Cal. Perianth inferior, of one leaf, bell-lhaped, 

 divided half way down into five little, awl-fhaped, nearly 

 equal, teeth. Cor. papilionaceous, apparently tripetalous $ 

 ftandard nearly ovate, obtufe, reflexed and fpreading ; 

 wings two, ovate-oblong, reflexed and fpreading outwards, 

 in fuch a manner that, with the ftandai-d, they almoft con- 

 ftitute a regular tripetalous corolla ; keel very fliort, obtufe, 

 occupying the centre of the flower. Stam. Filaments in 

 two diflinft fets, one fimple, one in nine fegments, (liort, 

 afcending ; anthers fimple, roundifh. Pifl. Germen ovate- 

 oblong, comprelTed ; ftyle fimple, afcending ; ftigma fimple. 

 Pcric. Legume linear-oblong, fomevvliat curved, compreffed, 

 much longer than the calyx, of one cell, and two valves. 

 S^eds numerous, roundifli. 



Eft". Ch. Standard and wings nearly equal, fpreading, 

 in the form of a tripetalous corolla. Stigma fmooth. Le- 

 gume of one cell, with many feeds, compreffed, longer than 

 the calyx. 



Obf. Linnseus remarks that the figure of the corolla 

 alone ftamps tliis as a diftind genus. We could vvifh that 

 charader were more evident. The clofcil affinity exifts 

 between fome TrigonelU, which we ftiall indicate, and the 

 Meliloti, conftituting the firft fedion of TlUFOLlUM. Sec 

 that article. 



Tlie fpecies of this genus, feventeeu in Willdenow, are her- 

 baceous, for the moil part annual, ftrong-fcented plants, 

 with three leaflets on a ftalk, and indeed the general habit 

 of Trifolium. Their flowers are tufted rather than properly- 

 capitate, ufually yellow. T. Fecnum grttcum has blucifh 

 flowers, which are quite ieffile, or truly capitate, their 

 corolla that of a Trifolium, and their beaked /c^uwc fplitting 

 chiefly along its upper margin. This fpecies certainly docs 

 not well accord with the reft.. 



Kk2 I- T. 



