G N O 



OnfT-mVi conftrufted a gnomon of 50 feet in the " College 

 ■d>- rOr-atoirs" at Marfeilles, and obferved the folftices with 

 it in 1636. 



Ignatius Dante, in 157;. a Dominican friar, and after- 

 wards bilhop of Alatri, tirll conllrucled the celebrated 

 gnomon in the church of St. Petronius, at Bologna : it was 

 originally only about 70 feet high, but was raifed by 

 Caifini, in 16;^, to the height of 90 feet, and it was with 

 this that his valuable folar obforvations were made. 



Mofl of thefe gnomons are accompanied by a meridian 

 line, often liighly ornamented with the names of tlie months 

 and ligns of the 7.odiac, fo as to ferve the purpofe of a calen- 

 dar. One of the finell in Italy is in the Carthulian convent 

 at Rome, budt on the ancient TherniE of Diocletian. 

 I'here are two, one to the north 62 feet, another on the 

 fouth 75 feet high : they were conltructed by Bianchini in 



1701. 



Picard crefted a gnomon in the royal obfervatory at 

 Paris, which flill exifts. 



The meridian hue in St. Snlpice, at Paris, was begun in 

 1727 by Sully, a watchmaker, and has iince been much 

 improved and highly ornamented : its height is 80 French 

 feet. 



M- de Cefaris and Reggio in 1786, conftrufted a gnomon 

 and meridian line in the cathedral at Milan : its height is 73 

 feet. 



The reafon why none of thefe gnomons are to be found in 

 England, is that' we do not regulate our clocks by folar, 

 but by mean time : to this may be added our unfavourable 

 climate, which would very much diminifh the interell which 

 they obtain in other countries, where they are made to ferve 

 the ufeful purpofes of civil life. 



NotwithftandLiig the great attention that has been paid 

 to the conftruftion of thefe gnomons, their ufe has been 

 entirely fuperfeded by modern inflruments of fmall dimen- 

 fions, fo that to the allronomer they are now become merely 

 objefts of curiofity : yet to no inftrument has aftronomy 

 greater obligation. The folar theory was firft brought to a 

 very improved ftate by the ufe of this funple inlirument 

 alone. The length of the year, and the obliquity of the 

 ecliptic, were determined by it with very great exaftnefs. 

 The ancient method of deducing the folar theory was nearly 

 as follows. 



To determine the obliquity of the ecliptic, the length of 

 the {hadows were obferved at the period of the follHces, and 

 the extreme points to which the longell and Ihortell Ihadows 

 extended at the moment of noon were carefully obferved, 

 and the altitudes of the fun deduced at e;ich of thefe periods : 

 half the difference of the greatelt and leall altitudes would 

 be the obliquity of the ecliptic, fubjeft to a fmall error 

 arifino- from refraftion, the effefts of which were not known 

 at that early period, nor were they very confiderable in thofe 

 fonthern portions of the globe in which allronomy had its 

 origin. But thougli it was not difficult to deduce the 

 length of the folftitial (hadows, it was by no means eafy to 

 determine the day on whieh either of the folilices actually 

 Jiappened, becaufe the length of the (liadow v/ould appear 

 to be the fame at thofe feafons for feveral fucceffive days. 

 It appears, by fome very ancient Chinele oblcrvations ilili 

 on record, that this inconvenience was remedied by obfervir.g 

 the length of the fhadow, ten or tv.-enty days before tl/e 

 foIlHce ; then waiting till they found the meridian ihadow 

 again of the fame length, they concluded that the moment 

 of the foUlice happened on the intermediate day between 

 thefe correfponding obfervations. 



The leno-th of the year was determined from obfervations 

 of the equinoxes, which could be niucli more cxaftly ob- 



G N O 



ferved ; for the obliquity of the ecliptic being determined 

 by the method above defcribed, tlie length of the equinoc- 

 tial was known, and the day on which the fliadow corre- 

 fponded to the computed (liadow was evidently the day of 

 the equinox. 



It was by examining a fucceflion of thefe fimple obferva- 

 tions, that Hipparchus fird difcovered the great inequality 

 in tlie length of the four feafons, arhing from what is now 

 called the equation of the orbit: and from the difcover>- of 

 this important faft, we may date the origin oi phyllcal 

 ailronomy. 



GNOMONIC Column. See Column". 



Gno.monic, Polyhedron. See Polyheduo.v. 



Gnomonic, or Gnomor.kal projccfiov., that which reprefents 

 the circles of a hemifphere, upon a plain touching it in the 

 vertex, by lines or rays from the centre of the hemifphere to 

 all the points of the circles to be projefted. 



In this proje&ion, all the great circles of the fphere are 

 projefted into right lines. Any letter circle parallel to the 

 plane of projcftlon is projected into a circle. And any leffer 

 circle not parallel to the plane of proje£lion, is projefted into 

 a conic feCtion. 



The gnomonic projeftion is alfo called the " horologiogra- 

 phic projeftion/' becaufe it is the foundation of dialling. In 

 other refpefts it is not much ufed, becaufe the circles of the 

 fphere are projefted into conic feftions, which are diflicult to 

 be defcribed. However, this projeAion has its conveniences in 

 tlie-folution of fome problems of the fphere, on account of 

 the great circles being all projefted into right lines. 



Mr. Eroerfon, known by an ingenious Treatife upon Flux- 

 ions, and a variety of other publications, has given the theory 

 and praftice of the gnomonic projeftion, iu his Treatife on 

 the Projection of the Sphere, Lond. 1749, 8vo. See Pi;o- 



JECTION. 



GNOMONICA, riiMiovixy, or Gnomomcs, the art of dial- 

 ing, or of drawing fun and moon-dials, &e. on any given 

 plane. 



It is thus called, as it fhews how to find the hour of the 

 day, &c. by the fhadow of a gnomon or llyle. 



GNOSIMACHI, \n Ecchfiajlkal H'ljhry, an ancient fcA 

 in religion, whofe diftinguidiing eharatter was, that they 

 were profeffed enemies of all ftudied knowledge in divinity. 



The word is •) va3-i/iK%0:-, q, d. an enemy of wifdom or 

 knowledge. 



Damafcenus fays, that they were perfectly averfe from all 

 the gnofes of Chrillianity, i. e. all the fcience or technical 

 knowledge thereof. They held it an ufelefs labour to feek 

 for gnolts in the holy fcriptures ; and faid, that God requires 

 nothing of men but good works ; that it were, therefore, 

 I .uch better to walk with more (Implicity, and not to be fo 

 folicitous about the dogmata of the gnoftic hfe. 



GNOSSUS, in Ancient Geography, a town of the ifland of 

 Crete, more anciently called " Cccratus," from the name of 

 the river which watered it. It was the fixed refidence of 

 Mir.os, once the capital of the iiland, and, according to 

 Strabo, 2 wealthy and populous place, being 30 furlongs in 

 c-ompafs, and full of inhabitants. This writer places it 20 

 furlongs from the .(Egean or Archipelago, and 90 from the 

 Atncan fea. Its port, called Heracltum, was at a confider- 

 able dulance, according to Olivier about four or five leagues 

 to the eallward. Paiifinias (Attic.) fays, that it had a la- 

 byrinth. From this town Ariadne, fo much celebrated by the 

 poets, derived the name of Gnodis. When the ifland was 

 reduced by the Romans, GnolTus was humbled, and Gortyna, 

 her rival, raifed upon her ruins. Sonnini fays, that a fmall 

 village, Cnodbn, near Candia, would rccal to mind the fcite 

 of the ancient town, were it not difcovernble, in a manner no 



lefs 



