JET 11 



JET^EI, in j-fiiclfnt Geography., a people, iccoiiliiig to 

 rtolcmy, of Arabia Ftlix. 



ATiVRA, a town of Africa, placed by Ptolemy between 

 the town of Tnbraca and the river Ampfagas. ," 



ATATE prohanja, in Laiv, a writ that lay to inquire 

 whether the king's tenant, holding In chief by chivaliy, 

 were of full age to receive his lands into his own hands. It 

 was directed to the cfcheator of the county ; but is now 

 difulcd, fince wards and liveries are taken av>-ay by the lla- 

 tute Car. II. Reg. Orig. 294. 



.^TH, or Ath, in Geography, a ftrong little town of 

 the county of Hainault, in the Auftrian Netherlands, 

 fituate on the river Dender, about 20 miles fputh-well of 

 BrufTels. It had formerly an abbey of nuns, and fomc 

 good linen manufactures. 



^CTH^A, a town of Laconia. 



.('ETHALE, in Natural llijlory, a name given by fome 

 writers to the caihm.i fornatum, or ti;tty. 



It had this name from its being the concreted foot, or va- 

 pour of the h.p'is cahuniimr'is, and copper, melted together, 

 in the making of brafs. 



-/ETHALIA, or Ilva, now Ella, in /incic7it Geography, 

 an ifland on the coaft of Etriu-ia, about 100 miles in com- 

 pafs, and abounding witli iron. It was fo called from 

 aivaXn, fmoke, ifliiing from the fliops of Vidcan. 



.(ETHALIDiE, a people of Attica, in the tribe of 

 Leontides. 



^THALOEIS, a town of Myfia, eail of mount Ida, 

 and fouth of Scepfis. 



iETHELING, in Briiylj Hi/lory. See Atheling. 



jETHER, in P/'v/io/ogy, is ufually underllood of a thin, 

 •fubtle matter, or medium, much finer and rarer than air ; 

 which, commencing from the limits of our atmofphere, 

 pofTefTes the whole heavenly fpace. 



The wortl is luppofed to be formed from the verb atduv, 

 to burn, to Jlo.me ; fome of the ancients, partitiularly Anax- 

 agoras, fuppofing it of the nature of fire. 



The philofophers cannot conceive that the largefl part of 

 the creation fliould be perfeftly void ; and therefore fill it 

 with a fpecies of matter under the denomination of sether. 

 — But they vary extremely as to the nature and charafter 

 of this a3ther. — Some conceive it is a body fu'i generis, ap- 

 pointed only to fill up the vacuities between the heavenly 

 bodies ; and tlierefore confined to the regions above our 

 atmofphere. — Othei-s fuppofe it of fo fubtile and penetrating 

 a nature, as to pervade the air, and other bodies ; and 

 poffefs the pores and intervals thereof. — Others deny the ex- 

 iftence of any fuch fpecific matter ; and think the air itfclf, 

 by that immenfe tenniiy and expanfion of v.hich it is found 

 •capable, may difi'ufe itfelf through the interftellar fpaces, 

 and be the only matter found in them. 



In effeft, xther being no objeft of our feiife, but the 

 mere produft of imagination, introduced only for the fake 

 of hypothefis, or to iolve fome phenomenon, real or ima- 

 ginary ; authors take the liberty to modify it how they 

 pleafe. — Some fuppofe it of an elementary nature, like other 

 todies, and only dliUnguifhed by its tenuity, and the otlier 

 affeftions refulting from, it ; which is the philofophieal 

 aether. — Others will have it of another fpccits, and not 

 elementary ; but rather a fort of fifth element, of a pure, 

 more refined, and fpirituous naturt than the fubftances about 

 cur earth ; and void of the common properties of matter, as 

 gravity, &c. — Such is the ancient idea of oetlicr or aetherial 

 matter. 



The term xther being thus embarrafTed with a variety of 

 ideas, and arbitrarily applied to fo many different things, 

 the later philofophers choofe to fct it afide ; and accord- 



M T H 



jngly, the Cartefians ufc the tenn tyiitlerin futlWu, which 

 is their sether ; and Sir Ifaac Newton fomctimes a fukile 

 fplr'it, as in the clofe of his Principia ; (apnd Oper. torn. iii. 

 p. 174. Ed. Horfi.) and fomctimes ^fubtile or ethereal me- 

 dittm; as in his Ojnics. Qiieries 18 — 24, apud Oper, torn, 

 iv. p. 223 — 226. Sec alfo his letter to Mi. Boyle, apud 

 Oper. tom. iv. p. 385, &c. 



The truth is, there are numerous confidcrations, which 

 fcem to evince the cxiftence of fome matter in the air miicli 

 finer than the air itfelf. There is an un.known fomelhing 

 which remains behind when the air \% taken away ; as ap- 

 pears from certain efiefts which we fee produced in Tacvo. — 

 Heat, Sir Ifaac Newton obfcrves, is communicated through 

 a vacuum, almoll as readily as through air ; but fuch com- 

 munication cannot be without fome interjacent body, to ail 

 as a medium. And fuch body mull be fubtile enough to 

 penetrate the pores of glafs ; and may be very well con- 

 cluded to penetrate thofe of all other bodies, and confe- 

 quently be ditfufed through all the parts of fpace j which 

 anfwers to the full character of a;i aither. He fuppofes 

 that it is rarer in the pores of bodies than in open fpaces, 

 and even rarer in fmall pores and denfe bodies than in large 

 pores and rare bodies ; and alfo, that its denfity increafcs 

 in receding from grofs matter, fo as to be greater, e.g. 

 at the T-J^yth of an inch from the furface of any body than 

 at its furface ; and fo on. 



The cxiftence of fuch an tetherial medium being fettled, 

 that author proceeds to its properties ;• inferring it to be 

 not only rarer and more fluid than air, but exceedingly- 

 more elafi;ic and aftive : in virtue of which properties, he 

 ftiews, that a gi-eat part of the phenomena of nature may 

 be produced by it. The elaftic force of this medium, in 

 proportion to its denfity, according to his mode of eftimat- 

 ing it, muft be above 700000 x 700000 times greater than 

 the elaftic force of the air in proportion to its denfity. If, 

 he fays, any one fliould fuppofe that sether, like our air, 

 may contain particles which endeavour to recede from one 

 another, and that its particles are exceedingly fmaller than 

 thofe of air, or even than thofe of light ; the exceeding 

 fmallnefs of its particles may contribute to the greatnefs of 

 the force, by which thofe particles may recede from onf 

 another, and thereby make that medium exceedingly more 

 rare and elaftic than air, and by confequence exceedingly 

 lefs able to refift the motions of projeiSliles, and exceedingly 

 more able to prefs upon grofs bodies by endeavouring tt» 

 expand itfelf. The rtfiftance of this medium, he fuppofes, 

 to be very inconfiderable. If this xther iliould be fuppofed 

 700000 times more elaftic than our air, and above 700000 

 times more rare, its refiftancc would be above 6000C0000 

 times lefs than that of water ; and a refiftance fo fraall 

 would fcarcely make any fenfible alteration in the motions 

 of the planets in 10,000 years. 



To the acStion of this medium he afcrihes the attraiaions 

 of gravitation and cohefion, the attraftio.is and repulfions of 

 electrical bodies, the elaftic force of the air, and of nervous 

 fibres, and the emiffion, refraijlion, reflection, and other 

 phenomena of light, the effects and communication of heat ; 

 as alfo fenfation, mufcular motion, &c. In fine, this fame 

 matter feeins the primum mobile, the firll fourcc or fpring, of 

 phyfical aiftion in the modern fyfteni. 



The Carlclian sether is fiippoled not onl)' to pei-vade, but 

 adequately to fill all the vacuities of bodies ; and thus to 

 make an abfolute pletium in the univerfe. See Materia 

 fuhtilis. 



But Sir Ifaac Newton overturns this opinion, from divers 



confidcrations; by (hewing that the ccleftial fpaces arc 



void of all fenfible refiftance; for, hcnee it follows, that 



S f ? tkc 



