T W I 



Bui this philofopher might have fatiafied himfelf with 

 j-efpeft to this hypotVcfis, by looking at the ftars near the 

 aenith, when the light iraverfes but a fmall part of the at- 

 iriofphere, and therefore might be expefted to affeft the eye 

 moft fenfibly. For he would not have perceived them to 

 twinkle near fo much as they do near the horizon, when 

 much more t)f their light is intercepted by the atmofphere. 



Some aftronomers have lately endeavoured to explain the 

 twinkling of the fixed ftars, by the extreme minutenefs of 

 their apparent diameter ; fo that they fuppofe the fight 

 of them is intercepted by every mote that floats in the air. 

 To this purpofe Dr. Long obferves { Aitron. vol. i. p. 170.) 

 that our air near the earth is fo full of various kinds of par- 

 ticles, which are in continual motion, that fome one or other 

 of them is perpetually pafhng between us and any ftar 

 which we look at, and this makes us every moment alter- 

 nately fee it and lofe fight of it : and this twinkUng of the 

 ftars, he fays, is greateil in thofe which are neareft the 

 horizon, becaufe they are viewed through a great quantity 

 of thick air, where the intercepting particles are moft nu- 

 merous ; whereas ftars that are near the zenith do not 

 twinkle fo much, becaufe we do not look at tliem through 

 fo much thick air, and therefore the intercepting pailicles 

 being fewer, come lefs frequently before them. 



With refpeft to the planets, it is obferved, that tliey, be- 

 caufe they are much nearer to us than the ftars, have a fen- 

 fible apparent magnitude, fo that they are not covered by 

 the fmall particles floating in the atmofphere, and therefore 

 do not twinkle, but fliine with a fteady hght. The fallacy 

 of this hypothefis appears from the obfervation of Mr. 

 Michell, that no objeft can hide a ftar from us that is not 

 large enough to exceed tlie apparent diameter of the ftar, by 

 the diameter of the pupil of the eye ; fo that if a ftar was a 

 mathematical pomt, the interpofing objeft muft ftill be 

 equal in fize to the pupil of the eye : and, indeed, it muft 

 be large enough to hide the ftar from both eyes at the fame 

 time. 



The principal caufe, therefore, of the twinkling of the 

 ftars is now acknowledged to be, the unequal refradlion of 

 light, in confequeiice of inequalities and undulations in the 

 atmofphere. 



Befides a variation in the quantity of light, it may here be 

 added, that a momentary change of colour has likewife been 

 obferved in fome of the fixed ftars. Mr. Melville (Edinb. 

 Eft. vol. ii. p. 81.) fays, that when one looks ftedfaftly at 

 Sirius, or any bright ftar, not much elevated above the 

 horizon, its colour feems not to be conftantly white, but 

 appears tinftured, at every twinkling, with red and blue. 

 Mr. Melville could not entirely fatisfy himfelf as to the 

 caufe of this phenomenon ; obfei-ving, that the feparation 

 of the colours by the refraftive power of the atmofphere 

 is, probably, too fmaU to be perceived. Mr. Michell's 

 hypothefis above-mentioned, though inadequate to the exph- 

 cation of the twinkling of the ftars, may pretty well ac- 

 count for this circumftance. For the red and blue rays 

 being much fewer than thofe of the intermediate colours, 

 and therefore much more hable to inequalities, from the 

 common effeft of chance, a fmall excefs or defed in either 

 of them, will make a very fenfible difference m the colour of 

 the ftars. 



TWINS, two young ones delivered at a birth by an 

 animal which ordinarily brings forth but one. _ 



It has been greatly difputed, which of two twins is to be 

 efteemed the elder ? The faculty of Montpelier have given 

 it, that the latter born is to be reputed the elder, becaule 

 iirft conceived : but by all the laws which now obtain, the 



Two 



fifft-born enjoys the privilege of feniority ; and the cuftom 

 IS confirmed by the fcripture inftance of Efau and Jacob. 



But if two twins be born fo intermixed, that one cannot 

 diftinguifli which of the two appeared firft, it fhould feem 

 that neither the one nor the other can pretend to the right 

 of primogeniture, which ought to remain in fufpence by 

 reafon of their mutual concourfe. In fuch cafe, fome would 

 have the decifion left to the father, and others to the chance 

 of a lot. 



In catttle, twins are feldom defirable, as they can rarely 

 be well fupported, though occafionally cows are capable of 

 fuckhng two calves ; but in ftieep, it is often greatly de- 

 firable in different breeds or varieties. The frequency of 

 twins in ftieep is much influenced both by the condition of 

 the ewes, and the ftate of vigour in the rams. Such flocks 

 as are well fed, and pafture eafily about the tupping time, 

 ufually produce a far larger proportion of twins, than thofe 

 which have higher walks, or pafturcs which are of inferior 

 quality. Twins too are the moft frequent in the early part 

 of the lambing feafon : all which fliew the propriety of 

 keeping the ewes well fupported, and of not letting the 

 rams have too great a number put to them at fuch feafons. 



Nice attention is neceflary to twins at the time of 

 lambing, by the perfon who has the charge of the bufinefs, 

 to fee that they are in every way properly taken care of. 

 See Sheep. 



Twins, in AJlronomy. See Gemini. 

 TvviN,s, in Geography, two fmall iflands in the Eaft 

 Indian fea, near the ifland of Paraguay. N. lat. 9° 18'. 



E. long. 118° 3' Alfo, two fmall iflands in the Eaft 



Indian fea, near the north eoaft of the ifland of Flores. 

 S. lat. 8° 2'. E.long. 1 22° 33'. 



TWIST of a Rope, Cord, &c. See Rope. 

 Twist, again, is ufed for the infide, or fiat part of a 

 man's thigh, upon which a true horfeman refts when on 

 horfeback. 



Twist a Horfe, To, is violently to wring or twift his tef- 

 ticles twice about, which caufes them to dry up, and de- 

 prives them of nourifliment, and reduces the horfe to the 

 fame ftate of impotency with a gelding. 



TWISTE, in Geography, a river of Germany, which runs 

 into the Erpe, near Valckmarfen, in the duchy of Weft- 

 phaha. 



TWISTED Column. See Column. 

 Twisted Silks. See Silk. 



Twisted Thumb-Band, in Rural Economy, a term ufed to 

 fignify the band of hay or ftraw which is formed by means 

 of the thumb and fingers, by twifting it with them, and em- 

 ployed in binding up trufl"es of thefe kinds for fale. See 

 Truss. ,. , 



TWITCH, \a Agriculture, a name often applied to a 

 very troublefome plant of the weed kind in tillage-land, of 

 which there are feveral forts, as the black, white, creeping, 

 and fome others. See CouCH-Gz-^yJ-. 



Tviircn-Rake, a name fometimes applied to a large tool 

 of this kind, made ufe of m dragging and colleaing the 

 roots of couch together in arable land, by means of a horfe. 

 See Couch and Rake. 



TWITE, in Ornithology. See Linnet. , . , 



TWO Brothers, in Geography, two fmall iflands in the 

 Chinefe fea. N. lat. 8=^ 30'. E. long. 105° 48'.— Alfo, 

 two fmall iflands in the Eaft Indian fea, near the W. coatt 

 of Borneo. S. lat. 1° 32'- E. long. 109° 1 3'.--Alfo, two 

 fmall iflands in the Eaft Indian fea, 27 miles from the 

 E. point of the ifland of Madura. S. lat. 6 50. iL. 

 long. 114° 43'. — Alfo, two fmall iflands m the Eaft 



