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Seam'ing or faU-mnkr's twine, for fewing the feams of fails, 

 is made of the bell long hemp, beaten, fpun fine, and well 

 dreffed over a fine clearer : eighteen threads are fpun out of 

 half a pound of hemp, every thread being fifty yards in 

 length : two threads are twiftjd together ilack, and wound 

 on a reel, in half-pound il^ains containing four hundred 

 and fifty yards : but twine of three threads is ufed in the navy. 

 Sean'-t-ivine is made from good long hemp, each thread 

 fpun fifty -four yards : three threads are hid together. 

 When hardened and Rretched, each cord ftands fifty yards ; 

 nine hundred yards are wound on a reel, and eighteen cords 

 weigh two pounds. 



Store-tivine, ufed by fail-makers for old work and on board 

 of (hips, is made from good long hemp, well drefled. Four- 

 teen threads are fpun from half a pound ; two threads are 

 twifted together, and wound into half-pound Ikains of three 

 hundred and fifty yards. 



Turtle-f'wine, for turtle-nets, is made of good bar hemp, 

 fpun one hundred yards : three threads are laid together, 

 ftand ninety yards, and weigh one pound. 



Whipping- twine, the fame as bolt -rope twine. 

 TWINING Irons, fquare bars with an efs-hook at 

 one end, which g^afp the porter or the fhank of an anchor 

 to turs it over. 



TWINKLING of the Stars, denotes that tremulous, 

 vibratory, intermitting motion, which is obferved in the Light 

 proceeding from the fixed ftars : Alhazen, an Arabian phi- 

 lofopher of the twelfth century, confiders refraftion as the 

 caufe of this phenomenon. 



Vitellio, in his Optica, publilhedin 1 270, p. 449, afcribes 

 the twinkling of the ftars to the motion of the air in which 

 the light is refrafted ; and he obferves, in confirmation of 

 this hypothefis, that they twinkle ftill more when tliey are 

 viewed in water put into motion. 



Dr. Hooke (Microgr. p. 231, &c.) very reafonably attri- 

 butes this phenomenon to the inconftant and unequal refrac- 

 tion of the rays of light occafioned by the trembling motion 

 of the air and interfperfed vapours, in confequence of variable 

 degrees of heat and cold in the air, producing correfpond- 

 ing variations in its rarity or denfity, and alio of the aftion 

 of the wind, which muft caufe the fucceffive rays to fall 

 upon the eye in different direftions, and confequently upon 

 different parts of the retina at different times, and alfo to hit 

 and mifs the pupil alternately ; and this is alfo the reafon, 

 he fays, why the limbs of the fun, moon, and planets appear 

 to wave or dance. 



Thefe tremors of the air are manifeft to the eye by the 

 tremulous motion of fhadows caft from high towers ; and 

 by looking at objefts through the fmoke of a chimney, or 

 through ftreams of hot water, or at objeAs fituated be- 

 yond hot fands, efpecially if the air be moved tranfverfely 

 over them. But when ftars are feen through telefcopes that 

 have large apertures, they twinkle but little, and fometimes 

 not at all. For, as fir Ifaac Newton has obferved, (Opt. 

 p. 98.) the rays of light which pafs through different parts 

 of the aperture, tremble each of them apart, and by means 

 of their various, and fometimes contrary tremors, fall at one 

 and the fame time upon different points in the bottom of the 

 eye, and their trembling motions are too quick and confufcd 

 to be feparately perceived. And all thefe illuminated points 

 conftitute one broad lucid point, compofed of thofe many 

 trembhng points confufedly and infenfibly mixed with one 

 another by very ftiort and fwift tremors, and thereby caufe 

 the ftar to appear broader than it is, and without any trem- 

 bling of the whole. 



Dr. Jurin, in his Effay upon Diftina and Indiftina Vi- 

 fion, has rccourfe to fir Ifaac Newton's hypothefis of fits of 



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eafy refraftion and refleftion for explaining the twinkling oi 

 the liars : thus, he fays, if the middle part of the image of 

 a ftar be changed from light to dark, and the adjacent ring 

 be at the fame time changed from dark to light, as muft 

 happen from the leaft motion of the eye towards or from the 

 ftar, this will occafion fuch an appearance as twinkling. 



Mr. Michell (Phil. Tranf. vol. Ivii. p. 262.) fuppofes that 

 the arrival of fewer or more rays at one time, efpecially fron\ 

 the fmaller or more remote fixed ftars, may make fuch an un- 

 equal impreflion on the eye, as may, atleaft, have fome fhare in 

 producing this effeft : fince it may be fuppofed, that even :i 

 fingle particle of light is fufBcient to make a fenfible impref- 

 iion upon the organs of fight ; fo that a very few particles , 

 arriving at the eye in a fecond of time, perhaps not more 1 

 than three or four, may be fufficient to make an objeft con- 

 ftantly vifible. See Light. 



Hence, he fays, it is not improbable, that the number oi 

 the particles of light which enter the eye in a fecond of 

 time, even from Sirius himfelf, may not exceed three or 

 four thoufand, and from ftars of the fecond magnitude they 

 may, therefore, probably not exceed a hundred. Now the 

 apparent increafe and diminution of the light, which we ob- 

 ferve in the twinkling of the ftars, feem to be repeated at 

 not very unequal intervals, perhaps about four or five times 

 in a fecond. He, therefore, thought it reafonable to fup- . 

 pofe, that the inequalities which will naturally arife from 

 the chance of the rays coming fometimes a little denfer, and 

 fometimes a little rarer, in fo fmall a number of them, as mufh 

 fall upon the eye in the fourth or fifth part of a fecond, may 

 be fufBcient to account for this appearance. An addition 

 of two or three particles of light, or perhaps a fingle one, 

 upon twenty, efpecially if there fliould be an equal defici- 

 ency, out of the next twenty, would, he fuppofed, be very 

 fenfible, as he thought was probable from the very great 

 difference in the appearance of ftars, the light of which 

 does not differ fo much as is commonly imagined. The 

 light of the middlemoft flar in the tail of the Great Bear 

 does not, he thinks, exceed the light of the very fmall ftar 

 that is next to it, in a greater proportion than that of about 

 16 or 20 to I ; and M. Bouguer found, that a difference in 

 the light of objefls, of one part in fixty-fix, was fufficiently 

 diftinguifhable. 



Since thefe obfervations were publilhed, Mr. Michell (as 

 we are informed by Dr. Prieftlcy, Hift. of Light, p. 49J. ) 

 has entertained fome fufpicion, that the unequal denfity of 

 light does not contribute to this effeft in fo great a degree 

 as he had imagined ; efpecially in confequence of obferving 

 tliat even Venus does fometimes twinkle. This he once 

 obferved her to do remarkably when flie was about fi.x 

 degrees high, though Jupiter, which was then about fixteen 

 degrees high, and was fenfibly lefs luminous, did not twinkle 

 at all. If, notwithftanding the great number of rays, 

 which, without doubt, come to the eye from fuch a furface 

 as this planet prefents, its appearance be liable to be affedled 

 in this manner, it muft be owing to fuch undulations in the 

 atmofphere, as will probably render the eSaSt of every other 

 caufe altogether infenfible. 



M. Mufchenbroeck (Introd. ad Phil. Nat. vol. ii. feft. 

 1 741, p. 707.) fufpefts, that the twinkling of the ftars 

 arifes from fome affeftion of the eye, as well as the ftate of 

 the atmofphere. For, he fays, that in Holland, when the 

 weather is frofty, and the fl<y very clear, the ftars twinkle 

 moft manifeftly to the naked eye, though not in telefcopes ; 

 and fince he does not fuppofe there is any great exhalation, 

 or dancing of the vapour at that time, he queftions wliether 

 the vivacity of the light, affedling the eye, may not be con- 

 cerned in the phenomenon. 



But 



