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that is a prime, or the first minute, ; a second = js Vb ; athird Anciently, there were no other than sexagesi-

mals used in astronomy ; and they are still retained in many cases,though decimal arithmetic begins to grow in use now in astrono-mical calculations. In these fractions, which some call astrono-mical fractions, the denominator being always CO, or a multiplethereof, is usually omitted, and the numerator only written down :thus, 4° 59' 32" 30" 16”" is to be read, 4 degrees, 59 minutes,32 seconds, 50 thirds, 16 fourths, &c.

SEXTANS, SEXTANT , a sixth part of certain things. TheRomans having divided theirs as into 12 unci* or ounces, the sixthpart of that, or two ounces, was the sextans.

Sextans, in astronomy, a constellation of the southern hemis-phere, made by Hevelius out of unformed stars. See Astro-nomy .

Sextant , in mathematics, denotes an arch comprehending 60decrees.

Sextant , is also particularly used for an astronomical instrumentmade like a quadrant, excepting that its limb only comprehends60 degrees. The use and application of the sextant is the samewith that of the quadrant. See Quadrant, and Naviga-tion.

SEXTIEE, [sextilis, Lat.] such a position or aspect of twoplanets, when at 60 degrees distant, or at the distance of two signsfrom one another, and is marked thus See Aspect, and As-tronomy .

SEXTILIS, [Lat. i. e. the sixth month, from March,] inchronology, the name given by the Romans to the month of Au-gust, during the whole time of the kingdom and republic, anduntil the reign of the emperor Augustus , when it was changed incompliment to him, as Quintilis had been previously changed toJuly, in honour of his uncle Julius Caesar .

SEXTIUS, Quintus, a Pythagorean philosopher, who flourish-ed under Augustus . He seemed formed to rise in the republic;J>ut he shrunk from civil honours, and declined accepting the rankof senator when it was offered him by Julius Cxsar, that he mighthave time to apply to philosophy. It appears that he wished toestablish a school at Rome , and that his tenets, though chieflydrawn from the doctrines of Pythagoras , in some particulars re-sesembled those of the Stoics . His laws were tinctured with greatseverity; and in an early period of his establishment, he foundhis mind so harassed, and the harshness of the doctrines which he■"fished to establish so repulsive to his feelings, that he had nearlyWorked himself up to such an height of desperation, as to put aPeriod to his existence. Of the school of Sextius were Fabianus ,Sotion, Flavianus, Crassitus, and Celsus . Of his works only a*ew fragments remain ; and whether any of them formed a part of'he work which Seneca admired so much, cannot now be deter-mined. Some of his maxims are valuable. He recommended anexamination of (he actions of the day to his scholars when they re-nted to rest; he taught, that the road to Heaven (ad astra) was byril gality, temperance, and fortitude.

SEXTON, is thus called by corruption of the Latin sacrista,. 0r . Saxon segcrstone, which denotes the same. His office is to®ke care of Ihe vessels, vestments, belonging to the church ; andattend the minister, churchwarden, &c. at church. He isdually chosen by the parson only. Sextons, as well as parish-frx*’ are re S ai 'ded by the common law as persons who haveehold in their offices; and, therefore, though they may be pu-Vpv cannot he deprived, by ecclesiastical censures,is l ^XTUPLE, in music, denotes a mixed sort of triple, whichg?at e » i n double time.

an , h-XriJS, a Stoic philosopher, born at Chxronxa in Boeotia,said to be nephew of Plutarch . He was preceptor to the era-g° rs Marcus Aurelius, and Lucius Verus .jjv ? x . TUs Empiricus, a famous Pyrrlionian philosopher, whocian ln 'he second century, under Antoninus . He was a physi-tlie 'he sect of the Empirics, and is said to have been one oftantj^pl’tors °f Antoninus the philosopher. There are still ex-thein'u "yrrhonian Institutions, and a large work against the ma-of p r ltlc ! a » s , &c. The best edition of Sextus Empiricus i3 thatSFvtt C J us * n Greek and Latin , printed at Leipsic in 1718, fol.covevpi.'T SYSTEM, the beautiful system of botany, dis-SEYSsirr arran ® e< ' kj Dinnxus. See Botany .

SEL, a town of France, in the department of Ain ; and

province of Bresse, on the Rhone, 14 miles N. E. of Belley,Lon. 7. 45. E. Lat. 46. 6. N.

SEYZU, a singular kind of fruit, without kernels. SeeSamar.

SEZANNE, a town of France, in the department of Marne ,and province of Champagne, 27 miles N. W. of Troyes , and 65S. E. of Paris . Lon. 4. 10. E. Lat. 4S. 41. N.

SEZAWUL, [Hindoo,] in Bengal, an officer employed occa-sionally to collect the revenue, and enforce payment.

SEZZA, a town of Naples, in Lavora; 29 miles N. N. E. ofNaples, and 72 S. E. of Rome . 4

SFAX, or SFAKUSSE, a town of Africa, in Tunis , 45 milesS. S. E. of Cairoan. Lon. 10. 56. E. Lat. 34. 39. N.

SGIGATA, or STORA, a town of Africa, in Algiers , ancientlycalled Rusciada, 45 miles W. of Bona. Lon. 7. 6. E. Lat. 36.43. N.

SGIGATCHE, a (own of Asia, in Thibet , near the Sanpoo ;128 miles VV. of Lassa, and 244 N. E. of Catinandu. Lon. 88.59- E. Lat. 30. 40. N.

SHAB, a town of Africa , in Nubia; 400 miles S. S. W. ofCairo . Lon. 30. 30. E. Lat. 23. 33. N.

SHABUR, a town of Egypt, on the W. branch of the Nile ;50 miles N. N. E. of Cairo , and 48 S. E. ot Alexandria.

SHACK, in ancient customs, a liberty of winter-pasturage. Inthe counties of Norfolk and Suffolk, the lord of the manor hasshack, i. e. a liberty of feeding his sheep at pleasure in his tenants’lands during the six winter-months.

SHACKLES, aboard a ship, are those oblong iron-rings, big-ger at one end than at the other, with which the ports are shut fast,bv thrusting the wooden bar of the port through them v

SHAD, in ichthyology. See Clupea.

SHAD DOCK, a species of citrus.

SHADOW, in optics, is a privation or diminution of light bythe interposition of an opaque body : or it is a plane where thelight is either altogether obstructed, or greatly weakened, by theinterposition of some opaque body between it and the luminary.

Shadow, in painting, is an imitation of a large shadow, effectedby gradually heightening and darkening the colours of such figuresas by their dispositions cannot receive any direct rays from theluminary that is supposed to enlighten the piece.

Shadow, in perspective, the appearance of an opaque body,and a luminous one, whose rays diverge, (e. gr. a candle, lamp,&c.) being given; to find the just appearance of the shadow, ac-cording to the laws of perspective. See Perspective. The sha-dows of the sun are equal in objects of the same height, though ata distance from each other. See Plate CLVII. fig. 1. Expe-rience teaches, that stiles, or elevations of the same height, re-moved to a distance from each other, do yet project equal shadowsat tlie same lime: for they are lengthening and shortening, in pro-portion as the sun comes nearer, or recedes further off; one orother of which he is continually doing. For this reason, when theshadow of an object is to be cast any tvay, you must determinethe place of the sun, and the point underneath, to draw two occultlines from the same, for the extremity of the shadow ; as here thepalisade A gives the extreme of its shadow in B : and if from thispoint B, you draw to the point of sight C, (his line BC will be theshadow of the palisade D, as well as that of A, and of all the rest inthe same line to the very point of sight. In effect, it must beheld for a certain maxim, that shadows always retain the samepoint of sight as the objects. On the footing of this observation,that objects of the same height give equal shadows: if you wouldgive the shadow of the palisades, E, F, which are of the sameheight as A, D, lake in your compasses the distance AD, and setit on the foot of the palisade E, by which you will have EG ;then front G draw a line to the point of sight C ; and thus you areto proceed, let the number of walks be ever so great. Thoughthe sun is made to appear in this figure, it must not be supposedthat he is so near the objects ; the design being only to shew, thatthe rays proceed from him in this manner, when at such a height,though far without the limits of the piece ; as in fig. 2. which yethas the line for the foot of the object AB ; and those of the raysof the sun C, C, C, because these are always required for findingthe extremities of the shadows. The shadow of the object O isfound by containing the line AB, and making it rise over the steps,and against the wall,.till cut by the ray in the point S, by the rays

passing