Lettuce.

Letter, than the other. However, this makes no odds as to the kind of ftyle; for every one would choose to speak as correctly as he writes, if he could. And therefore all fuch words and expreffions, as are unbecoming in converfation, fhould be avoided in letters; and a manly fimplicity free of all affectation, plain, but decent and agreeable, fhould run through the whole. This is the ufual ftyle of Cicero's epiftles, in which the plainnefs and fimplicity of his diction is accompanied with fomething so pleasant and engaging, that he keeps up the attention of his reader, without fuffering him to tire. On the other hand, Pliny's ftyle is fuccinct and witty; but generally fo full of turns and quibbles upon the found of words, as apparently render it more ftiff and affected than agrees with converfation, or than a man of fenfe would choofe in difcourfe, were it in his power. You may in fome meafure judge of Pliny's manner, by one fhort letter to his friend, which runs thus: "How fare you? As I do in the country? pleasantly? that is, at leisure? For which reafon I do not care to write long letters, but to read them; the one as the effect of nicenefs, and the other of idleness. For nothing is more idle than your nice folks, or curious than your idle ones. Farewell." Every fentence here confifts of an antithefis, and a jingle of words, very different from the ftyle of converfation, and plainly the effect of ftudy. But this was owing to the age in which he lived, at which time the Roman eloquence was funk into puns, and an affectation of wit; for he was otherwife a man of fine fenfe and great learning.

LETTER of Attorney, in law, is a writing by which one perfon authorifes another to do fome lawful act in his ftead; as to give feifin of lands, to receive debts, fue a third person, &c.

The nature of this inftrument is to transfer to the perfon to whom it is given, the whole power of the maker, to enable him to accomplish the act intended to be performed. It is either general or fpecial: and fometimes it is made revocable, which is when a bare authority is only given; and fometimes it is irrevocable, as where debts, &c. are affigned from one perfon to another. It is generally held, that the power granted to the attorney must be ftrictly pursued; and that where it is made to three perfons, two cannot execute it. In most cafes, the power given by a letter of attorney determines upon the death of the perfon who gave it. No letter of attorney made by any feamen, &c. in any fhip of war, or having letters of marque, or by their executors, &c. in order to empower any perfon to receive any fhare of prizes or bounty-money, fhall be valid, unless the fame be made revocable, and for the ufe of fuch feamen, and be higned and executed before, and attefted by, the captain and one other of the figning officers of the fhip, the mayor or chief magiftrate of fome corporation. LETTER of Mart or Marque. See MARQUE. LETTERS Patent or Overt, are writings fealed with the great feal of England, whereby a man is authorifed to do, or enjoy any thing, which, of himself, he could not do. See PATENT. They are fo called, by reafon of their form; as being open, with the feal affixed, ready to be fhown for the confirmation of the authority given by them.

LETTUCE, in botany. See LACTUCA.

Leucata.

LEVANT, in geography, fignifies any country Levant fituated to the east of us, or in the eastern fide of any continent or country, or that on which the fun rises. LEVANT, is also a name given to the eastern part of the Mediterranean fea, bounded by Natolia or the Leffer Afia on the north, by Syria and Palestine on the eaft, by Egypt and Barca on the fouth, and by the island of Candia and the other part of the Mediterranean on the west.

LEVATOR, in anatomy, a name given to several mufcles. See ANATOMY, Table of the Mufcles.

LEUCA, in antiquity, a geographical measure of length in ufe among the later Gauls; which, according to Jornandes, who calls it leuga, contained fifteen hundred paces, or one mile and a half. Hence the name of league, now reckoned at three miles; in the lower age, called leuva.

LEUCADENDRON, in botany: A genus of the monogynia order, belonging to the tetrandria class of plants; and in the natural method ranking under the 48th order, Aggregate. The florets are tripetalous, with one petal of each trifid; the receptacle is a little villous; there is no proper calyx; the antheræ are almoft coalited.

LEUCADIA, formerly called Neritis, a peninfula of Acarnania, (Homer); but afterwards, by cutting through the peninfula, made an island, as it is at this day, called St Maura.

LEUCAS, (anc. geog.), formerly called Neritos and Neritum, a town of Leucadia or Leucas; fituated near a narrow neck of land, or ifthmus, on a hill facing the eaft and Acarnania: the foot or lower part of the town was a plain lying on the fea by which Leucadia was divided from Acarnania, (Livy); though Thucydides places Leucas more inward in the island, which was joined to the continent by a bridge. It was an illuftrious city, the capital of Acarnania, and the place of general assembly.

LEUCATA, or LEUCATE, (anc. geog.); a promontory of Leucadia according to Strabo, a white rock projecting into the fea towards Cephalenia, on which stood a temple of Apollo furnamed Leucadius. At his feftival, which was annually celebrated here, the people were accuftomed to offer an expiatory facrifice to the god, and to avert on the head of the victim all the calamities with which they might be threatened. For this purpose, they made choice of a criminal condemned to die; and leading him to the brink of the promontory, precipitated him into the fea amidst the. loud fhouts of the fpectators. The criminal, however, feldom perished in the water: for it was the cuftom to cover him with feathers, and to faften birds to his body, which by fpreading their wings might ferve to break his fall. No fooner did he touch the fea, than a number of boats ftationed for the purpose flew to his affiftance, and drew him out; and after being thus faved, he was banished for ever from the territory of Leucadia. (Strabo, lib. 10. p. 452.)

According to ancient authors, a ftrange opinion concerning this promontory prevailed for fome time among the Greeks. They imagined that the leap of Leucata was a potent remedy against the violence of love. Hence disappointed or defpairing lovers, it is faid, were often known to have come to Leucadia;, and, having afcended the promontory, offered facri

Leucippus fices in the temple, and engaged by a formal vow to Leucoma, perform the defperate act, to have voluntarily preci#1 pitated themselves into fea. Some are reported to have recovered from the effects of the fall; and among o§ Ptolem. thers mention is made § of a citizen of Buthroton, in Hepbaf. Epirus, whofe paffions always taking fire at new obap. Phot. jects, he four times had recourfe to the fame remedy, and P. 491. always with the fame fuccefs. As those who made the trial, however, feldom took any precaution to render their fall less rapid, they were generally deftroyed; and women often fell victims to this act of defperation.--At Leucata was shown the tomb of Artemifia, that celebrated queen of Caria who gave fo many + Herodot proofs of courage at the battle of Salamist. Inflamed lib. 8. with a violent paffion for a young man who inflexibly cap 87. refused her love, she surprised him in his fleep and put out his eyes. Regret and defpair foon brought her to Leucata, where the perished in the waves notwithstanding every effort to fave her ‡. Such likewife was the end of the unhappy Sappho. Forfaken by her lover Phaon, the came hither to feek relief from her fufferings, and found her death. (Menand. ap. Strab.

Ptolem.

Hepbaft. ibid.,

lib. 10. p. 452.) LEUCIPPUS, a celebrated Greek philofopher and mathematician; firft author of the famous fyftem of atoms and vacuums, and of the hypothefis of ftorms; fince attributed to the moderns. He flourished about 428 B. C.

LEUCOGÆUS, (anc. geog.), a hill fituated between Puteoli and Neapolis in Campania, abounding in fulphur; now l'Alumera. Whence there were alfo fprings called Leucogai fontes; the waters of which, according to Pliny, gave a firmnefs to the teeth, clearnefs to the eyes, and proved a cure in wounds.

LEUCOJUM, GREAT SNOW-DROP: a genus of the monogynia order, belonging to the hexandria clafs of plants; and in the natural method ranking under the ninth order, Spathacea. The corolla is campanulated, fexpartite, the fegments increased at the points, the ftigma fimple.

Species. 1. The vernum, or fpring leucojum, has an oblong bulbous root, fending up feveral flat leaves fix or eight inches long; and amidst them an upright, channelled, hollow, naked ftalk, about a foot high, terminated by a fpatha, protruding one or two white flowers on flender footftalks drooping downwards, and appearing in March. 2. The æftivum, or fummer leucojum, has a large, oblong, bulbous root, crowned with feveral long, flat, broad leaves; and amidft them an upright, thick, hollow stalk, 15 or 18 inches high; terminated by a fpatha, protruding many white flowers, on flender footftalks, drooping down wards; flowering in May. 3. The autumnale, or autumnal leucojum, hath a large oblong bulbous root, crowned with many narrow leaves, an upright, naked, hollow ftalk, terminated by a fpatha protruding many white flowers on long weak footftalks, hanging down wards, and flowering in autumn.

Culture. All the three fpecies are very hardy, durable in root, and increase exceedingly by offsets, which may be feparated every two or three years.

LEUCOMA, in antiquity, was a public regifter amongst the Athenians, in which were inferted the names of all the citizens, as foon as they were of age to enter upon their paternal inheritance..

Leuctra.

LEUCOMA, in furgery, a diftemper of the eye, Leucopetra. otherwife called albugo. See ALBUGO, and SURGERY. LEUCOPETRĂ, (anc. geog.) fo called from its white colour (Strabo); a promontory of the Bruttii, in the territory of Rhegium: the termination of the Apennine. The outmoft extremity of the Bruttii, or the modern Calabria Ultra; as the Japygium is of the ancient Calabria, or the modern Calabria Citra. LEUCOPETRIANS, in ecclefiaftical hiftory, the name of a fanatical fect which sprang up in the Greek and Eastern churches towards the clofe of the 12th century; the fanatics of this denomination profeffed to believe in a double Trinity, rejected wedlock, abftained from flesh, treated with the utmost contempt the facraments of Baptism and the Lord's Supper, and all the various branches of external worship; placed the effence of religion in internal prayer alone, and maintained, as it is faid, that an evil being, or genius, dwelt in the breast of every mortal, and could be expelled from thence by no other method than by per petual fupplication to the Supreme Being. The founder of this enthusiastical fect is faid to have been a perfon called Leucopetrus, and his chief difciple Tychicus, who corrupted, by fanatical interpretations, feveral books of fcripture, and particularly St Matthew's gofpel.

LEUCOPHLEGMATIA, in medicine, a kind of dropfy, otherwise called anafarca. See (Index fubjoined to) MEDICINE.

LEUCOTHOE, or LEUCOTHEA (fab. hift.), the wife of Athamus, changed into a sea deity; fee Ino. She was called Matuta by the Romans. She had a temple at Rome, where all the people, particularly women, offered vows for their brother's children. They did not intreat the deity to protect their own. children, because Ino had been unfortunate in hers. No female flaves were permitted to enter the temple; or if their curiofity tempted them to tranfgrefs this. rule, they were beaten with the greatest severity. To this fupplicating for other people's children, Övid alludes in these lines;

Faft. 6.

Non tamen banc pro ftirpe fua pia mater adorat, Ipfa parum felix vifa fuiffe parens. LEUCTRA, (anc. geog.), a town of Bœotia, to the weft of Thebes, or lying between Platex and Thefpiæ, where the Lacedemonians had a great defeat given them by Epaminondas and Pelopidas the Theban generals. The Theban army confifted at moft but of 6000 men, whereas that of the enemy was at least thrice that number: but Epaminondas trufted most in his horfe, wherein he had much the advantage, both in their quality and good management; the rest he endeavoured to fupply by the difpofition of his men, and the vigour of the attack. He even refused to fuffer any to serve under him in the engagement, but such as he knew to be fully refolved to conquer or die. He put himself at the head of the left wing, oppofite to. Cleombrotus king of Sparta, and placed the main ftrefs of the battle there; rightly concluding, that if he could break the body of the Spartans, which was but 12 men deep, whereas his own was 50, the rest would be foon put to flight. He clofed his own with: the facred band, which was commanded by Pelopidas; and placed his horfe in the front. His right, from which he had drawn fo many men, he ordered to fall

back,,

Leudra, back, in a flanting line, as if they declined to fight, Level. that they might not be too much exposed to the enemy, and might ferve him for a corps of referve in cafe of need. This was the wife difpofition which the two Theban generals made of their few but refolute forces; and which fucceeded in every part, according to their wish. Epaminondas advanced with his left wing, extending it obliquely, in order to draw the enemy's right from the main body; and Pelopidas charged them with fuch defperate speed and fury, at the head of his battalion, before they could reunite, that their horse, not being able to ftand the fhock, were forced back upon their infantry, which threw the whole into the greateft confufion: fo that though the Spartans were of all the Greeks the most expert in recovering from any surprise, yet their skill on this occafion either failed them or proved of no effect; for the Thebans, obferving the dreadful impreffion they had made on them with their horse, pushed furiously upon the Spartan king, and opened their way to him with a great flaughter.

Upon the death of Cleombrotus, and feveral officers of note, the Spartans, according to cuftom, renewed the fight with double vigour and fury, not so much to revenge his death as to recover his body, which was fuch an established point of honour as they could not give up without the greatest difgrace. But here the Theban general wifely chofe rather to gratify them in that point, than to hazard the fuccefs of a fecond onfet; and left them in poffeffion of their king, whilft he marched ftraight against their other wing, commanded by Archidamus, and confifted chiefly of fuch auxiliaries and allies as had not heartily engaged in the Spartan intereft: these were fo difcouraged by the death of the king and the defeat of that wing, that they betook themselves to flight, and were prefently after followed by the reft of the army. The Thebans, however, pursued them fo closely, that they made a fecond dreadful flaughter among them; which completed Epaminondas's victory, who remain ed mafter of the field, and erected a trophy in memory of it. This was the conclufion of the famed battle of Leuctra, in which the Lacedemonians loft 4000 men, and the Thebans but 300.

LEVEL, an instrument wherewith to draw a line parallel to the horizon, by means of which the true level, or the difference of afcent or descent between feveral places, may be found, for conveying water, draining fens, &c. There are feveral inftruments of different contrivance and matter, invented for the perfection of levelling; all of which, for the practice, may be reduced to the following.

Air-LEVEL, that which fhows the line of level by means of a bubble of air inclosed with fome liquor in a glass-tube of an indeterminate length and thickness, whose two ends are hermetically fealed. When the bubble fixes itself at a certain mark, made exactly in the middle of the tube, the plane or ruler wherein it is fixed is level. When it is not level, the bubble will rife to one end. This glafs-tube may be fet in another of brafs, having an aperture in the middle, whence the bubble of air may be obferved. The liquor wherewith the tube is filled is oil of tartar, or aqua fecunda; thefe not being liable to freeze as common water, nor to rarefaction and condenfation, as fpirit of wine is.

This application of a bubble of air was the invention of Dr Hooke.

There is one of these inftruments made with fights, being an improvement upon that last described, which, by the addition of more apparatus, becomes more commodious and exact. It confifts of an air-level, fig. 1. about eight inches long, and feven or eight lines in diameter, fet in a brass tube, 2, with an aperture in the middle, C. The tubes are carried in a strong straight ruler, a foot long; at whofe ends are fixed two fights, 3, 3, exactly perpendicular to the tubes, and of an equal height, having a fquare hole, formed by two fillets of brafs croffing each other at right angles, in the middle whereof is drilled a very little hole, through which a point on a level with the inftrument is defcried. The brafs tube is fastened on the ruler by means of two fcrews; one whereof, marked 4, ferves to raise or deprefs the tube at pleasure, for bringing it towards a level. The top of the ball and focket is rivetted to a little ruler that springs, one end whereof is faftened with fcrews to the great ruler, and at the other end has a fcrew, 5, ferving to raise and depress the inftrument when nearly level.

The inftrument juft defcribed, however, is yet lefs commodious than the following one; because though the holes be ever so small, yet they will ftill take in too great a space to determine the point of level precifely.

The inftrument alluded to confifts of an air-level, with telescope fights. This level (fig. 2.) is like the last; with this difference, that, instead of plain fights, it carries a telescope to determine exactly a point of level at a good diftance. The telescope is a little brass-tube, about 15 inches long, fastened on the fame ruler as the level. At the end of the tube of the telescope, marked 1, enters the little tube 1, carrying the eye-glafs and an hair horizontally placed in the focus of the object-glafs, 2; which little tube may be drawn out, or pushed into the great one, for adjusting the telescope to different fights: at the other end of the telescope is placed the object-glafs. The fcrew 3, is for railing or lowering the little fork, for carrying the hair, and making it agree with the bubble of air when the inftrument is level; and the fcrew 4, is for making the bubble of air, D or E, agree with the telescope: the whole is fitted to a ball and focket. M. Huygens is faid to be the first inventor of this level; which has this advantage, that it may be inverted by turning the ru ler and telescope half round; and if then the hair cut the fame point that it did before, the operation is just.

It may be observed, that one may add a telescope to any kind of level, by applying it upon, or parallel to, the base or ruler, when there is occafion to take the level of remote objects.

Dr Defaguliers contrived an inftrument, by which the difference of level of two places, which could not be taken in lefs than four or five days with the best telescope-levels, may be taken in as few hours. The inftrument is as follows. To the ball C (fig. 3.) is joined a recurve tube B A, with a very fine bore, and a fmall bubble at top A, whofe upper part is open. It is evident from the make of this inftrument, that if it be inclined in carrying, no prejudice will be done to the liquor, which will always be right both in the ball and tube when the inftrument is fet upright. If the air at C be fo expanded with heat, as to drive the

6

liquor

Level. liquor to the top of the tube, the cavity A will receive the liquor, which will come down again and fettle at D, or near it, according to the level of the place where the instrument is, as soon as the air at C returns to the fame temperament as to heat and cold. To preferve the fame degree of heat, when the different obfervations are made, the machine is fixed in a tin veffel EF, filled with water up to g h, above the ball, and a very fenfible thermometer has also its ball under water, that one may obferve the liquor at D, in each experiment, when the thermometer ftands at the fame height as before. The water is poured out when the inftrument is carried; which one may do conveniently by means of the wooden frame, which is fet upright by the three fcrews, S, S, S, fig. 4. and a line and plummet P P, fig. 5. At the back part of the wooden frame, from the piece at top K, hangs the plummet P, over a brafs point at N; Mm are brackets to make the upright board K N continue at right. angles with the horizontal one at N. Fig. 6. reprefents a front view of the machine, fuppofing the fore part of the tin-veffel transparent; and here the brafs-focket of the recurve-tube, into which the ball is fcrewed, has two wings at II, fixed to the bottom, that the ball may not break the tube by its endeavour to emerge when the water is poured in as high as g h. After the Doctor had contrived this machine, he confidered, that as the tube is of a very small bore, if the liquor should rife into the ball at A (fig. 3.) in carrying the inftrument from one place to another, fome of it would adhere to the fides or the ball A, and upon its descent in making the experiment, fo much might be left behind, that the liquor would not be high enough at D to show the difference of the level: therefore, to prevent that inconveniency, he contrived a blank screw, to shut up the hole at A, as foon as one experiment is made, that, in carrying the machine, the air in A may balance that in C, fo that the liquor fhall not run up and down the tube, whatever degree of heat and cold may act upon the inftrument, in going from one place to another. Now, because one experiment may be made in the morning, the water may be fo cold, that when a fecond experiment is made at moon the water cannot be brought to the fame degree of cold it had in the morning; therefore, in making the first experiment, warm water must be mixed with the cold, and when the water has ftood fome time, before it comes to be as cold as it is likely to be at the warmeft part of that day, obferve and fet down the degree of the thermometer at which the fpirit ftands, and likewife the degree of the water in the barometer at D; then screw on the cape at A, pour out the water, and carry the inftrument to the place whofe level you would know; then pour in your water, and when the thermometer is come to the fame degree as before, open the screw at top, and obferve the liquor in the barometer.

The Doctor's fcale for the barometer is ten inches long, and divided into tenths; fo that fuch an inftrument will ferve for any heights not exceeding ten feet, each tenth of an inch anfwering to a foot in height.

The Doctor made no allowance for the decrease of denfity in the air, because he did not propofe this maehine for measuring mountains (though, with a proper allowance for the decreafing denfity of the air, it will do very well), but for heights that want to be known

in gardens, plantations, and the conveyance of water, Level where an experiment that anfwers two or three feet in a diftance of 20 miles, will render this a very useful inftrument.

Artillery Foot-LEVEL is in form of a fquare, having its two legs or branches of an equal length; at a juncture whereof is a little hole, whence hangs a thread and plummet playing on a perpendicular line in the middle of a quadrant. It is divided into twice 45 degrees from the middle. Fig. 7.

This inftrument may be used on other occafions, by placing the ends of its two branches on a plane; for when the thread plays perpendicularly over the middle divifion of the quadrant, that plane is affuredly level. To ufe it in gunnery, place the two ends on the piece of artillery, which you may raife to any propofed height, by means of the plummet, whofe thread will give the degree above the level.

Carpenter's and Pavior's LEVEL, confifts of a long ruler, in the middle whereof is fitted, at right angles, another fomewhat bigger, at the top of which is faftened a line, which, when it hangs over a fiducialline at right angles with the bafe, shows that the faid bafe is horizontal. Sometimes this level is all of one. board. Fig. 8.

Gunner's LEVEL, for levelling cannons and mortars, confifts of a triangular brafs plate, about four inches. high, fig. 9. at the bottom of which is a portion of a circle, divided into 45 degrees; which number is fufficient for the highest elevation of cannons and mortars, and for giving fhot the greatest range: on the centre of this fegment of a circle is screwed a piece of brafs, by means of which it may be fixed or fcrewed at pleafure: the end of this piece of brafs is made fo as to ferve for a plummet and index, in order to show the different degrees of elevation of pieces of artillery. This inftrument has also a brafs foot, to fet upon cannons or mortars, fo as, when thofe pieces are horizontal, the inftrument will be perpendicular. The foot of this inftrument is to be placed on the piece to be elevated, in fuch a manner, as that the point of the plummet may fall on the proper degree: this is what they call levelling the piece.

Mafon's LEVEL, is compofed of three rules, fo joined as to form an ifofceles-rectangle, fomewhat like a Roman A; at the vertex whereof is fastened a thread, from which hangs a plummet, that paffes over a fiducial line, marked in the middle of the bafe, when the thing to which the level is applied is horizontal; bur declines from the mark, when the thing is lower on the one fide than on the other.

Plumb or Pendulum LEVEL, that which fhows the horizontal lines by means of another line perpendicular to that defcribed by a plummet or pendulum. This inftrument, fig. 10. confifts of two legs or branches, joined together at right angles, whereof that which carries the thread and plummet is about a foot and a half long; the thread is hung towards the top of the branch, at the point 2. The middle of the branch where the thread paffes is hollow, fo that it may hang free every where but towards the bottom, where there is a little blade of filver, whereon is drawn a line per pendicular to the telefcope, the faid cavity is covered by two pieces of brafs, making as it were a kind of cafe, left the wind fhould agitate the thread; for which reafon the filver blade is covered with a glass. G, to the

end

Level. end that it may be feen when the thread and plummet play upon the perpendicular: the telescope is faftened to the other branch of the inftrument, and is about two feet long; having an hair placed horizontally acrofs the focus of the object-glafs, which determines the point of the level. The telescope must be fitted at right angles to the perpendicular. It has a ball and focket, by which it is fastened to the foot, and was invented by M. Picard.

Reflecting LEVEL, that made by means of a pretty long furface of water reprefenting the fame object inverted which we fee erected by the eye, fo that the point where these two objects appear to meet is a level with the place where the furface of the water is found. This is the invention of M. Marriotte.

There is another reflecting level confifting of a mirror of fteel, or the like, well polished, and placed a little before the object-glafs of a telescope, fufpended perpendicularly. This mirror muft make an angle of 45 with the telescope, in which cafe the perpendicular line of the faid telescope is converted into a horizontal line, which is the fame with the line of level. This is the invention of M. Caffini.

Water-LEVEL, that which shows the horizontal line by means of a surface of water or other liquor; founded on this principle, that water always places itself level.

The moft fimple is made of a long wooden trough or canal, whose fides are parallel to the bafe; fo that being equally filled with water, its furface fhows the line of level. This is the chorobates of the ancients. See CHOROBATA.

It is also made with two cups fitted to the two ends. of a pipe, three or four feet long, about an inch in diameter, by means whereof the water communicates from the one to the other cup; and this pipe being *moveable on its ftand by means of a ball and focket, when the two cups become equally full of water, their two furfaces mark the line of level.

This inftrument, instead of cups, may also be made with two fhort cylinders of glafs three or four inches long, fastened to each extreme of the pipe with wax or maftic. Into the pipe is poured fome common or coloured water, which shows itself through the cylinders, by means whereof the line of level is determined; the height of the water, with refpect to the centre of the earth, being always the fame in both cylinders; this level, though very fimple, is yet very commodious for levelling fmall distances.

:

LEVEL of Mr Huygens's invention, confifts of a telefcope a, fig. 11. in form of a cylinder, going through a ferril, in which it is fastened by the middle. This ferril has two flat branches bb, one above, and the other below at the ends whereof are fastened little moving pieces, which carry two rings, by one of which the telescope is fufpended to an hook at the end of the fcrew 3, and by the other a pretty heavy weight is fufpended, in order to keep the telescope in aquilibrio. This weight hangs in the box 5, which is almost filled with linfeed oil, oil of walnuts, or other matter that will not easily coagulate, for more aptly fettling the balance of the weight and telescope. The inftrument carries two telescopes clofe and very parallel to each other; the eye-glafs of the one being against the object glafs of the other, that one may fee each way No 181.

without turning the level. In the focus of the object. Level. glafs of each telescope muft a little hair be strained horizontally, to be raised and lowered as occafion requires by a little fcrew. If the tube of the telescope be not found level when fufpended, a ferril or ring, 4, is put on it, and is to be flid along till it fixes to a level. The hook on which the inftrument is hung is fixed to a flat wooden cross; at the ends of each arm whe of there is a hook ferving to keep the telescope from too much agitation in ufing or carriage. To the faid flat crofs is applied another hollow one, that ferves as a cafe for the inftrument; but the two ends are left open, that the telescope may be fecured from the weather and always in a condition to be used. The foot of this inftrument is a round brass plate, to which are faftened three brass ferrils, moveable by means of joints whereon are put ftaves, and on this foot is placed the box.

Fig. 12. marked I, is a balance-level; which being fufpended by the ring, the two fights, when in aquilibrio, will be horizontal, or in a level.

Spirit LEVEL. The moft accurate levelling inftrument, and that poffeffed of the greatest essential advantages in ufe, is the fpirit-level; which was firft conftructed by the late Mr Siffon, and to which fome fmall additions and improvements have been fince made. The following is a description of one of the best of thefe levels, as made by the principal mathematical inftrument makers.

Fig. 13. is a reprefentation of the inftrument mounted on its complete ftaves, copied (except the letters) from Mr Adams's Graphical Effays, Plate xvii. fig. 3.. The telescope (ABC) is made from 15 inches to 2 feet in length, as may be required. It is achromatic, of the best kind, and shows the objects erect. In the focus of the eye-glaffes are exceedingly fine cross wires, the interfection of which is evidently shown to be perfectly in the axis of the tube; for by turning it round on its two fupporters DE, and looking through the telescope, the interfection of the wires will conftantly cut the fame part of the object viewed. By turning the fcrewa at the fide of the telescope, the objectglass at g is moved; and thus the telescope is exactly adapted to the eye. If these crofs wires are at any time out of their adjustment, which is discovered by their interfection not cutting the fame part of the object during the revolution of the telescope on its axis, they are easily adjusted by means of the four fcrews bbb, placed on the telescope about an inch from the end for the eye. Thefe fcrews act in perpendicular directions to one another, by unscrewing one and tightening the other oppofite to the wire, fo that if connected with it, it may be moved either way at pleasure; and in this manner the other wire perpendicular to it may be moved, and thus the interfection of the wires brought exactly in the axis of the tube.

To the telescope is fixed, by two small fcrews cc, the level tube containing the spirits, with a small bubble of air: This bubble of air, when the inftrument is well adjusted, will fettle exactly in the fame place, in or near the middle of its tube, whether the telescope be reverfed or not on the fupporters, which in this cafe are kept unmoved.

It is evident, that the axis of the telescope, or the interfection of the wires, as before shown, must be in this