LEVEL

, an instrument used to make a line parallel to the horizon, and to continue it out at pleasure; and by this means to find the true level, or the difference of ascent or descent between two or more places, for conveying water, draining sens, &c.

There are several instruments, of different contrivance and matter, invented for the perfection of levelling, as may be seen in De la Hire's and Picard's treatises of Levelling, in Biron's treatise on Mathematical Instruments, also in the Philos. Trans. and the Memoirs de <*> Acad. &c. But they may be reduced to the following kinds.

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

The most simple kind is made of a long wooden trough or canal; which being equally filled with water, its surface shews the line of level. And this is the chorobates of the ancients, described by Vitruvius, lib. viii. cap. 6.

The water-level is also made with two cups sitted to the two ends of a straight pipe, about an inch diameter, and 3 or 4 feet long, by means of which the water communicates from the one cup to the other; and this pipe being moveable on its stand by means of a ball and socket, when the two cups shew equally full of water, their two surfaces mark the line of level.

This instrument, instead of cups, may also be made with two short cylinders of glass three or four inches long, fastened to each extremity of the pipe with wax or mastic. The pipe is filled with common or coloured water, which shews itself through the cylinders, by means of which the line of Level is determined; the height of the water, with respect to the centre of the earth, being always the same in both cylinders. This level, though very simple, is yet very commodious for levelling small distances. See the method of preparing and using a water-level, and a mercurial Level, annexed to Davis's quadrant, for the same purpose, by Mr. Leigh, in Philos. Trans. vol. XL. 417, or Abr. viii. 362.

Air-Level, that which shews the line of Level by means of a bubble of air inclosed with some fluid in a glass tube of an indeterminate length and thickness, and having its two ends hermetically sealed: an invention, it is said, of M. Thevenot. When the bubble fixes itself at a certain mark, made exactly in the middle of the tube, the case or ruler in which it is fixed, is then level. When it is not level, the bubble will rise to one end.— This glass-tube may be set in another of brass, having an aperture in the middle, where the bubble of air may be observed.—The liquor with which the tube is silled, is oil of tartar, or aqua secunda; those not being liable to freeze as common water, nor to rarefaction and condensation as spirit of wine is.

There is one of these instruments with sights, being an improvement upon that last described, which, by the addition of other apparatus, becomes more exact and commodious. It consists of an air-Level, n° 1, (fig. 1, Plate XIV) about 8 inches long, and about two thirds of an inch in diameter, set in a brass tube, 2, having an aperture in the middle, C. The tubes are carried in a strong straight ruler, of a foot long; at the ends of which are fixed two sights, 3, 3, exactly perpendicular to the tubes, and of an equal height, having a square hole, formed by two fillets of brass crossing each other at right angles; in the middle of which is drilled a very small hole, through which a point on a level with the instrument is seen. The brass tube is fastened to the ruler by means of two screws; the one of which, marked 4, serves to raise or depress the tube at pleasure, for bringing it towards a level. The top of the ball and socket is rivetted to a small ruler that springs, one end of which is fastened with springs to the great ruler, and at the other end is a screw, 5, serving to raise and depress the instrument when nearly level.

But this instrument is still less commodious than the following one: for though the holes be ever so small, yet they will still take in too great a space to determine the point of Level precisely.|

Fig. 2, is a Level with Telescopic Sights, first invented by Mr. Huygens. It is like the last; with this difference, that instead of plain sights, it carries a telescope, to determine exactly a point of Level at a considerable distance. The screw 3, is for raising or lowering a little fork, for carrying the hair, and making it agree with the bubble of air when the instrument is Level; and the screw 4, is for making the bubble of air, D or E, agree with the telescope. The whole is fitted to a ball and socket, or otherwise moved by joints and screws.—It may be observed that a telescope may be added to any kind of Level, by applying it upon, or parallel to, the base or ruler, when there is occasion to take the level of remote objects: and it possesses this advantage, that it may be inverted by turning the ruler and telescope half round; and if then the hair cut the same point that it did before, the operation is just. Many varieties and improvements of this instrument have been made by the more modern opticians.

Dr. Desaguliers proposed a machine for taking the difference of Level, which contained the principles both of a barometer and thermometer; but it is not accurate in practice: Philos. Trans. vol. xxxiii. pa. 165, or Abr. vol. vi. 271. Fig. 3, 4, 5, 6.

Mr. Hadley too has contrived a Spirit Level to be fixed to a quadrant, for taking a meridian altitude at sea, when the horizon is not visible. See the description and figure of it in the Philos. Trans. vol. xxxviii. 167, or Abr. viii. 357. Various other Spirit Levels, and Mercurial Levels, are also invented and used upon different occasions.

Reflecting Level, that made by means of a pretty long surface of water, representing the same object inverted, which we see erect by the eye; so that the point where these two objects appear to meet, is on a Level with the place where the surface of the water is found. This is the invention of M. Mariotte.

There is another reflecting Level, consisting of a polished metal mirror, placed a little before the object glass of a telescope, suspended perpendicularly. This mirror must be set at an angle of 45 degrees; in which case the perpendicular line of the telescope becomes a horizontal line, or a line of Level. Which is the invention of M. Cassini.

Artillery Foot-Level, is in form of a square (fig. 7), having its two legs or branches of an equal length; at the junction of which is a small hole, by which hangs a plummet playing on a perpendicular line in the middle of a quadrant, which is divided both ways from that point into 45 degrees.

This instrument may be used on other occasions, by placing the ends of its two branches on a plane; for when the plummet plays perpendicularly over the middle division of the quadrant, the plane is then Level.

To use it in Gunnery, place the two ends on the piece of artillery, which may be raised to any proposed height, by means of the plummet, which will cut the degree above the Level. But this supposes the outside of the cannon is parallel to its axis, which is not always the case; and theresore they use another instrument now, either to set the piece Level, or clevate it at any angle; namely a small quadrant, with one of its radii continued out pretty long, which being put into the inside of the cylindrical bore, the plummet shews the angle of elevation, or the line of Level. See Gunner's Quadrant.

Carpenter's, Bricklayer's, or Pavior's Level, consists of a long ruler, in the middle of which is sitted at right angles another broader piece, at the top of which is fastened a plummet, which when it hangs over the middle line of the 2d or upright piece, shews that the base or long ruler is horizontal or Level. Fig. 8.

Mason's Level, is composed of 3 rules, so jointed as to form an isosceles triangle, somewhat like a Roman A; from the vertex of which is suspended a plummet, which hangs directly over a mark in the middle of the base, when this is horizontal or Level. Fig. 8.

Plumb or Pendulum Level, said to be invented by M. Picard; fig. 10. This shews the horizontal line by means of another line perpendicular to that described by a plummet or pendulum. This Level consists of two legs or branches, joined at right angles, the one of which, of about 18 inches long, carries a thread and plummet; the thread being hung near the top of the branch, at the point 2. The middle of the branch where the thread passes is hollow, so that it may hang free every where: but towards the bottom, where there is a small blade of silver, on which a line is drawn perpendicular to the telescope, the said cavity is covered by two pieces of brass, with a piece of glass G, to see the plummet through, forming a kind of case, to prevent the wind from agitating the thread. The telescope, of a proper length, is sixed to the other leg of the instrument, at right angles to the perpendicular, and having a hair stretched horizontally across the focus of the object-glass, which determines the point of Level, when the string of the plummet hangs against the line on the silver blade. The whole is fixed by a ball and socket to its stand.

Fig. 12, is a Balance Level; which being suspended by the ring, the two sights, when in equilibrio, will be horizontal, or in a Level.

Some other Levels are also represented in plate xiv.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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LEMNISCATE
LENS
LEO
LEPUS
LEUCIPPUS
* LEVEL
LEVELLING
LEVER
LEVITY
LEUWENHOEK (Antony)
LEYDEN Phial