horny, with short jaws, the mandible thick, and three toothed at the tip; lip longer than the jaw, membranaceous and emarginate at the tip; four feelers; short, equal, filiform; antennæ short, clavate; thorax with a long lanceolate scale beneath; wings folded; sting reflected, and concealed in a groove of the abdomen. There are four species, all foreign insects. LEVEL, an instrument constructed for the purpose of ascertaining the exact level of any fluid, building, &c. Of these there are two distinct kinds, viz. the horizontal, and the perpendicular: the first sort, which comprises spirit and air levels, is chiefly in use among surveyors; the latter is ordinarily employed by artificers, and depends for exactness on a plumb line. The instruments used by persons taking the levels of lands, waters, &c. whereby to ascertain the comparative heights of dif ferent spots, or tracts, are simple in the extreme, being generally made with a telescope of about fifteen inches long, fixed above a circular opening in a brass plate, so as to show a compass that traverses immediately below its centre, and gives not only the number of points, i. e. thirty-two, according to the mariner's division, but by means of a neat brass rim, graduated with three hundred and sixty degrees, divided into thirty-six portions of ten degrees each, and numbered, shows the exact angle made between any two sights taken by the telescope, which traverses on two legs, support ed in grooves on the outer edge of the brass plate, and allowing it to move round in a direction perfectly parallel thereto. The plane thus described by the circular motion of the telescope is made to correspond with that of the horizon by the aid of a small brass tube, about six or eight inches in length, fixed exactly parallel with the line of sight through the telescope, and screwed to its cylinder in such manner as to remain firm. This little tube has on its upper side, or surface, an opening into which a piece of clear glass, corresponding with the cylindrical curve of the tube, is fitted and properly cemented. This piece of glass being perfectly centrical, serves to show how the fluid, generally alcohol (or pure spirits), with which the tube is filled, with the exception of a very minute portion, stand in respect of inclination with, or from the horizon. When the bubble of air left in the tube floats exactly centrical, in that portion which is covered with glass, the tube itself must be level; and as it is affixed at an exact parallel with the line of sight, which passes through the axis, or centre of the telescope, from the eye to the crossing of two hairs, at right angles, within the telescope, the instrument itself must then be level, and that part of any object, however distinct, which is cut or indicated by the line of sight, is ascertained by the centre of the cross made by the hairs being on a rectilinear level with the line of sight. But in consequence of the curvature of the earth's surface, the horizontal level will be different from the rectilinear level, and will describe an arc parallel with the surface of the earth. This curvature amounts to about eight inches in every mile; or, in more minute parts, may be taken at four and a half lines for every hundred yards. The usual mode of taking a level is by means of a painted board, about a foot square, having a broad white stripe drawn horizontally across its centre. This board slides up and down a long pole, which being held perpendicularly by an assistant, at any appointed spot of which the level is to be ascertained, the instrument is brought to the exact direction in which the pole is situated; so that the latter may coincide, or as it is technically called, “be in one" with that basis which is vertical within the tube. The legs on which the level is supported, (generally the same as in theodolites, &c.) are spread so as to be firm; after bringing the compass as nearly as may be practicable to a level: by means of four screws, which serve to raise the different sides of the plate at pleasure, the utmost precision is attainable. The board is then moved up or down on the pole, which is marked all the way up in feet, inches, halves, and quarters, until the centre painted line" is in one" with the horizontal hair within the telescope. The height of the telescope above the surface on which it stands must be deducted from the num ber of feet and inches, at which the line on the board stands above the spot where the pole is fixed: the residue shows how much that is below the place where the instrument stands. But if the height of the line on the board be less than that at which the line of sight in the level stands from the ground, then the difference be tween those two heights will exhibit how much the former is above the latter. By this simple mode the level of any intended land, &c. may be correctly taken, observing to limit the sight as much as possible; indeed, it is always best to confine them to distances not exceeding threehundred yards; because the difference between the rectilinear and the horizontal levels are then greatly diminished, and the whole survey will prove far more correct. This will be easily seen from the following sketch. (Plate VIII. Miscellanies, fig. 2). Let A B represent a sight taken at 2100 yards, and let AC be an equal distance measured on the surface of the segment AD, but broken off at every three-hundred yards, i. e. into seven portions: it must be obvious that the line BC will give a greater length than would result from the proportion already stated, the perpendicular falling so much beyond D; and that such difference would increase in its disproportions according as the range of sight might be enlarged. To prove this still more clearly, let us state that the quadrant OP (fig 3), of the earth's surface stands on a radius PS of four-thousand miles. Now the first taken from the sunmit O of that quadrant would be a parallel to that horizontal radius, and a tangent to the arc at its summit, as from O to X. It is evident that if a sight of four-thousand and one miles could be taken in the direction OX, a perpendicular falling from X, would not even touch the point P, from which it would be a mile distant. It is true, that our sights are not to be compared with the foregoing extents; but it is equally true, that we verge towards the error above shown when we take too long sights. Perhaps nothing can be more deceptive than the common mode of estimating levels more than once we have witnessed the opinions of smatterers in this branch of surveying, who having levelled the instrument with great exactness, and directed the telescope to a very distant hill, on seeing the point of intersection cut near its summit, have concluded the spot so indicated to be on a level with that where the instrument stood. This mis conception arises purely from a long rectilinear sight, without considering that the base of a remote hill is a plane whose surface stands at a very great angle from that on which the level is placed; as shown by the dotted lines representing a hill R intercepted by the line of sight O X. A very good kind of level is made on a portable plan, by several mechanics in this branch. It consists of a small tube of glass let into a plate with which it is exactly VOL. IV. parallel. This is the surface, shewing the bubble in the tube, as before described: the under part of the box, which may be about eight inches long, two broad, and two deep, has a spring and screw that cause the box to change its direction from either above or below the horizon, to an exact rectilinear level, as indicated by the air left in the glass tube. At the centre of the bottom of the box, is a brass stud, serving to fix into a hole made in the top of a walking stick, &c. This kind of level, in the hands of a skilful surveyor, may be used to great advantage where very great precision is not required; but as the sight is taken only from the upper edges of the box's ends, it does not admit of that great nicety which is indispensable in many operations, and where the smallest deviation from a true level might occasion immense expence and inconvenience. The level represented in Plate Level, was made by the justly celebrated Jesse Ramsden, F. R. S, and considered by him as the most complete. It stands on three legs, which fold up into the size and appearance of a moderately thick round staff, three sliding rings, or leather bands, suffice to keep them compact and firm. The upper parts of the legs fit into a brass plate, as in theodolites, on the surface of which is a strong male screw, serving to fasten the working part of the instrument at pleasure, to its centre. The female screw is cut withinside a projection b b of a brass plate aa: this projection has a hole through its top, and contains a brass ball d, screwed into another similar plate ee; by which means the two plates are connected together, and the upper one can be turned about in any direction, while the lower one remains fixed. Four screws n passing through the lower plate, being worked in until they touch the bottom of the upper plate, serve either to fix the latter firmly; or, by unthreading one or two, and working in the opposite ones, to change the inclination of the upper plate, and to bring it to an exact level. These are called the parallel plates, though they often stand at an angle, the one with the other, when the direction of the upper one requires changing as above shown. The ball d is perforated with a conical hole, to receive an axis l, that is screwed to the bottom of the compass box, f, on which are two square brass arms F C, projecting from it diametrically opposite to each other. H is a small brass angle, or H frame, called a Y, screwed to the end of the arm C: it supports in its forked termination one end of the telescope K, of which the other end rests in a Y, (lettered N), similar to H, but which can be raised or lowered, by means of a screw having a milled head; as seen at y. The spirit level L is fixed to the telescope by two screws at its ends, whereby can be brought into exact adjustment with the culmination of the telescope. The level, which is the essential part of the instrument, has been already described; but it is proper to add, that the ends of the tube containing the alcohol, when made of glass, should be hermetically sealed at its ends, which should then be cased in brass. Mr. Ramsden preferred a very slight convexity in the tube; considering it best adapted for shewing the most trifling deviation from a perfect level, and causing the bubble to become more accurately centrical. With respect to the telescope, it is similar to those generally fitted to theodolites, &c. and has been described in the preceding part of this article. It is laid on in the two Y pieces, and kept In in by two curved pieces of brass. figure h is the achromatic object glass, fixed within the end of a tube, sliding with in the external cylinder of the telescope, and moved very gradually by a rack and pinion on the mill headed nuti: the distance of the glass h from the eye-piece, is thereby adjusted to a suitable focus. The eye-piece K contains two lenses, sliding in a tube fixed to the telescope, for adjusting them to a distinct vision of the I cross wires, or hairs, which are held in a proper state of tension in the frame t, and regulated to the axis, or line of sight, by four minute screws passing through the outside of the telescope. We have before shown how the four screws act upon the two parallel plates, a a and ee, while the axis b can be set very nearly perpendicular; then by the screw y the telescope can be set very nearly level. To turn the telescope about horizontally upon its axis 7, a screw m, works in a fixed collar v; its nut p, is fastened upon an arm which projects from a clanıp g, embracing a collar upon the axis 1, and is tightened by a screw r; which being unscrewed the clamp springs open, and the telescope, together with the level, moves round with freedom upon the axis 1, According to the pleasure of the operator. When the screw r is tightened, the clamp holds the telescope fast, but admits of a slight movement, either way, when acted upon by the mill-headed screw m. Our readers will readily perceive the simplicity of this level, beyond any others in use; and will lament the demise of a gentleman who, to profound theory, added the most ingenious and skilful practice. Where a very long, and continued range of brickwork is to be raised, it is often advisable to use a water level, made by laying a ridge of mortar along the centre of the wall, and opening a very narrow channel throughout its centre longitudinally; so as to form a kind of trough, let the ends be stopped, and the trough be filled with water, as far as it will flow. The surface of the water will give a true horizontal level; which if continued for miles would conform exactly to the curvature of our globe. To continue the level along the rest of the trough, stop it at the place where the water reached, and raising the adjacent part with more mortar, let the trough of the superior level be filled, and thus in succession. The difference between the end of one trongli, and the beginning of another will shew the respective levels; from which parallels may be set off at any height above by plumb lines of equal length. This mode is often practised in large works, such as fortifications, and when strata of masonry are to be regularly disposed; also to prevent those irregular breaks, and partial connections, that are almost inevitable where small sallows, or triangular levels, with plumb weights are in use. When no instrument can be obtained, and where it is not easy to draw an exact level by the foregoing means, take the hose of an engine, and having fixed one end at the spot whose level is to be sought, (on any opposite bank for instance,) carry the other end to the place where the corresponding height is to be established. Fill the hose with water until it ceases to require raising at the further end. When both ends show full to the brim, and that the water is retained at both, then they are on the same level: for it is a maxim in HYDROSTATICS (which see), that water, or indeed, any fluid heavier than atmospheric air, will, when at liberty, always find its own level. Where a succession of contiguous levels are wanted, it will often be found convenient to use a small leaden pipe, of about half an inch bore, which should be applied as above described; or even a common gutter, made of two pieces of planks, like those under the eaves of houses, may be made to answer the purpose; by support ing either end, until the water may come to a level in every part. Where works of moderate extent are carried on, and where the perfect level of each stratum of materials is not an object of importance the common bricklayer's level, made in the form of an inverted T, thus L, having a plumb suspended from the top, and received in an opening at the junction of the perpendicular with the horizontal piece, will answer well enough. The principle on which this acts, is, that as all weights have a tendency to gravitate towards the centre of the earth, so as the plumb line, is a true perpendicular, any line cutting that at right angles must be a horizontal line at the point of intersection. LEVEL, artillery foot, is in form of a square, 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 forty-five degrees from the middle. This instrument may be used on other occasions, by placing the ends of its two branches on a plane; for when the thread plays perpendicularly over the middle division of the quadrant, that plane is assuredly level. To use it in gunnery, place the two ends on the piece of artillery, which you may raise to any proposed height, by means of the plummet, whose thread will give the degree above the level. LEVEL., carpenter's and paviour's, consists of a long ruler, in the middle whereof is fitted, at right angles, another somewhat bigger, at the top of which is fastened a line, which, when it hangs over a fiducial line at right angles with the base, shows that the said base is horizontal. Sometimes this level is all of one board. LEVEL, gunner's, for levelling cannons and mortars, consists of a triangular brass plate, about four inches high, at the bottom of which is a portion of a circle, divided into forty-five degrees, which number is sufficient for the highest elevation of cannons and mortars, and for giving shot the greatest range: on the centre of this seg ment of a circle is screwed a piece of brass, by means of which it may be fixed or screwed at pleasure; the end of this piece of brass is made so as to serve for a plum. met and index, in order to show the different degrees of elevation of pieces of artillery. This instrument has also a brass foot, to set upon cannons or mortars, so as when those pieces are horizontal, the instrument will be perpendicular. The foot of this instrument is to be placed on the piece to be elevated, in such a manner as that the point of the plummet may fall on the proper degree: this is what they call levelling the piece. LEVEL, mason's, is composed of three rules, so joined as to form an isosceles-triangle, somewhat like a Roman A, at the vertex whereof is fastened a thread, from which hangs a plummet, that passes over a fiducial line, marked in the middle of the base, when the thing, to which the level is applied, is horizontal; but declines from the mark, when the thing is lower on one side than on the other. LEVEL, plumb or pendulum, that which shews the horizontal line by means of another line perpendicular to that described by a plummet or pendulum. This instrument consists 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. 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 little blade of silver, whereon is drawn a line perpendi cular to the telescope, the said cavity is covered by two pieces of brass, making as it were a kind of case, lest the wind should agitate the thread; for which reason the silver blade is covered with a glass to the end, that it may be seen when the thread and plummet play upon the perpendicular. The telescope is fastened to the other branch of the instrument, and is about two feet long; having an hair placed horizontally across the focus of the object-glass, which determines the point of the level. The telescope must be fitted at right angles to the perpendicular. It has a ball and socket, by which it is fastened to the foot. LEVELLING. See LEVEL. LEVELLING staves, instruments used in levelling, serving to carry the marks to be observed, and at the same time to measure the heights of those marks from the ground. They usually consist each of two long wooden rulers, made to slide over one another, and divide into feet, inches, &c. LEVER, in mechanics, an inflexible right line, rod, or beam, supported in a single point on a fulcrum or prop, and used for the raising of weights; being either void of weight itself, or at least having such a weight as may be commodiously counterbalanced. The lever is the first of those called mechanical powers, or simple machines, as being of all others the most simple; and is chiefly applied for raising weights to small heights. See MECHANICS. LEVISANUS, in botany, so called from the Rev. Mr. Lewis, a genus of the Pentandria Monogynia class and order. Essential character: flowers aggregate; calyx oneleafed, superior, five-cleft; corolla five petalled, superior; filaments inserted into the base of the perianth; styles two, conjoined; berry two celled; seeds five or six, compressed. There are five species, which are all shrubs, and natives of the Cape of Good Hope. LEYSERA, in botany, a genus of the Syngenesia Polygamia Superflua class and order. Natural order of Compositæ Discoideæ. Corymbiferæ, Jussieu. Essential character: calyx scariose; down chaffy; in the disk feathery also; receptacle subpaleaceous. There are three species. LEY, or lees, a term usually applied to any alkaline solution made by levigating any ashes that contain an alkali. Soaplees is an alkali used by soap-boilers, or potash or soda in solution, and made caustic by lime. Lees of wine are the refuse, or sediment, that deposits from wine by standing quiet. LEYDEN phial, in electricity, is a glass phial or jar, coated both within and with out with tin foil, or other conducting substance, that it may be charged, and employed in a variety of experiments. Flat glass, or glass of any shape, may be used in the same way. LIATRIS, in botany, a genus of the Syngenesia Polygamia Æqualis class and order. Natural order of Composite Capitatæ. Cinarocephala, Jussieu. Essential charac. ter: calyx oblong, imbricate, awnless, coloured; down feathered, coloured; receptacle naked, hollow, dotted. There are eight species. LIBELLULA, in natural history, dragonfly, á genus of insects of the order Neuroptera. Mouth armed with jaws, more than two in number; lip trifid; antennæ very thin, filiform, and shorter than the thorax; wings expanded; tail of the male insect furnished with a forked process. There are about sixty specics, divided into two families. A. wings expanded when at rest. B. wings erect when at rest; eyes distinct; outer divisions of the lip bifid. The whole tribe of the libellula are 'remarkable for being ravenous: they are usually to be seen hovering over stagnant waters, and may, in the middle of the day, be observed flying with great rapidity in pursuit of the smaller insects. These brilliant and beautiful animals were once, and for a considerable time, inhabitants of the water in that state, as larva, they are six-footed, active, and furnished with an articulate forcipated mouth. They prey upon aquatic insects, and the larva of others: the pupa resembles the larva, but has the rudiments of wings. The most remarkable of the English species is the L. varia, or great variegated libellula, which makes its appearance towards the decline of summer, and is an animal of singular beauty. Its length is about three inches; and the wings, when expanded, measure nearly four inches from tip to tip. The female libellula drops her eggs in the water, which, on account of their specific gravity, sink to the bottom: after a certain period they are hatched into larvæ, having a singular and disagreeable aspect: they cast their skins several times before they arrive at their full size, and are of a dusky brown colour: the rudiments of the future wings appear on the back of such as are advanced to the pupa state in the form of oblong scales, and the head is armed with a singular organ for seizing its prey. They continue in the larva and pupa state two years; when having attained to their full size, they prepare for their ultimate change, and creeping up the stem of some water plant, and grasping it with their feet, they make an effort, by which the skin of the back and head is forced open, and the enclosed libellula gradually emerges. This process takes place in a morning, and during a bright sunshine. The remainder of the animal's life is short, the frosts of autumn destroying them all. "It is impossible,” says Dr. Shaw," not to be struck with admiration on contemplating the changes of the libellula, which, while an inhabitant of the water, would perish by any long exposure to the air, while the complete animal, once escaped from the pupa, would as ef fectually be destroyed by submersion under water, of which not an hour before it was the legitimate inhabitant." In this, and other species of the libellula tribe, the structure of the eye is deserving of notice. |