We must now give a few directions respect ing the practice of levelling. LEVELLING is either simple or compound. The former is when the level points are determined from one station, whether the level be fixed at one of the points or between them. Compound levelling is nothing more than a repetition of many simple operations. An example of simple levelling is given in Pl. 95. fig. 1. where A B are the station-points of the level; CD the two points ascertained. Let the height Feet. Inches. The difference 3 0 0 shows that B is three feet lower than A. If the station-points of the level are above the line of sight, as in fig. 2. and the distance from A to C be six feet, and from B to D nine feet, the difference will still be three feet which B is higher than A. As an example of compound levelling, suppose it were required to know the difference of height between the point A on the river Zome, and N on the river Belann, fig. 3. (copied from M. le Febure). In this operation stakes should be driven down at A and N, exactly level with the surface of the water; and these stakes should be so fixed, that they may not be changed until the whole operation be finished: a plan of the ground between the two rivers should then be made, by which it will be discovered, that the shortest way between the rivers is by the dotted line AC, CH, HN; from whence also the number of stations ne cessary to be taken will be determined. The operator will also be enabled to distribute them properly according to the nature and situation of the ground. In the figure 12. stations are marked. Stakes ought then to be driven in at the limits of each station, as A, B, C, D, &c. They ought to be about two or three inches above the ground, and driven 18 inches into it. Stakes should also be driven in at each station of the instrument, as 1, 2, 3, 4, &c. The operation may be begun in the following manner: Let the first station be at 1, equally distant from the two points A and B, which themselves are distant 166 yards. Write down then in one column the first limit A; in another, the number of feet, inches, and tenths: with the points of sight indicated on the station staff at A. viz. 7. 6. 0). In the third column, the second limit B; in the fourth, the height in dicated at the station-staff B, viz. 6. 0. 0. Lastly, in the fifth column, the distance from one station-staff to the other; which in this case is 166 yards. Remove now the level to the point marked 2, which is in the middle between B and C, the two places where the sta tion-staves are to be held; observing that B, which was the second limit in the former operation, is the first in this. Then write down the observed heights as before; in the first column B; in the second 4. 6. 0.; in the third C; in the fourth 5. 6. 2.; in the fifth 560, the distance between B and C. It being impossible, on account of the inequality of the ground at the third station, to place the instrument in the middle between the two station-staves, find the most convenient point as at 3; then measure exactly how far this is from each station-staff, and you will find that from 3 to C is 160 yards; from 3 to D, 80 yards; and the remainder of the operation will be as in the preceding station. In the fourth operation, we must endeavour to compensate for any error which might have happened in the last. Mark out, therefore, 80 yards from the station-staff D to the point 4; and 160 yards from 4 to E; and this must be carefully attended to, as by such compensations the work may be much facilitated. Proceed in the same manner with the eight remaining stations, observing to enter every thing in its proper column; and when the whole is finished, add the sums of each column together, and then subtract the lesser from the greater: the difference, which in the present case is 5. 4. 8. shows the ground at N to be thus much lower than the ground at A. To obtain a section of this level, draw the dotted line o o, fig. 4. either above or below the plane; which may be taken for the level or horizontal line. Let fall then perpendiculars upon this line from all the station-points and places where the station-staves were fixed. Beginning now at A, set off 7 feet 6 inches upon the line from A to a: for the height of the level-point determined on the staff at this place, draw a line through a parallel to the dotted line o o, which will cut the third perpendicular at b, the second station-staff. Set off from this point downwards six feet to B, which shows the second limit of the first operation; and that the ground at B is one foot six inches higher than at A: place your instrument between these two lines at the height of the level line, and trace the ground according to its different heights. Now set off, on the second station-staff B, four feet six inches to C, the height determined by the level at the second station; and from C draw a line parallel too, which will cut the fifth perpendicular at d, the third station-staff. From this point set off 5 feet 6 inches downwards to C, which will be our second limit with respect to the preceding one, and the third with respect to the first. Then draw your instrument in the middle between B and C, and delineate the ground with its inequalities. Proceed in the same manner from station to station, till you arrive at the last N, and you will have the profile of the ground over which the level was taken. This method answers very well where only a general profile of the different stations is required; but where it is necessary to have an exact detail of the ground between the limits, we must then go to work more particularly. Suppose, therefore, the level to have been taken from A to N by another route, but no more uniform ground, in order to form a canal marked O, P, Q, R, S, T, U, X, Y. Draw at pleasure a line Z, Y, fig. 5. to represent the level, and regulate the rest; then let fall on this line perpendiculars to represent the staves at the limits of each station, taking care that they be fixed accurately at their respective distances from each other. The difference between the extreme limits, in this case, ought to be the same as in the former, viz. 5 feet 4 inches. Set off this measure upon the perpendicular o, the first limit; and from o, prolonging the perpendicular, mark off at a the height determined at the first stationstaff; then do the same with the second and third, and so on with the following, till this part of the work is finished: there remains then only to delineate in detail the ground between the station-staves, the distances in this example being assumed larger on account of the detail. To obtain the section of the ground between O and P, place your instrument at one of the limits, as P, fixing it so that the cross hairs may answer to the point C; then look towards the first limit o, raising or depressing the vane till it coincides with the intersection of the cross hairs; and the line of sight from one point to the other will mark the level or horizontal line. To set off the height of the brink of the river above the first limit, drive a stake down close to the ground at a ; and place your station-staff upon it, observing where the hairs intersect the vane, which will be at 4 feet 10 inches; then, laying off upon the line oz the distance from the first to the last stake, let fall from thence a perpendicular, and set off thereon 4. 10, 0, to a, which gives the height at the first stake; or, which is the same, the height from the edge of the river above the surface of the water, as is evident from the section. Drive a second stake at 6, in a line between the limits; place the station-staff upon this stake, and observe the height 4. 6. intersected by the cross hairs, the instrument still remaining in the same situation. Set off on the level-line the distance from the first stake a to the second b; and then let fall a perpendicular, and mark upon it 4. 6. to b, which gives the height of the ground at this place. ed The small hollow e is marked out by driving down a third stake even with the ground, in the middle of it at e; but the exact distance of the second stake from the third c, must be markupon the level line: then let fall a perpendicular from e, and set off upon it 6. 8. 0. pointed out by the cross hairs on the staff, which determines the depth of the hollow, as appears from the figure. As the distances between the stakes are now very short, they can easily be marked by the operator, who can settle any little inequalities by a comparison with those already ascertained. Proceed thus with the other stations till you arrive at the last, and you will always obtain an accurate section of your work; by which it is easy to form a just imation of the land to be dug away, in order to form the canal, by adding the depth to be given to it. Fig. 6. gives an example of compound levelling, where the situation is so steep and mountainous, that the staves cannot be placed at equal distances from the instrument, or where it is even impossible to make a reciprocal levelling from one station to the other. Thus suppose the point K to be the bottom of a basin where it is required to make a fountain, the reservoir being at A; so that, in order to know the height to which the jet d'eau will rise, it is necessary to know how high the point A is above K. In great heights such as this, it will be necessary to proceed by small descents, as from A to D. The instrument must be adjusted with all possible care; and it will even be proper, in some part of the work, to use a smaller instrument. The following is a table of the different operations used in making this level, it having been taken from M. le Febure's practice. In this case only two levellings are made be tween A and D, though more would have been necessary; but they are omitted to avoid confusion. In the fourth station the height found was 16 feet 8 inches; but on account of the great length, it was requisite to reduce the apparent level to the true one, which is always necessary where the length is considerable. At the last limit we get the height from N to o; then from o to I; from I to K, fig. 7. &c. ; all which added together, and then collected for the curvature, gives 47 feet 3 inches. Now, by adding each column together, and subtract ing one from the other, we have 51 feet 9 inches for the height which the point A is above the bottom of the bason, and which will cause the jet d'eau to rise about 45 feet. The general section of this operation is shown at fig. 7, 8. but an exact profile of the mountain is more difficult, as requiring many opera tions; though some of these might be obtained by measuring for the level line without moving the instrument. The last example given by Mr. Adams is like wise from M. le Febure, and includes a length of near five German miles (25 of ours) in a straight line, and 9 or 10 (45 or 50 English) including the turnings and windings. In this the declivity of the river Haynox was measured from Lignebruk to Villebourg. The first operation was to drive stakes at several parts of the river even with the water's edge; the first of which, a little above the mills of Lignebruk, showed the upper water-mark, and another showed the lower water-mark at the same mills. Two stakes above and below the mills of Ma. zurance, somewhat more than half way between Lignebruk and Villebourg, pointed out the difference between high and low water there, and formed likewise the third and fourth limits of the operation; while the stakes above and below the mills of Villebourg pointed out the difference between high and low water, and likewise formed the last limits of the operation. These marks were all made at the edge of the water, exactly even with its surface, and all made at the different parts of the river nearly at the same instant of time. "The principal limits of the levelling (says Mr. Adams) being now determined and fixed, it only remains to find the level between the limits, according to the methods already pointed out, using every advantage that may contribute to the success of the work, and at the same time avoiding all obstacles and difficulties that may retard or injure the operations. The first rule is always to take the shortest possible way from one limit to another, though this rule ought not to be followed if there are considerable obstacles in the way, as hills, woods, marshy ground, or if, by going aside, any advantage can be obtained." In the present case it was found necessary to deviate very considerably from the general rule, in order to take in several ponds, the surfaces of which might have all been taken for a perfect level; and thus levels were frequently taken across the country for a considerable way. The difference of height between the mills of Lignebruk and Villebourg was at last found to be about 19 feet, indicating a descent of not quite a foot in a mile. LEVELLING-STAVES, instruments used in levelling, serving to carry the marks to be observed, and at the same time to measure the height of those marks from the ground. They usually consist of two mahogany staves ten feet long, in two parts, that slide upon one another to about 5 feet, for the more portable carriage. They are divided into 1000 equal parts, and numbered at every tenth division, by 10, 20, 30, &c. to 1000; and on one side the feet and inches are also sometimes marked. A vane slides up and down upon each set of these staves, which by brass springs will stand at any part. These vanes are about 10 inches long and 4 inches broad; the breadth is first divided into three equal parts, the two extremes are painted white, the middle space divided again into three equal parts, which are less; the middle one of them is also painted white, and the two other parts black: and thus they are suited to all the common distances. These vanes have each a brass wire across a small square hole in the centre, which serve to point out the height correctly, by coinciding with the horizontal wire of the telescope of the level. For the method of levelling by means of the barometer and thermometer, see BAROMETER. LE VELNESS. 8. (from level.) 1. Even ness; equality of surface. 2. Equality with something else (Peacham). LE'VEN. 8. (levain, French.) 1. Ferment; that which being mixed in bread makes it rise and ferment. 2. Any thing capable of changing the nature of a greater mass (Wiseman). LEVEN, a river of Dumbartonshire, in Scot land, which issues from Loch Lomond. LEVEN (Loch), a beautiful lake in Kinrossshire, twelve miles in circumference, and somewhat of a circular form. It has several small islands, on one of which is a ruinous castle. Here the unfortunate Mary, queen of Scots, was confined by the confederate lords, after the murder of her husband lord Darnley, and her marriage with Bothwell; but she escaped in 1568 by the assistance of the brother of the governor of the castle. Another island, named St. Serf's Isle, is said to have been a residence of the Pictish priests: it was afterwards the seat of a priory, of which some remains are to be seen. This lake produces trout of a peculiar excellence; of which great quantities, at certain seasons, are sent to the Edinburgh markets. In autumn, a singular species, called the gully trout, is here salted and dried for winter provision. LEVER, a straight bar of iron or wood, &c. supposed to be inflexible, supported on a ful crum or prop by a single point, about which all the parts are moveable. The lever is the first of those simple machines called mechanical powers, as being the simplest of them all; and is chiefly used for raising great weights to small heights. The lever is of three kinds. First the common sovt, where the weight intended to be raised is at one end of it, our strength or another weight called the power is at the other end, and the prop or fulcrum is between them both. In stirring up the fire with a poker, we make use of this lever; the poker is the lever, it rests upon one of the bars of the grate as a prop, the incumbent fire is the weight to be overcome, and the pressure of the hand on the other end is the force or power. In this, as in all the other machines we have only to increase the distance between the force and the prop, or to decrease the distance between the weight and the prop, to give the operator the greater power or effect. To this kind of lever may also be referred all scissars, pincers, snuffers, &c. The steel-yard and the common balance are also levers of this kind. In the lever of the second kind the prop is at one end, the force or power at the other, and the weight to be raised is between them. Thus, in raising a water-plug in the streets, the workman puts his iron bar or lever through the ring or hole of the plug, till the end of it reaches the ground on the other side; then making that the prop, he lifts the plug with his force or strength at the other end of the lever. In this lever, too, the nearer the weight is to the prop, or the farther the power from the prop, the greater is the effect. To this second kind of lever may also be referred the oars and rudder of a boat, the masts of a |