Part 252

LETTRES en chiffre, Fr. Cyphers. Baron Espagnac in the continuation of his Essai sur l’operation de la guerre, tom. 1, page 269, gives the following instructions relative to this acquirement. He observes that writing in cypher may be practised in two different ways. First by means of distilled vinegar, which is boiled with silver litharge, one ounce of the latter to a pint of the former. When this mixture has stood some time, it must be carefully poured off from the sediment, and it will appear as clear as rock water. Intelligence or information may be conveyed by writing with this water in the blank spaces of an ordinary letter, on wrapping paper, or on the blank leaves of a book. The instant the writing dries, not the least trace appears of what has been marked. To render the writing legible, you must make use of a water in which quick lime has been dissolved with a mixture of orpiment. This water is as clear as rock water; and if you steep a sheet of paper in it, and lay it upon the letter, book, &c. on which any thing has been written, the different characters will instantly appear.

The first of these distilled liquids is so powerful and searching, that by putting the written letter upon several other sheets of paper, after having rubbed the top sheet with the second water, the writing will be clearly seen in almost all of them. The same circumstance will occur, if you rub the leaf of a book or any piece of paper which you may spread upon it. These waters, especially the last, should be kept in bottles that are well corked up, to prevent the spirituous particles from evaporating. A fresh composition must, indeed, be made, if the old one should seem weakened. The letters that are written must likewise be carefully penned, and kept free from blots, &c. The paper must not be turned, nor rubbed with the hand until the writing be thoroughly dry. This is the author’s first proposed mode of writing in cyphers, the second may be seen in page 270 of the work already quoted.

LETTRES de représailles, Fr. Reprisals. See LETTERS of marque.

LETTRES de santé, patentes de santé, Fr. letters of health.

LEVANT, the countries bordering upon the Mediterranean are so called. It appears to be derived from le vent, the wind, or country to windward, in relation to Italy.

LEVANTIN, Fr. A word generally used among the French to distinguish any person from the Levant.

LEVANTINE nations, (Nations Levantines, Fr.) Nations belonging to the East, or to those countries which border on the Mediterranean. The French likewise say, Peuples Levantines.

LEVANTIS, Fr. The soldiers belonging to the Turkish gallies are so called.

LEVEE de troupes, Fr. See LEVY.

LEVEE en Masse, Fr. a general rising of the people of any country, either for the purposes of self defence, or to answer the intentions of its governing powers.

LEVEE d’une siege, Fr. The raising of a siege. See SIEGE.

LEVEL, an instrument to draw a line parallel to the horizon, whereby the difference of ascent or descent between several places may be found, for conveying water, draining fens, &c.

Air-LEVEL, that which shews the line of level by means of a bubble of air, inclosed with some liquor in a glass tube of an indeterminate length and thickness, whose two ends are hermetically sealed. When the bubble fixes itself at a certain mark, made exactly in the centre of the tube, the plane or ruler wherein it is fixed is 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, whence the bubble of air may be observed. There is one of these instruments with sights, being an improvement upon the last described, which by the addition of more apparatus, becomes more commodious and exact: it consists of an air-level about eight inches long, and 7 or 8 lines in diameter, set in a brass tube, with an aperture in the middle: the tubes are carried in a strong straight ruler, a foot long, at whose ends are fixed two sights, 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 whereof is drilled a very little hole, through which a point on a level with the instrument is described: the brass tube is fastened on the ruler by means of two screws, one whereof serves to raise or depress the tube at pleasure, for bringing it towards a level. The top of the ball and socket is riveted to a little ruler that springs, one end whereof is fastened with screws to the great ruler, and at the other end is a screw serving to raise and depress the instrument when nearly level.

Artillery foot-LEVEL, is in form of a square, having its two branches or legs of an equal length, at the angle of which is a small hole, whence hang a line and plummet, playing on a perpendicular line in the middle of a quadrant: it is divided into twice 45 degrees from the middle.

Gunner’s-LEVEL, for levelling pieces of artillery, consists of a triangular brass plate, about 4 inches, at the bottom of which is a portion of a circle divided into 45 degrees; which angle is sufficient for the highest elevation of cannons, mortars, and howitzers, and for giving shot and shells the greatest range: on the centre of this segment 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 plummet and index, in order to shew the different degrees of elevation of pieces of artillery. This instrument has also a brass foot, to act upon cannon or mortars, so that when these 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, &c.

The most curious instrument for the use of the artillerist, was lately invented by the very ingenious colonel Congreve, of the British artillery; having the following qualifications, viz. 1. It will find the inclination of any plane, whether above or below the horizon. 2. By applying it either to the cylinder, or outside of any piece of ordnance, angles of elevation or depression may be given to the 60th part of a degree, with less trouble than the common gunner’s quadrant, which only gives to the 4th part of a degree. 3. It will give the line of direction for laying either guns or mortars to an object above or below the horizon. 4. It will find the centre of metals of any piece of ordnance. 5. With it, a point may be found in the rear of a mortar-bed, in the vertical plane of the mortar’s axis; consequently a longer line of sight is given for directing them to the object than the usual way. 6. It answers all the purposes of a pair of callipers, with the advantage of knowing (to the 100th part of an inch) diameters, whether concave or convex, without the trouble of laying the claws upon a diagonal scale. 7. On the sides of the instrument are the following lines, viz. equal parts, solids, plains, and polygons, logarithms, tangents, versed sines, and numbers, plotting scales, and diagonal scale of inches for cutting fuzes by. 8. In the lid of the instrument-case is a pendulum to vibrate half seconds. It is likewise of singular use in surveying; as, 1. It takes horizontal angles to the 60th part of a degree. 2. Vertical angles. 3. Levels. 4. Solves right angled plane triangles. 5. Oblique-angled plane triangles. 6. Answers all the purposes of a protractor, with the advantage of laying down angles exactly as taken in the field. N. B. captain Jordane’s ingenious instrument answers nearly the same purposes.

Spirit-LEVEL. See AIR LEVEL.

By the term level is also to be understood the line of direction in which any missive weapon is aimed.

LEVELLING, the finding a line parallel to the horizon at one or more stations, and so to determine the height of one place in regard to another.

A truly level surface is a segment of any spherical substance, which is concentric to the globe of the earth. A true line of level is an arch of a great circle which is imagined to be described upon a true level surface.

The apparent level is a straight line drawn tangent to an arch or line of true level. Every point of the apparent level except the point of contact, is higher than the true level.

The common methods of levelling are sufficient for laying pavements of walks, for conveying water to small distances, for placing horizontal dials, or astronomical instruments; but in levelling the bottoms of canals or ditches in a fortification, which are to convey water to the distance of many miles, the difference between the apparent and true level must be taken into the account.

Dr. Halley suggests a method of levelling, which is performed wholly by the barometer, in which the mercury is found to be suspended to so much the less height, as the place is more remote from the centre of the earth. Hence it follows, that the different height of the mercury in two places gives the difference of level.

Mr. Derham, from some observations at the top and bottom of the monument in London, found that the mercury fell ¹⁄₁₀th of an inch at every 82 feet of perpendicular ascent, when the mercury was at 30 inches. Dr. Halley allows of ¹⁄₁₀th of an inch for every 30 yards; and considering how accurately barometers are now made, we think this method sufficiently exact to take levels for the conveyance of water, or any other military purposes, and indeed less liable to errors than the common levels. Mr. Derham also found a difference of 3 inches ⁸⁄₁₀ths between the height of the mercury at the top and bottom of Snowdonhill in Wales.

For the common occasions of levelling, set a pole upright in a spring, pond, &c. and mark how many feet and inches are above water; then set up another pole of equal length with the other, in the place to which the water is to come. Place the centre of a quadrant on the top of this last pole, the plummet hanging free; spy through the sights at the top of the pole in the water, and if the thread cuts any degree of the quadrant, the water may be conveyed by a pipe laid in the earth. If you cannot see from one extreme to the other, the operation may be repeated.

LEVELLING.--Table shewing the difference between the true and apparent level.

+------+------+------+------+------+--------+------+-------+ | | Dif- | | Dif- | | Dif- | | Dif- | | | fer- | | fer- | | fer- | | fer- | | | ence | | ence | | ence | | ence | | Dis- | of | Dis- | of | Dis- | of | Dis- | of | |tance.|level.|tance.|level.|tance.| level. |tance.| level.| +------+------+------+------+------+--------+------+-------+ | Yds. | Inch.| Yds. | Inch.| Mls. |Ft. in.| Mls. |Ft. in.| | 100 | 0.026| 1000 | 2.570| ¹⁄₄ | 0 0¹⁄₂| 7 | 32 6 | | 200 | 0.103| 1100 | 3.110| ¹⁄₂ | 0 2 | 8 | 42 6 | | 300 | 0.231| 1200 | 3.701| ³⁄₄ | 0 4¹⁄₂| 9 | 53 9 | | 400 | 0.411| 1300 | 4.344| 1 | 0 8 | 10 | 66 4 | | 500 | 0.643| 1400 | 5.038| 2 | 2 8 | 11 | 80 3 | | 600 | 0.925| 1500 | 5.784| 3 | 6 0 | 12 | 95 2 | | 700 | 1.260| 1600 | 6.580| 4 |10 7 | 13 |112 2 | | 800 | 1.645| 1700 | 7.425| 5 |16 7 | 14 |130 1 | | 900 | 2.081| | | 6 |23 11 | 15 |150 | | | | | | | | 16 |175 | +------+------+------+------+------+--------+------+-------+

This table will answer several useful purposes.