LEV 



LEV 



lumbricus terristris, and nais littoralis : it 

 is very pellucid, shining like silver, and 

 is propagated by a transverse division ; 

 oval when young, and growing more ob- 

 long with age ; truncate at the tip. 



LEUCOPSIS, in natural history, a ge- 

 nus of insects of the order Hymenoptera : 

 mouth horny, with short jaws, the mandi- 

 ble thick, and three toothed at the tip ; 

 Jip longer than the jaw, membranaceous 

 and emarginate at the tip; four feelers ; 

 short, equal, filiform ; antennae short, cla- 

 vate ; thorax with a long lanceolate scale 

 beneath ; wings folded ; sting reflected, 

 and concealed in a groove of the abdo- 

 men. There are four species. 



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 hori- 

 zontal and the perpendicular : the first 

 sort, which comprises spirit and air levels, 

 is chiefly in use among surveyors; the 

 latter is ordinarily employed by artifi- 

 cers, and depends for exactness on a 

 plumb line. 



The instruments used by persons tak- 

 ing the levels of lands, waters, &c. where- 

 by to ascertain the comparative heights 

 of different 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 tra- 

 verses immediately below its centre, and 

 gives not only the number of points, i. e. 

 thirty-two, according to the mariner's di- 

 vision, 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, supported 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 mo- 

 tion of the telescope is made to corres- 

 pond 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 teles- 

 cope, 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 ce- 

 mented. This piece of glass being per- 

 fectly centrical, serves to show how the 

 fluid, generally alcohol (or pure spirit), 



with which the tube is filled, with the ex- 

 ception 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 af- 

 fixed 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 ascertain- 

 ed 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 hori- 

 zontal level will be different from the rec- 

 tilinear 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 



instrument is brought to the exact direc- 

 tion in which the pole is situated; so that 

 the latter may coincide, or as it is techni- 

 cally called, *' be in one" with that basis 

 which is vertical within the tube. The 

 legs on which the level is supported, (ge- 

 nerally the same as in theodolites, &c.) 

 are spread so as to be firm ; after bringing 

 the compass as nearly as may be practi- 

 cable to a level : by means of four screws, 

 which serve to raise the different sides of 

 the plate at pleasure, the utmost preci- 

 sion 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 horizon- 

 tal hair within the telescope. The height 

 of the telescope above the surface on 

 which it stands must be deducted from 

 the number 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 



