LEVEL, a mathematical instrument, used for drawing a line parallel to the horizon and continuing it out at pleasure, and by this means, for finding the true level, or the difference of ascent or descent between any two places, for conveying water, levelling the surface of floors, and for various other purposes in architecture, hydraulics, surveying, &c.
LEVEL, consists of a long rule, straight on its lower edge, about ten or twelve feet in length, with an upright piece fixed to its upper edge, perpendicular to, and in the middle of the length, having its sides in the same plane with those of the rule, and a straight line drawn on one of its sides perpendicular to the straight edge of the rule. This standing piece is generally mortised into the other, and firmly braced on one side, in order to secure it from acci dents, and has its upper end kerfed in three places, viz., through the perpendicular line, and on each side. The straight edge of the transverse piece has a hole or notch cut out on the under side, equal on each side of the perpendi cular line. A plummet is suspended by a string from the middle kerf at the top of the standing piece, so that, when hanging at full length, it may vibrate freely in the hole or notch. When the straight edge of the level is applied to two distant points, with its two sides placed vertically, if the plummet hangs freely, and coincides with the straight line on the standing piece, the two points are level ; but if not, suppose one of the points to be at the given height, the other point must be lowered or heightened, as the case may require, until the third is brought to a coincidence with the perpendicular line. By two points is meant two surfaces of contact, as two blocks of wood, or chips, or the upper edges of two distant beams.
The use of the level in carpentry, is to lay the upper edges of joints in naked flooring horizontal, by first levelling two beams as remote from each other as the length of the level will allow ; the plummet may then be taken oil; and the level be used as a straight edge. In the levelling of joints, it is best to make two remote joints first level in themselves, that is, each throughout its own length, then the two level with each other ; after this, bring one end of the intermediate joists straight with the two which have been levelled ; then the other end in the same manner ; then try the straight edge longitudinally on each intermediate joist, and such as are found to be hollow must be furred up straight.
To adjust the level.—Place it in its vertical situation upon two phis or blocks of wood ; then, if the plummet, hanging freely, settle upon the line on the standing piece, (or, if not, one end being raised, or the other end lowered, to make it do so,) torn the level end for end, and if the plummet fall upon the line, the level is just ; but if not, the bottom edge must be shot straight, and as much taken off the one end as you may think necessary ; then trying the level first one way and then the other, as before, if a coincidence takes place between the thread and the line, the level is adjusted ; but if not the operation must be repeated till it come true.
The most convenient class of levels is the spirit level, called also the air Iced, which is more accurate than any other kind, and is most extensi, ely used. The invention of this instru ment has been ascribed to M. Thevenot. Others have attributed this application of a bubble of air to Dr. Hooke. The instrument consists of a cylindrical glass tube filled with spirits of wine, except leaving in it a small bubble of air ; its ends being hermetically sealed to keep in the fluid. This bubble, being the lightest of the contents of the tube, will, by the laws of hydrostatics, always run towards that end of the tube which is most elevated ; but when the tube is per fectly horizontal, the bubble will have no tendency towards either end. The tube is not strictly cylindrical withinside, though it bears that appearance, but is slightly curved, the convex side being upwards, and by this means the bubble will rest in the middle of the tube when it is horizontal, but approaches either end if elevated above the other. The simplest form of a spirit level for fixing any plane truly horizontal, consists of a glass tube of the above description, called a bubble tube, fixed into a block of wood, as at A B, Figure 1. The lower surface, D E, of the block is made flat ; and when the bubble, c, stands between two scratches marked on the glass at a b, the line D E is horizontal. The method of making it correct is this : the tube is first fitted into the block, the lower edge, D E, of which is placed on a bench or table as nearly horizontal as eon be determined, so that the bubble stands between the scratches a b. The level is now reversed, that is, the end D is put where E was at first. In this position, if the bubble stands in the middle, it proves the level to be correct, and the table horizontal ; but if it runs to either end of the tube, it shows that end to be too much elevated : suppose it B, for instance ; this end of the tube must therefore be let deeper into the wood, or the surface D E rectified to produce the same effect : one-half the error must be compensated by this means, and the other half by rectifying the table or support ; for D E, the level, must now be reversed again to verify these corrections ; and when they are so made that the bubble stands at a b, either way, the level is correct. To illustrate this more plainly, see Figure 2, which represents a section of the bubble tube ; but, for elucidation, is shown as if curved much more than they are ever made. Suppose the convex or upper surface of the tube to be a segment of a large circle, BC D; from the laws of hydrostatics, it is plain that the bubble of air, being the lightest body in the tube, will certainly occupy the highest point of the circle at c; and the two points, B, D, being equally distant therefrom, will be in the same horizontal line E D. The larger the radius of the circle D 13, so will the level be the more sensible of any deviation from the hori zontal, because the bubble will have to traverse a greater distance along the tube, in proportion to any partial elevation of either end.