   right|thumb|a tubular spirit level  thumb|fell all-way precision level a spirit level, bubble level or simply a level is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some photographic or videographic work. early spirit levels had banana-shaped curved glass vials at each viewing point. these vials are incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube. they have a slight upward curve, so that the bubble naturally rests in the centre, the highest point. at slight inclinations the bubble travels away from the marked center position. where a spirit level must also be usable upside-down or on its side, the banana-shaped tube is replaced by a barrel-shaped tube. alcohols such as ethanol are often used rather than water. alcohols have low viscosity and surface tension, which allows the bubble to travel the tube quickly and settle accurately with minimal interference with the glass surface. alcohols also have a much wider liquid temperature range, and won't break the vial as water could due to ice expansion. a colorant such as fluorescein, typically yellow or green, may be added to increase the visibility of the bubble. an extension of the spirit level is the bull's eye level: a circular, flat-bottomed device with the liquid under a slightly convex glass face with a circle at the center. it serves to level a surface across a plane, while the tubular level only does so in the direction of the tube. ==checking and adjustment== to check the accuracy of a carpenter's type level (i.e. whether the level indicates that a truly horizontal surface is, in fact, level), it is placed on a flat and roughly level surface and the reading on the bubble tube is noted. this reading indicates to what extent the surface is parallel to the horizontal plane, according to the level, which at this stage is of unknown accuracy. the spirit level is then rotated through 180 degrees in the horizontal plane, and another reading is noted. if the level is accurate, it will indicate the same orientation with respect to the horizontal plane. a difference implies that the level is inaccurate. adjustment of the spirit level is performed by successively rotating the level and moving the bubble tub
Geodesy Inclinometers Surveying Woodworking_measuring_instruments