<P> 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 . </P> <P> A variant of the linear 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 . </P> <P> To check the accuracy of a carpenter's type level, a perfectly horizontal surface is not needed . The level 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 . </P> <P> Adjustment of the spirit level is performed by successively rotating the level and moving the bubble tube within its housing to take up roughly half of the discrepancy, until the magnitude of the reading remains constant when the level is flipped . </P>

What are the lines on a spirit level