Editing Spirit level (section)

==Design and construction==
[[File:5Sec3Tenths.JPG|right|thumb|Machinist's precision level]]

Early tubular spirit levels had very slightly curved glass [[vial]]s with constant inner diameter at each viewing point. These vials are filled, incompletely, with a {{nowrap|liquid{{hsp}}{{mdash}}{{hsp}}}}usually a [[Tincture|colored]] [[Aqua vitae|spirit]] or {{nowrap|[[ethanol|alcohol]]{{hsp}}{{mdash}}{{hsp}}}}leaving a [[liquid bubble|bubble]] in the tube. They have a slight upward curve, so that the bubble naturally rests in the center, 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 curved constant-diameter tube is replaced by an uncurved barrel-shaped tube with a slightly larger diameter in its middle.

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 from the glass surface. Alcohols also have a much wider liquid temperature range, and will not break the vial as water could due to ice expansion. A [[Colorants|colorant]] such as [[fluorescein]], typically yellow or green, may be added to increase the visibility of the bubble.

A variant of the linear spirit level is the [[bull's eye level]]: a circular, flat-bottomed device with the liquid under a slightly [[wikt:convex|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.