Noun
spirit level (plural spirit levels)
(tool) A tool employing a chamber with a colored liquid (usually alcohol, hence spirit) and an air bubble, used to determine a horizontal or vertical reference line.
spirit-level
A spirit level or watch timing machine can achieve a higher accuracy than relying on the sound of the beat; precision regulators often have a built in spirit level for the task. Source: Internet
The mechanism – hydraulics controlled by a steersman watching a spirit level – worked in model form and in a trial version built in his garden in Denmark Hill, London. Source: Internet