It is a glass tube so nearly filled with water that it contains only a bubble of air. This tube is fastened in a piece of wood, as shown in [Fig. 22].
Fig. 22.
How can we tell when the instrument is level?
By the bubble of air, which will always be at the centre of the tube when it is level.
What other liquid may be used to fill the tube?
Alcohol may be used to fill the tube, and it is then called a spirit-level.
Of what use is the spirit-level?
It is used for levelling the walls and wood-work of houses, bridges, and other structures.