SM099     Spirit Level     circa 1950     Hydrostatics

  The spirit level (or water level) is a device, using the principle of equilibrium conditions of a liquid in communicating vessels, to accurately check that a surface is level or a predetermined angle to the horizontal.   In use, the level of the liquid in two connected tubes is used to define a horizontal level.

  The spirit level is more delicate and more accurate than a water level but is used for the same purpose.   It consists of a glass tube, very slightly curved, so that instead of being a true cylinder, it has its axis on the circumference of a circle of very large radius.   The tube is filled with spirit, except for a small air bubble, and sealed.   The liquid will fill the lower parts of the tube at equilibrium, leaving the bubble at the highest part.   The tube is placed in a brass case and adjusted so that the position of the bubble between two markings will show when the whole instrument is perfectly horizontal.

  To take levels in surveying, the spirit level is placed on a telescope, which can be accurately calibrated for inclination.