level stopped around 1970, and was restarted in the 1980s by Thomas Butler Technology, also of Rockford, Illinois, but finally ended in the mid-1990s. However, there are still hundreds of the devices in existence.

Design and construction

Early tubular spirit levels had very slightly curved glass vials with constant inner diameter at each viewing point. These vials are filled, incompletely, with a liquid{{hspmdash}}hsp}}}}usually a colored spirit

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