The 'Chorobates,' described by Vitruvius in Book VII of the Architecture, was used to measure horizontal planes and was especially important in the construction of aqueducts. Similar to modern spirit levels, the 'Chorobates' consisted of a beam of wood 6 m in length held by 2 supporting legs and equipped with 2 plumb lines at each end. The legs were joined to the beam by two diagonal rods with carved notches. If the notches corresponding to the plumb lines matched on both sides, it showed that the beam was level. On top of the beam, a groove or channel was carved. If the condition was too windy for the plumb bobs to work effectively, the surveyor could pour water into the groove and measure the plane by checking the water level. Vitruvius instructs that the water level groove was to be "five feet long, one digit wide, and a digit and a half deep". By using two or more 'Chorobates,' established levelly, the vertical distance between instruments could be established by sighting along the depth of the uphill instrument, to a rod placed atop the lower 'Chorobates.'
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