Theater of Machines, Part 2

Corobates is more certain in the leveling of water than other instruments.

one brings such into a determined sum through the rules of arithmetic, whereby one learns the decline for the fall of the water. However, there are three types of instruments and methods for such leveling. That he calls it a dioptra a surveying instrument is marked in the following figure with A; this instrument has its name from "sighting," which is why all geometric instruments that we use for perspective, or otherwise for various measurements of height, width, plane, depth, and length, may be called dioptrae. Such instruments are mentioned by Ptolemy, Theon, Pericles, and Pliny. But Vitruvius wants the chorobates to be the most certain among other instruments. But if you wish to make such an instrument, note above all things: the longer it is, the more certain you will find it. Therefore, you should not make it under 20 feet, for such a length does much for the straightness; we note this not only with bird pipes, but also with the barrels of cannons, as we know well enough how to attest this from geometric principles. The ancones elbows or supports (which we will call little arms) are shown in the following figure from K to F and G, or with K and N, or as on the other side with L, M, N, O. These little arms should be well cut and closed according to the proper measure of the square. The hooks between them are denoted with F and K, but from O to N. And at the outermost part of both places hang the perpendicular weights, as denoted from F to... on one side. But when one wishes to use this instrument as a water level, it is necessary that it be completely still, for if the wind blows, the measurement will be inaccurate. Therefore, align and fasten the little rod of such water levels (denoted in the following figure with v) first straight and upright; such a beam should have two supports below, like vaulted corbels or