Theater of Machines, Volume II

the water is learned. However, there are three types of instruments and manners for such leveling. That which he calls Dioptra sighting instrument is marked in the following figure with A., and this instrument has its name from sighting. Therefore, all geometric instruments which we use for perspective, or otherwise for various measurements of height, width, plane, depth, and length, may be called Dioptrae sighting instruments. Ptolemy, Theon, Pericles, and Pliny mention such instruments. But Vitruvius maintains that among other instruments, the Corobates leveling instrument is the most certain of all. If, however, you wanted to make such an instrument, then note above all things that the longer it is, the more certain you will find it to be; therefore you should not make it less than 20 feet long. For we notice that such length adds much to the straightness, not only with bird-pipes but also with the barrels of artillery, as we know well enough how to testify to this from geometric principles. The Anconen brackets/supports, which we call little arms, are shown in the following figure from K. to F. G., or with K. N., or as on the other side with L. M. N. O. These little arms should be well cut and closed according to the correct measure of the square. The between-hooks are marked with F. K., but from O. to N. And at the furthest end at both places hang the

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