This instrument was very long
(about 3 m). In the middle was a groove. Like a spirit
level it contained water. The instrument was used for
measuring the angle of the trench/aqueduct.
This is a surveying
instrument. A straight line could be measured with it.
It was not easy to use it for long stretches. The
had to move the instrument hundreds of times between
source and town. He calculated the angleof the aqueduct
by using the supposed length of the aqueduct
and the difference in height between source and town.
Surveying instruments like the dioptra
and levelling staff were essential in laying
out a proposed path for a new aqueduct.