The plate water level. A long plate having 4 sides would be set atop an item needing to be level, such as in ancient times an aquaduct. Water would be poured into the plate and the item beneath adjusted until additional water evenly reached the top of all four sides (or scribed marks partway up the sides) of the plate. Slopes and angles could also be duplicated over and over by having a taller plate with higher sides that could be set on the sloped item. When the water leveled at marks placed around the high sides, the leveled water would indicate the slope or angle was correct. I looked for a picture example of one, but apparently the device is so obsolete and unknown now that it’s not even mentioned on the internet until this answer I’m giving. Now that’s obscure.
A vestige of the plate water level lives on in the tube water level available at hardware stores or made from clear plastic tubing with water (and food coloring for easy view). All of these can be very accurate measuring devices:
Today, of course, spirit bar levels (the alcohol solutions in the sealed sight glasses don’t freeze) are common, but even those are being superseded by electronic/digital levels that can be faster and easier to use.