Above the flight engineer's panel in a 747-100 there is an instrument that looks like a curved spirit level.
What was it used for?
It's for measuring the airplane's inclination on the ground for the longitudinal axis. There will be another one on a lateral bulkhead for measuring lateral inclination.
When doing fuel quantity calculations from the manual dipsticks, you have to take into account the effects of the plane's tilt on the fuel depth measurements, so you will read the values on the inclinometers and consult some kind of documentation that applies corrections for off-level conditions.
There are also other situations where they come in handy, like when you want to park in a level spot, or to keep the plane level during jacking.
Just about all Transport Category airplanes have inclinometers like that somewhere in the cockpit.