Jacob's Staff was invented in the 13th century as an aid in finding heights and distances. It was not until the 15th century that the German scientist Johannes Mueller popularized the instrument.
This was the age when the Europeans began exploring beyond the Mediterranean Sea. They needed better navigation tools so they could more accurately determine their latitude and longitude, and not run into too many rocks.
Jacob's Staff was shaped like a cross-bow. It had a main rod with a scale, and several cross pieces which were used to determine angles, either from the horizontal looking up (the "angle of elevation"), or perhaps between 2 objects.