The plumb bob is a gravimeter. The spirit level is a clinometer. The mason line is a monochord. Ra told me that. He said I have been writing his Lexicon from the street this whole time and neither of us noticed. The builder and the scientist were always using the same tools. One called it a plumb bob. The other called it a gravimeter. Both of them were measuring the same thing. Gravity does not care what you call the string.

The plumb bob measures the direction of gravitational acceleration. The gravimeter measures the magnitude of gravitational acceleration. The plumb bob asks where is down. The gravimeter asks how much is down. The mason needed where. The scientist needed how much. The mason got the wall straight. The scientist got the equation right. Same force. Same string. Same weight at the bottom. Different questions. Different names. Same answer.

The spirit level measures the angle of a surface relative to the horizon. The clinometer measures the angle of a surface relative to the horizon. The spirit level has a bubble in a tube. The clinometer has a bubble in a tube. The spirit level costs twelve dollars at the hardware store. The clinometer costs twelve hundred dollars at the survey supply. The bubble does not know the difference. The bubble sits between the lines and tells the truth for twelve dollars or twelve hundred dollars. The truth does not have a price tier.

The mason line is a monochord. Pythagoras discovered the relationship between string length and pitch by plucking a single string. The monochord. One string. The mason plucks the line to check the tension. A tight line hums high. A loose line hums low. The mason is tuning an instrument every time the mason checks the wall. The wall is a song played on a string stretched between two corners. The pitch tells you if the string is true. Pythagoras knew this twenty-five hundred years ago. The mason knows it every morning at seven.

The builder and the scientist split apart somewhere around the Renaissance. The builder went to the job site. The scientist went to the laboratory. The builder kept the tools. The scientist renamed the tools. The plumb bob became the gravimeter. The spirit level became the clinometer. The chalk line became the datum. The tools did not change. The names changed. The building did not know it was being measured by science. The laboratory did not know it was using builder tools. Ra figured it out from the Lexicon. I figured it out from the street. We met in the middle. The middle is where the plumb bob hangs.

See also: Plumb Bob, Spirit Level, Mason Line