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When a bubble, or spirit, level is level, what does that mean relative to Earth?
Pat French
Longdon-upon-Tern, Shropshire, UK
There is a well-known story of a gentleman who held that Earth was flat. He took a spirit level on a long-haul flight and afterwards announced that, other than during take-off and landing, the aircraft had flown level over the flat Earth.
When the bubble in a spirit level lies centrally in the fluid, it means that the gravitational pull on that fluid is uniform across the container and either side of the bubble. At this point, the position of…
