The bubble level

A curved vial of mineral spirits, sealed at both ends, holds a single bubble of air that a bubble level relies on entirely, the bubble rising to the vial's highest point and settling between two etched lines when the surface beneath is, within a small tolerance, flat. The bubble does not react quickly. Tilt the level and it drifts rather than jumps, taking a full second or two to arrive, an unhurried pace that has frustrated generations of people holding a shelf bracket in place with one hand while watching the vial with the other, willing the bubble to commit. Most levels carry three vials: one for level, one for plumb, one at forty-five degrees for the rare occasion a fence post needs to lean with intention, though the diagonal vial goes unused in most households for the tool's entire working life. Age fogs the mineral spirits slightly and can shift a vial's calibration by a degree that no eye would catch, and no level anywhere nearby would catch either, since the standard method for checking one level is to compare it against another level, an arrangement with an obvious flaw. Set two on a truly flat surface and if their bubbles disagree, the surface is not the problem being solved, though it is rarely the level that gets blamed first.

H. G. Plimsoll