Continuum — the series
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What a flow is
A fluid is made of molecules and nobody models it that way. Treating it as a continuous field with a velocity at every point is an approximation, an extremely good one, and knowing why it works is knowing where it stops.
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A gas's viscosity ignores its pressure until the free path reaches the walls
Maxwell predicted in 1860 that a gas's viscosity does not depend on its density — halve the pressure and half as many molecules carry momentum across the flow, each twice as far — and measured it with swinging discs in 1866: unchanged from an atmosphere to a sixtieth of one. Solved across every pressure with the kinetic equation, the shear a gas transmits between two plates obeys him over five decades and then, once the free path reaches the gap, falls in proportion to the pressure. A millimetre gap reads half the true viscosity at fifteen pascals, and a vacuum gauge reads pressure from exactly that fall.