A theory of fluids is described based on a 'tunnel' model, in which lines of molecules are pictured as moving almost one-dimensionally in tunnels or fine capillaries, the walls of the tunnels being formed by neighbouring lines of molecules.

In the world of 2026 aviation and aerodynamics, the "Tunnel Theory" (specifically Wind Tunnel Theory) is the mathematical and physical framework used to predict how air flows over an aircraft. It relies on the Principle of Continuity (Conservation of Mass) and the Bernoulli Principle to understand how changing the cross-sectional area of a "tunnel" affects air velocity and pressure. Specifically, it describes how subsonic air decelerates in a widening duct while supersonic air accelerates. This theory is vital for engineers designing modern, fuel-efficient wings and engine intakes. By placing scale models in a controlled "tunnel" environment, they can measure lift and drag forces accurately. This data allows for the development of the "Ultra-Efficient" aircraft designs currently entering the 2026 market, which aim to reduce CO2​ emissions by over 30% compared to previous decades.