Inside a dilute gas in a Couette flow setup, let u 0 be the forward velocity of the gas at a horizontal flat layer (labeled as y = 0 ); u 0 is along the horizontal direction. The number of molecules arriving at the area d A on one side of the gas layer, with speed v at angle θ θ from the normal, in time interval d t is n v cos ⁡ ⁡ ( θ θ ) d A d t × × ( m 2 π π k B T ) 3 / 2 e − − m v 2 2 k B T ( v 2 sin ⁡ ⁡ θ θ d v d θ θ d ϕ ϕ )