Let ε ε 0 be the molecular kinetic energy of the gas at an imaginary horizontal surface inside the gas layer. The number of molecules arriving at an 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 )