Simplification of the Knudsen equation.

The empirical Knudsen equation describing the flow of fluid in a channel of circular crossection for arbitrary Knudsen numbers is simplified by a slight change of one of the numerical coefficients occurring in this equation. The resulting equation is simpler and in limiting cases - continuum and free molecule regions - reduces exactly to Hagen-Poiseuille and Clausing law respectively. The simplified form of the Knudsen equation is simultaneously in slightly better agreement with the independent numerical solution of the Boltzman equation than the original one.