A new solution of Clausing’s equation has been obtained by iterative methods. In long cylindrical tubes it is found that the gradient of the molecular density is higher in the ends of the tube than in the center. Relations are developed for the measurement of the molecular flow rate by pressure gauges mounted in the wall of the tube. A simple function closely approximates the density distribution for both long and short tubes. The gauge position is calculated for the measurement of pumping speed at the end of the tube. The molecular transmission probability is found to agree with the results of Berman to within 0.1%.