AN article by J, Kemp (Electrical Communication, 20, No. 2, 1941) shows how the attenuation of electromagnetic waves propagated through the interior of rectangular metal tubes is calculated by the familiar telephone transmission formulæ, the top and bottom of the tube being regarded as zigzag flat-strip transmission lines, while the side walls, when suitably disposed, are regarded as transmission lines of another type. From the losses occurring in these lines, the attenuation offered by the tube is obtained in a simple manner. The general formula for the attenuation of all types of waves in metal tubes of all cross-sections has been established by Schelkunoff. Chu and Barrow have also derived attenuation formulæ for all types of waves supported by tubes of rectangular cross-section. The method so far followed has involved the computation of the energy flow in the direction of the tube and the power losses in the walls by means of the Poynting flux theorem.