It is shown that the optimum design of duct in a magnetohydrodynamic generator is close to the one in which the flow Mach number remains constant. This constant-Mach-number generator is analysed in some detail and it is shown that the optimum Mach number can be defined to within a few percent. For a γ of 1·25, this optimum is near 0·85. For very short ducts, the maximum power output is obtained near matched-load conditions but for rather longer ones maximum total power output is obtained by working as close to short-circuit conditions as is practicable. Against this, the minimum compressor requirements are found by working as close to open-circuit conditions as is practicable, and so a compromise must be reached for optimum overall generator design as far as load conditions are concerned. This will probably give an internal ohmic loss in the fluid of about one-third of the total output. Curves are presented which enable the optimum Mach number to be determined with greater precision when the optimum load conditions have been selected.

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The first arrivals (Pn) from the Hardtack series of underground explosions, at distances up to 1000 km, are analyzed in detail. There is no measurable dispersion in these arrivals, and the possibility that the observed decay of amplitude with distance is due to a low‐velocity layer below the Mohorovicic discontinuity is discussed.

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