The determination of the actual pressures produced by a blow such as th at of a rifle bullet or by the detonation of high explosives is a problem of much scientific and practical interest but of considerable difficulty. I t is easy to measure the transfer of momentum associated with the blow, which is equal to the average pressure developed, multiplied by the time during which it acts, but the separation of these two factors has not hitherto been effected. The direct determination of a force

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... ing for a few hundred-thousandths of a second presents difficulties which may perhaps be called insuperable, but the measurement of the other factor, the duration of the blow, is more feasible. In the case of impacts such as those of spheres or rods moving at moderate velocities the time of contact can be determined electrically with considerable accuracy.* The present paper contains an account of a method of analysing experi mentally more violent blows and of measuring their duration and the pressures developed. If a rifle bullet be fired against the end of a cylindrical steel rod there is a definite pressure applied on the end of the rod at each instant of time during the period of impact and the pressure can be plotted as a function of the time. The pressure-time curve is a perfectly definite thing, though the ordinates are expressed in tons and the abscissae in millionths of a second; the pressure starts when the nose of the bullet first strikes the end of the rod and it continues until the bullet has been completely set up or stopped by the impact. Subject to qualifications, which will be considered later, the result of applying this varying pres sure to the end is to send along the rod a wave of pressure which, so long as the elasticity is perfect, travels without change of type. If the pressure in different sections of the rod be plotted at any instant (fig. l) then at a later time the same curve shifted to the right by a distance proportional to the time will represent the then distribution of pressure. The velocity with which the wave travels in steel is approximately 17,000 feet per second. As the wave travels over any section of the rod, th at section successively experiences pressures represented