Abstract
By means of certain simplifying assumptions a formula is developed for the penetration into a ductile target by a high-velocity (`Munroe') jet. The action of the jet is divided into two stages, each making its contribution to the total penetration In the first stage a hole is formed by the lateral compression of the target as the jet penetrates it; the second stage begins when the last particle of the jet has ceased to act, the hole continuing to deepen until the residual energy in the target has been spent. At the high pressures set up by a Munroe jet the strength of the target plays only a subsidiary part in the phenomenon.