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 when it had thus endowed it with a perfect propellant. There remained the High Explosives with their tremendous pressures. Pressure in Explosives is measured by estimating the number of tons which each square inch of the containing wall has to bear when the explosion takes place. The pressures produced by the propellants of which I have been speaking may be taken to be of the order of 10 to 20 tons to the square inch. Recent measurements of those produced instantaneously by High Explosives point to a figure of 300 tons per square inch. But this inadequately expresses the contrast between them. It takes no notice of the rate at which the pressure rises. The rate at which that pressure comes on when a six inch gun is fired has been found to be about 10,000 tons per second so that it rises to the full pressure of 15 to 20 tons in something under the five hundredth part of a second. In a rifle the rate of rise is perhaps ten times as great but the period is proportionately shorter. But in a good high explosive the rate of rise per second is 24