On the firing of the propellent charge, the expanding gases, we are told, cause expansion of the gun, thereby simultaneously allowing proportionate expansion of the shell. We know already that erosion is most pronounced round the area within which the propellent charge is fired, that is to say, just at the commencement of the rifling of the bore; hence we see that in all probability the bore decreases gradually, even if imperceptibly, in the direction of the muzzle. We have, however, had experience of these varying and imperceptible degrees of graduation at Crewe, measuring them with our plus and minus gauges. What, therefore, if the shell, expanded in the commencement of the rifling, jams, or is momentarily checked, in its passage up the bore? A "premature"; for the pellet in the fuse is probably already free to jump forward against the detonating needle. These nightmare prematures! Thank goodness, after all, in one direction, at least, assurance has been rendered doubly sure; we used to wonder why all this fuss and trouble about a base-plate? Now we understand. Imperceptible must be our attribute again; metals are porous, imperceptibly so. Now imagine a sieve trellised with wires finely drawn as the threads of a spider's web, threads so closely woven and interlaced that only the most minute "teeny weeny" holes remain, a sieve in fact which though porous is not even transparent. Figuratively speaking this is the base of a shell-forging; a porous partition, the sole dividing line between the ignited propellent gases behind, and the high explosive bursting charge within the shell; in other words, between ourselves and "kingdom come"; hence our supplementary or protecting plate, the grains of whose metal run crosswise to those of the shell-forging base. By this method of reinforcing the shell-base, the odds in favour of a "premature" due to the penetration of the propellent gases to the explosive charge are reduced to a further irreducible minimum. The gun is, of course, designed to withstand the pressure within the breech and behind the shell, exerted by the firing of the propellent gases; this pressure naturally varies according to the size of the gun, and decreases proportionately as the shell shoots forward towards the muzzle with rapidly increasing velocity. When, however, we come to consider the pressure exerted by the detonation of the high-explosive charge within the shell itself, and the velocity acquired by the resultant explosive gases, we are apt to fidget about a trifle uneasily in spite of our efforts to remain at least outwardly cool and nonchalant.