Picric acid solidifies after being run in a molten condition into the shell, and every precaution has to be taken in order to prevent, during solidification, the formation of cavities, because the least tendency to friction that might occur due to the "setting back" of this hard explosive mass resultant on the sudden forward movement of the shell might easily give rise to a "premature." Herein we recognise the vital importance of wholly harmonious internal blending, absence of which might prove a further source of friction entailing imminent peril to ourselves as we stand beside the gun. Again, picric acid is ever seeking opportunities for combining with metal, whereby compounds of a nature most sensitive, and styled "picrates," are created, and of which the most sensitive is the lead-picrate. Hence we breathe a silent prayer that not only has the shell's inside been sprayed in the most efficacious and thorough manner possible, but that the composition of the varnish itself is entirely lead free.
Our wives, our families, our friends, we fain would turn to them as of a sudden a fresh suggestion, fraught with peril dire, creeps from out some hidden corner of our timorous mind.
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.
All the same, a matter of 300 tons per square inch, which is the pressure liable to be exerted by the detonation of an average charge contained within the high-explosive shell, can only be explained as "splitting"; and when our genial gunner-friends further assure us that 7000 metres or 21,000 feet per second is the velocity resulting from the detonation of "Trotyl," "staggering" is perhaps the most fitting epithet, and an American "Gee whiz" the only coherent sign of comprehension of which we are capable at the moment. For supposing a "premature" did chance to occur; well...! Mercifully enough perhaps for them and for their peace of mind, these intrepid individuals the gunners have little or no time, as a rule, to reflect upon the naked meaning of these figures and their attendant possibilities; for as Kipling has sung—
"The moril of this story, it is plainly to be seen,
You 'avn't got no families when serving of the Queen—
You 'avn't got no brothers, fathers, sisters, wives or sons—
If you want to win your battles take an' work your bloomin' guns."