We can thus sum up the chief types of shell as follows: the naval shell which has to be capable of
penetrating armour: the high-explosive shell which must be able to break up earthworks and blow down the walls of trenches: and the shrapnel shell which scatters a shower of bullets and is most useful in attacks upon bodies of men rather than upon material structures.
Some shells have their propellent explosive combined with them just as the familiar rifle cartridge contains the propellant combined with the bullet. In the larger sizes, however, it is much more convenient to have the propellant in a separate cartridge, which can be handled separately and loaded into the gun separately.
As has been already explained, the propellant is a "powder" which gives a steady push rather than a destructive blow: moreover, it is practically smokeless, so as not to "give away" the position of the gun to the enemy. The "high explosive," however, shatters and usually makes a dense smoke, so that the observers can see where it fell and report to the gunners whether or not they have got the range. Soldiers' letters have told us of the "black Marias" and "coal boxes" used by the Germans, those terms being simply soldiers' nick-names arising no doubt from the fact that certain particular shells are filled with "tri-nitro-toluene" which gives a black smoke. Clearly, smoke, which is most objectionable in the propellant, is a positive advantage in the bursting charge.
And now let us take a glimpse at the manufacture of one of these terrible missiles. An ingot of shell-steel is first cast as described in an earlier chapter
. Since impurities are apt to rise, while the metal is liquid, the top of the ingot is always cut off and discarded. This waste material is used for many other purposes, in which a chance flaw would not be a serious matter, under the title of "shell-discard" steel.
The lower part is then heated and passed through a rolling mill, a machine very similar in principle to the domestic mangle, the rollers being of iron with suitable grooves cut in them. A few passages through this machine transforms the ingot into a thick round bar. This is then sawn into short pieces called billets, each of which is the right size to form a shell. Again heated, a powerful press drives a pointed bar through the softened steel, thereby converting the short billet into a rough tube. Another press then slightly closes in one end, making it resemble a bottle without a bottom and with the neck broken off.
The rough forging is then ready to be machined, an operation which is performed in a lathe. The outside is made perfectly round and smooth and of precisely the right size. The inside is also bored out to the correct diameter and finished off to an exceeding smoothness so as to avoid the possibility of any rough places irritating the explosive which in due time will be filled into it. For the same reason, the inside, when finished, is varnished in a certain way and with a certain varnish. The formation of this varnish is one of those little thought of but highly important services which alcohol renders to us, as mentioned elsewhere.
The smaller end (that which has already been partially squeezed in) is bored out and screwed for the reception of the nose-bush, while the other end is recessed for the reception of the plate which forms the bottom.