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. Most of these operations have to be very accurately carried out and, to ensure that that is so, gauges are continually employed to check the work. These gauges are based upon a very simple principle, known as the "limit" principle. This is both interesting and important, sufficiently so to merit a more detailed reference. It must first be realized that no two things are alike and no measurement is perfectly correct. When we lightly speak of two things being "alike" we really mean that for the purpose contemplated they are nearly enough alike. Two things might be "alike" for one purpose and yet be so unlike as to be useless for another. What the authorities do in the case of shells, therefore, and what is done nowadays in many branches of engineering, is to recognize this fact and at the same time overcome the difficulty by stating what difference is permissible. In other words, instead of saying that a thing must be a certain size, it is required to fall between two limits: it must not be more than one or less than the other.