Gauges may be divided into two categories known as "working" and "inspecting" gauges; again each category is further divided into two sub-categories, designated in approved munitions' parlance as "to go" and "not to go." "Working" gauges being in more or less constant use, and subjected, not infrequently, to a certain measure of rough usage, were allowed a slightly less "tolerance" or margin, plus or minus, high or low, than were "inspecting" gauges; that is to say, given a certain specific measurement for any specific part of a shell, the gauge in proportion as it was "working" or "inspecting" was limited to a tolerance of a minute fraction of an inch either above or below the finished dimension; this was in fact considered the beau idÉal of good workshop practice.

Let us take for the sake of argument a shell body. If after being turned to presumably the required and finished diameter it was found to slip through the hole of a "not to go" working gauge, there was no alternative other than that of scrapping it then and there, because obviously once turned to too small a diameter, not "all the king's horses nor all the king's men" could ever increase that diameter by one single iota again; as a biblical parallelism one might even add that it would be "easier for a camel to pass through the eye of a needle" than for a "not to go" shell to enter into bond. If, on the other hand, the shell would not quite enter the "to go" hole of a "working" gauge, or even if it was a tight fit in this hole, it followed that there remained a certain tolerance or margin whereby the diameter could still be reduced to a trifle, and the shell per se qualified as a candidate for the final examination of the "inspecting" gauge. Picking up, then, our "inspecting" gauge we find that this, in like manner, has two holes, one "to go" and one "not to go"; but the gauge-maker has perforce been wily enough to leave the tolerance of these "inspecting" gauge holes a minute fraction of an inch greater, above and below the required finished dimension, than that allowed in the "working" gauge holes, his purpose being to ensure the inspector something up his sleeve in the way of control over the machinists who use the "working gauge" only; hence the "not to go" hole of the "inspecting" gauge is slightly less in diameter than the "not to go" hole of the "working" gauge; the "inspecting" "to go" hole is a tiny trifle larger than the diameter of the finished shell than is the "to go" hole of the "working" gauge.

The accompanying diagram scaled to convenient fractions of an inch will perhaps help to illustrate the meaning of these tolerances.

Suppose for instance that the correct finished diameter of our shell is 5-31/32 inches, the tolerances allowed by the Government (i.e. the final "inspecting" tolerances) can be represented by the two semicircles and dimensions shown below the centre line, which, as will be seen, are further from the finished dimensions than are the two semicircles and dimensions shown above the centre line, these latter representing the "working" tolerances, the difference between the two sets of figures representing the amount of margin kept up his sleeve by the Inspector by way of control over the machinists using the "working" gauges only.

In actual practice of course the fractions of an inch representing these tolerances are infinitely smaller than those shown above for the purpose of illustration only.

Some idea of the infinitely small fractions representing the tolerances allowed may be gathered by taking a sheet of ordinary thin note-paper, the thickness of which is about 3/1000 parts of an inch, or ·003 inch; try to imagine this sheet of thin note-paper made ten times thinner still, and you will find it reduced to a minimum of tissue thickness, 3/10000 parts of an inch or ·0003 inch; remember now that on good artillery depends not only the lives, maybe of your nearest and dearest, but the honour of the nation too; and you have firstly some idea of the imperceptible fractions of an inch to which a shell-gauging instrument is made; secondly the reason for the proportionate degree of accuracy demanded in the manufacture of artillery ammunition itself.