driving towards the enemy's position. The idea, of course, is to burrow until the end of the tunnel is just under some important part of the enemy's works or fortifications. When the end of the tunnel has reached the right spot explosives can be placed there, the tunnel partly stopped to prevent the explosion from driving back upon those who make it and the whole fired at the desired moment.
This tunnelling is also called "sapping" and the tunnel itself a sap. Military engineers are often spoken of as "sappers and miners" as if the two things were clearly different, but as a matter of fact both are often used to describe the same thing. Roughly, we may say that a mine which stays still in the hope that the enemy will walk upon it is a mine proper, while a mine which itself progresses towards the enemy until it ultimately goes off beneath him, is a "sap" and the making of such a thing is "sapping." Or we might say that sapping is under-mining, in which sense we use it in general conversation when we speak of something sapping a man's strength. Soldiers speak of their engineering comrades as "sappers" just as they term artillerymen "gunners," but the only reason why they call them by that name instead of miners is because the latter is a well-known term applied to those who work in coal mines.
A subterranean mine, then, is nothing more or less than a hole in the ground, made in any way that may be convenient, filled with explosives and fired at a suitable time to do damage to the enemy.
In other words, it is simply some explosive con
cealed in the ground with means for firing it, and when the sailor conceals explosives in the sea so that they may blow up the enemy's ships, he borrows his military comrades' term and calls it a "mine" too.
Counter-mining is the enemy's reply to mining. Suppose I was foolish enough to wish to blow up my neighbour who lives in the house opposite to mine. I might start from my cellar and dig a tunnel under the road until I knew that I had arrived under his dwelling. But suppose that he got to know of my little scheme: he could then try counter-mining. In this case it would mean starting a tunnel of his own from his cellar towards my tunnel: then, as soon as the two tunnels had come sufficiently near to each other, he could let off his explosives thereby wrecking my tunnel and putting an end to my operations while yet I was only half-way across the road. Thus he would stop me before I had had time to harm him, and since he need only tunnel just far enough to render the necessary explosion harmless to his house, while I to succeed would have to tunnel right across the road, the man who is counter-mining always has a slight natural advantage over the man who is doing the mining. If only he gets to know what is going on in time he can always retaliate.
All forms of land mine are improvised on the spot according to circumstances. Not so, however, with submarine mines on which much ingenuity has been expended, the mines being made in workshops ashore ready for laying and then laid by ships and sometimes by divers.
Of these there are two main kinds, those which are put in place in times of peace for the protection of particular harbours and channels, and those which are simply dropped overboard from a mine-laying ship during the actual war.
They all consist essentially of a case of iron or steel plates riveted together just as a steam boiler is made, in fact the cases are made in a boiler shop. The charge is gun-cotton fired by a detonator, the latter being excited by a stroke from a hammer, as in a rifle, or else by electricity. In the latter case, a tiny filament of platinum wire is in contact with the detonator, and the wire being heated by the current, just as the filament of a lamp is, the detonator is fired by the heat.