Trenches are exposed to attack not only from the surface but also from the air and from beneath the surface of the ground. Aeroplanes come over and drop steel darts or bombs, and only very strong head cover can give protection against them. But it is very hard to hit a trench from the air with a missile of this kind, and the danger to the men in the trenches is not very great. By far the greater danger comes from mining. Men must be on the watch all the time to detect these operations of the enemy and to forestall them where possible. Special instruments like giant stethoscopes have been invented and men are told off to hold them to the ground to detect the sounds of digging. But, while these instruments are of use, the best means is a well trained ear. If the enemy is discovered mining towards your position, the only thing to do is to countermine him and try and blow him up before he is ready to touch off the fuse that is intended for your destruction. It is not an easy matter to decide just where the countermine should be sunk or how far to go. Many combats have been fought with pick and shovel in the bowels of the earth in cases where one side or the other has broken through the gallery. But any risk must be taken rather than let the enemy enjoy uninterrupted his work of mining you. CHAPTER VIII ARTILLERY FIRE It may be desirable to indicate the various kinds of shell fire to which trench men are exposed. (I pass over rifle fire which is harmless so long as men keep their heads down and avoid corners from which they can be enfiladed.) No amount of caution will save a man from shells if they happen to be falling in his neighbourhood. The most frequent visitor of the shell variety is of course the shell from the field gun. The British use an 18-pounder, the French a 75 millimetre, and the Germans a 77 millimetre--all shells of about the same calibre (3 inches). Of these the best gun is undoubtedly the French, which is a perfectly marvellous piece of mechanism. But all of them are deadly in their effects. They may fire either shrapnel or high explosive--always spoken of in the army as H. E. In the former case, the shell that is fired contains a nose that comes off at the time for which it is set, and liberates hundreds of small round leaden bullets. These go out in cone shape and spray the ground round about. Shrapnel is very effective against men in the open but of little use when they are entrenched, for very few of the bullets from any shell will enter the ditch itself. When the British army first took the field they had very little H. E. Shrapnel had been used successfully in South Africa, and it was thought it would do in France. The proportions used were 96 per cent shrapnel to 4 per cent H. E. We learnt our lessons dearly--as we usually do--and in time we came to realise that for breaking down parapets shrapnel was absolutely useless. The proportion that is now used is about 90 per cent H. E. to 10 per cent of shrapnel.