The Strombos horns are mounted in the trenches in such a way as to protect them from shell splinters as far as possible. This is generally done by packing them round carefully with sandbags, only the mouth of the horn being displayed and pointing toward the rear. Every sentry must know how and when to sound the horn. All he has to do when he realises that a gas attack is being made, or on receiving instructions from an officer to do so, is to loosen the tap on the cylinder one complete turn, when the horn will sound continuously for more than a minute. The noise is terrific and in an enclosed space or in a quiet region it is absolutely deafening. In the trenches, however, it is none too loud, and the distances between the horns in the front system of trenches are never more than four or five hundred yards. Farther back in the chain, toward the rear, the distances can be increased. Horns are now installed at battalion, brigade and divisional headquarters. By turning them on when the noise of those in front is heard it is possible to pass the alarm in an incredibly short space of time and thus forestall the cloud of gas to such an extent that every man in the support trenches or in rest billets or the villages behind the firing line is aware that an attack is in progress and gets ready to protect himself. Naturally, things don’t always work out exactly according to schedule. The horns are frequently damaged. In one place I was at, just this side of the canal, near Boesinghe, a heavy German trench mortar wrecked three of our Strombos horns within a week, and another and less suitable position had to be found for the alarm. Then there are occasional false alarms. These sometimes arise from individual men “getting the wind up” from a bombardment by gas shell and thinking that a cloud attack is being made. Others I am afraid have been more in the nature of experiments “to see how it works.” After all, it must be a great temptation to a sentry to be in charge of a Strombos horn and never have the pleasure of turning it on.