Soon we came to a fire bay, specially well placed to obtain a good view of the enemy trenches, which had been converted into what is known as an O.P., _i.e._, an artillery observation post. These O.P.’s are manned during daylight by the F.O.O. (Forward Observation Officer) and his signaller assistants. Their job is to keep a close watch on hostile trenches, watching for any unusual movement or for the appearance of new constructive works, such as machine gun emplacements or new saps. The O.P. has numerous wires leading into it, and these come from all the batteries in immediate support of that part of the line, which are jointly responsible for its defence. Our own signallers had been out early, and a wire had already been carefully laid and labelled from our gun position to the O.P., so we were now ready to register our howitzers on some definite object behind the enemy lines. A house, or some such landmark which is shewn on our trench maps, is usually chosen to calibrate upon. There is little trouble in effecting this, but, at first, there is some difficulty in following the rounds as they fall, through a periscope, owing to its small field of vision. It was, however, imperative to make use of that instrument, in this case, as an enemy sniper, watchful and on the alert, had already seen the top of it, and from time to time a bullet passed overhead unpleasantly close. This served to remind us to be discreet and to run no risks by exposing ourselves in the slightest degree above the parapet. Sometimes it is very difficult to restrain one’s enthusiasm when there is an interesting shoot taking place. The pieces being duly registered, the Battery is now ready for any emergency, and theoretically we can engage any target in our arc of fire. It is then essential to learn the country in hostile territory, and one looks out for likely targets and for points at which one can inconvenience the enemy by keeping him under constant harassing fire. This work must necessarily be done from a point of vantage where a good wide view can be obtained, and, in most cases, a house, tree, or high piece of ground well behind the lines, is selected for a Rear O.P.