=18. "S. O. S." AND "TEST" SIGNALS.=--In the event of an infantry attack by the enemy, a mine being fired, or other emergency, the S. O. S. signal will be sent by the quickest route to the Field Artillery battery covering the trenches concerned. _The signal will be followed by the number of the trench, e.g., "S. O. S. B4."_ The signal will be repeated to battalion H. Q., who will transmit it to brigade H. Q. On receipt of the S. O. S. signal all batteries covering the trenches concerned will open a concentrated fire on the enemy's front line. When necessity for fire no longer exists, a message to this effect will be sent to the artillery and to brigade H. Q. In order to test the efficient working of the artillery lines, "test" messages will frequently be sent from the trenches to the supporting battery. The number of the trench will always be sent, e.g., "Test A 6." The test will consist of one round of shrapnel fired on the "night line" of the battery. No target will be given by the officer in the trenches. The time taken from the handing in of the message until the shell bursts will be carefully noted and reported to battalion H. Q. The O. C. battalion will enter the result of all tests in his daily report. He will give the exact time at which the test was sent and will state whether it was a "direct" or an "indirect" test. A "direct" test is from trench to battery. An "indirect" test is from trench via battalion H. Q. to battery. =19. VERMOREL SPRAYERS.=--One man will be detailed to look after each sprayer. A spare tin of solution will be kept with each sprayer. Medical officers will periodically inspect both sprayers and solution. =20. HOSTILE ARTILLERY FIRE.=--In reporting activity by the enemy's artillery it is necessary to state: (i) The time at which shelling began and when it ceased. (ii) Your own position. (iii) Whether howitzer or gun. (iv) Direction from which shells arrive. Compass bearing should be given if possible. (v) Whether shells burst in the air or on "graze." In reporting results of our own fire, state: (i) Your own position. (ii) Estimate distances short, over, right, or left, in yards. Avoid vague statements. (iii) Whether gun or howitzer. (iv) Whether shrapnel or high explosive. If shrapnel bursts in the air, judge whether range is correct by the splash of the bullets on the ground and not by the burst. =21. AIRCRAFT.=--On the approach of any of the enemy's aircraft three blasts will be blown on a whistle. This will be the signal for all ranks to keep perfectly still. One blast on a whistle will indicate that the aircraft has moved away.