The machine gunners are generally used at the rear of a party, so that they can bring their guns into action from behind and sweep the top of the ground around the trenches being attacked, in order to prevent an overland attack on the bombing party. In all these formations the number of men detailed must allow for casualties. Rapidity of movement is essential, as crawling and stalking will give the waiting enemy an advantage. The leading bayonet men generally move along the trench, from corner to corner, in a succession of rushes, followed by first bomber, and the thrower hurls as directed by the bayonet man. The duty of the bayonet man is to protect the thrower and carrier at all costs. Second bomber and carrier follow the leader, keeping one corner behind to replace casualties. Each party must be regarded as reserves to the party in front, and some method must be found every time a new attack is carried out for giving the aerial service notice of trenches occupied. If the head of a party is checked, that which has been gained must be held by throwing up a barricade. In all attacks bombing parties are supported by a party of sandbag men, under an experienced N. C. O., so that while bombers keep the enemy at bay a strong barracade may be put up as quickly as possible. This is generally done by placing what is known as a demolition tube about 1½ feet from the bottom of the trench and in each side of the trench. This will bring down enough of the sides of the trench to make a good enough barricade for the moment, but great care must be taken that while watching and protecting the barricade the enemy do not come overland and drop in behind the barricade, with disastrous results to the garrison. Although the main defense of a line of trenches is infantry supported by artillery and machine gun fire, parties of bombers should be distributed throughout the front system of trenches. The best position is in the support trenches close to the main communication trenches, where they can make an immediate counter-attack should the enemy succeed in gaining a footing. A bombing trench back about 20 yards in the rear from which bombs may be thrown into the front trench, is a distinct advantage. The bombs stored in the trench should be kept ready-fused and with detonators inserted. They must be distributed in a number of dry, enclosed, as nearly as bomb-proof depots as possible, established at frequent intervals along the trenches. A good type of grenade depot is one built in a “T” shaped trench, slightly off the main trench. EXPLOSIVES