In such an instance before the actual advance of the troops the fire of all these guns is concentrated along parallel lines to the enemy trenches, first, second and sometimes third. Each gun has its work mapped out for it in advance on a map covered with tiny squares. The actual point may be well beyond view of the gunners. The shell is landed in its appointed square solely on mathematical calculation. The commander of each gun knows, for instance, that he must fire into this, that or the other square for so many minutes or hours, and exactly at a given minute change his fire to another source.
RAIN OF SHELLS LIKE STREAMS OF WATER.
In effect on the enemy a continuous rain of shells, comparable to streams of water from hundreds of hoses is poured in a line right down the trench. At the same time a parallel line of fire is concentrated at a given distance back of the enemy's first trench and in front of the second, or in it. This means that the troops in the first line must not only take their bombardment without hope of retreat or escape, but that it is impossible to get reinforcements to them through the second curtain.
When it is calculated that the first line has been destroyed or demoralized, the troops leap from their trenches and advance strictly according to schedule over the ground between the opposing trenches. Their arrival at the enemy's first trench is timed to the second, and just as they are on the verge of plunging into their own curtain of fire this latter is gradually thrown forward, forming a screen between the newly captured trench and the enemy's second line. This means two curtains of fire through which the enemy would have to advance to counter-attack.
Time is given to rout out what remains of the enemy from the first line dugouts, and then the troops advance again. In the meantime the curtain of fire has preceded them as before, moving up to the line of drum fire which has been playing on the second line of trenches or just in front of it. If any of the enemy have attempted to flee before the attack from the first line they are caught between these two barrages which are gradually brought together.
When the first and second lines of fire have been brought together they are poured with redoubled fury into the second line of the enemy trenches, and then moved forward again just as the advancing troops reach this line.
DEPENDING ON LOCAL CONDITIONS.
The performance is made continuous so far as possible under the conditions peculiar to the given section in which the attack is being made. Sometimes it is possible to advance over three, four or five trenches in a single attack. At others it is as much as can be accomplished to capture one, which must be consolidated before further advance is made. It depends on the strength of the trenches, the nature of the ground, the distance apart that they are, and, of course, the amount of artillery fire which the enemy is able to concentrate in return.
When a sufficient advance has been made, it also becomes necessary to suspend operations for a time while the guns behind the lines are moved forward to new positions.
This is always the period of the counter-attack in force by the enemy, who seizes the opportunity when a certain proportion of the artillery is unable to fire because it is being moved. And it is during this period that the infantry have to do their hardest fighting, which consists, not in making the advance over no-man's land to the enemy trench, but in holding that trench afterward when the bringing up of their own artillery behind them to more advanced positions robs them of some of the support of the drum fire.