2. When placed at the foot of slopes that trend towards the enemy they have the advantage that fire from them is more grazing than it would be if they were placed higher up the slope, but a retreat under fire from them will probably involve heavy loss. It is, as a rule, easy to conceal them in such a position. On the other hand, the field of fire from low-lying trenches is often very limited, and it is generally more easy to open up communication with the rear when trenches are close to the crest-line than when they are much in advance of it. 3. Trenches which can bring fire to bear at decisive range on to the ground over which the attack must pass, and which are themselves concealed from the attackers in the early stages, are most valuable in surprising the enemy at the most critical period of the attack. 4. Trenches should always be concealed and head cover provided when possible, covered communication from the rear should also be provided when time is available. 5. Important tactical points and such others which, owing to the dead ground in their vicinity, constitute a weakness to position, should be further strengthened by placing barbed wire entanglements or abattis in front of them, trenches being so placed as to bring an effective fire to bear on such obstacles. 6. When time admits, deep trenches just in rear of the crest-line may be usefully provided to give cover to the supports or the garrisons of the advanced trenches till they are required. =163.= _Fire._ 1. As the difficulties of ammunition supply and want of knowledge of ranges are not so great as in the attack, it will often be expedient to open fire at long ranges in order to oblige the assailant to deploy and adopt a definite course of action which it will be difficult for him to rectify when exposed to fire. Long-range fire may also be used to deceive the enemy as to the dispositions and strength of the defender, and to check the advance of reinforcements. The employment of long-range fire must, however, be regulated by the effect produced on the enemy. If this is observed to be small, it will be wiser to reserve ammunition for closer ranges where better results may be expected, and on occasion it may be advisable to encourage the enemy’s advance by a weak fire or by withholding it entirely, and to receive him at decisive ranges with a fire of the greatest intensity possible. =164.= _Machine Guns._ 1. In defence, as in attack, machine guns may be employed singly, or in pairs.