(ii) In undulating or mountainous country it may be possible for detachments employed to provide covering fire to cover the advance from positions in rear, but in flat country it may be dangerous or impossible for infantry or machine-guns to fire over the heads of their own troops, and opportunities for providing covering fire should be sought on the flanks. (iii) Troops detailed to provide covering fire for the advance must take care to select as targets _those bodies of the enemy whose fire is chiefly checking the advance_. Great difficulty will often be experienced in discovering which these bodies are, and all ranks must be on the alert to notice any indication which may help to discover them. (iv) As soon as their fire ceases to be effective in aiding the advance of the firing-line, it is the duty of troops detailed to provide covering fire _at once to join in the advance_, unless definite orders to the contrary have been received. =14. Volume of Fire.=--In deciding on the volume of fire to be directed against the enemy at any particular time, a commander should consider chiefly the tactical situation, the target presented, the effect it is desired to produce, the range, and the state of the ammunition-supply. =15. Rates of Fire.=--_The rate of fire will always be regulated carefully according to tactical requirements._ (i) =Slow Desultory Fire= may disturb the enemy’s aim, but it is opposed to principles of surprise. (ii) =Deliberate Fire.=--The rate of deliberate fire should not exceed six rounds a minute. (iii) =Rate when Working in Pairs.=--Soldiers working in pairs for observation and mutual support (Sec. 47, para. 4) may _each_ fire about three rounds a minute. (iv) =Rapid Collective Fire.=--In rapid collective fire the rate will vary according to the visibility of the aiming-mark, the range, and the standard of training a man has reached. With a distinct aiming-mark within about 1,000 yards a well-trained man should be able to fire from twelve to fifteen rounds per minute without serious loss of accuracy. =16. Use of Rapid Fire.=--(i) _Rapid fire should be considered as a reserve of power to be used when the occasion demands it._ It must never be used except when occasion most fully justifies it, otherwise serious waste of ammunition will result. _Rapid fire must combine accuracy with rapidity, and never degenerate into a wild expenditure of ammunition at the fastest possible rate._ If rapid fire is ordered, _each man will fire at his own best rate for combining rapidity with accuracy_. (ii) Rapid fire may be employed generally when it is necessary to beat down the enemy’s fire quickly; when covering the withdrawal of other troops; when pursuing an enemy with fire; when meeting cavalry attacks; and when good targets are exposed. In attack, rapid fire is employed by all troops as final preparation for the assault. In defence, it is employed to beat off an enemy in the act of assaulting (Practices Nos. 5 and 6, p. 228).