The mobile machine guns of the fortress will be used on the advanced line of defence with the mobile troops, and should be divided into two--those allotted to the outposts and those allotted to the local reserve.[41] Those allotted to the outposts must be placed in carefully selected positions commanding the approaches to the section of the defence to which they have been posted. These positions will usually be in minor works such as redoubts, emplacements, and lunettes, and they will be selected for their good field of fire, particular attention being paid to their command of dead ground in front of other works. Great care and trouble must be taken in concealing the guns and providing them with good cover, not only from rifle fire, but also from artillery. An endeavour should be made to command all wire entanglements along the front with machine guns, and the angles of traverse of each gun should be carefully laid off and marked in white paint or tape, so that they may be used in the dark accurately to sweep their area of ground. Too much stress cannot be laid on the importance of the accurate laying and sighting of machine guns by day for use at night, and it must be remembered that they are the only weapons which are capable of bringing a rapid and concentrated rifle fire on a particular spot in the dark, and are therefore invaluable to the defence during a night attack. The detachments of guns on outpost duty at night should be told off into three watches of two men each, whose duty it will be to remain with the gun in readiness for instant action. The gun should be loaded and laid, and the men on duty should watch the front. Where the gun is in an emplacement or other loop-holed work, one man should watch through the loophole in turns of half an hour at a time. The strain of peering into the dark and listening for the sound of an approach at night is so great that no man should be required to do this duty for more than half an hour at a time, while the chance of a man dozing during a short spell is much reduced and the acuteness of the senses has not time to get dulled. Very strict orders must be given to insure that fire is not opened prematurely; and where infantry sentries are on duty near the gun, it may be advisable not to load the gun, but merely to insert the belt in the feed-block in readiness.