COMPOSITION.--Its composition depends upon the work to be performed, and this calls for detachments of all arms of the land service, in addition to _Aircraft_, which can prevent surprise by reconnaissance over the hostile area and can harass the pursuing columns by day and by night by fire-action with Lewis guns and bombs. _Mounted troops_ are required to extend the area watched and to prolong the resistance by reason of their superior mobility, in addition to their counter-action as cavalry. _Artillery_ are required to open long-range fire on the enemy's columns and so to cause delay by deployment; and to concentrate upon them while in, or emerging from, a defile. _Infantry_ and _Machine-gun Platoons_ are required for prolonged fire-fights and local counter-attacks, during which sudden bursts of machine and Lewis-gun fire will do the greatest execution. _Engineers_ provide sappers for the creation of obstacles and traps, and for the demolition of bridges and viaducts. _Mechanical Transport_ may be required to add to the mobility of the infantry. The _Medical Service_ is called upon to provide attention and ambulances for the wounded and for the sick and worn-out troops. DISTRIBUTION.--The Rear Guard is divided into two parts--the Rear Party and the Main Guard. The _Rear Party_ consists, like the Vanguard of the Advanced Guard, of patrols and supports; the rest of the force forms the _Main Guard_, and marches in the order in which the troops are required, viz.: Artillery (with escort), Mounted Troops (if any remain over from the Rear Party), Infantry, Medical Services and Ambulances, and the Sappers of the Royal Engineers. The guns can thus open fire whenever required, and the sappers, who are furthest away from the pursuit, will have the longer time to prepare obstacles and demolitions, the {121} latter being completed by the Rear Party. Communication must always be secured and maintained between the Rear Party and the Main Guard, and between the Rear Guard and the Main Body. DISTANCE.--The distance at which the Rear Guard works is governed by the duty it has to perform, viz.: to permit the withdrawal of the Main Body to be carried out without interruption by the enemy, and to effect this it will usually be necessary for the Machine Gun and Infantry Platoons of the Main Guard to keep within effective range of positions from which hostile artillery might molest the Main Body. The commander will probably remain with this part of his force, as its work is of the highest importance; in any case his position must be made known and there should be definite commanders of the Rear Party and the Main Guard. But while the distance separating the Rear Guard from the Main Body must be sufficient, it must not be too great, or the enemy may penetrate between it and the Main Body, and not only will the Rear Guard be cut off and liable to destruction but it will cease to protect the Main Body.