same time, the rear guard in mountain warfare is no place for them and they had best march with the baggage. During the expedition to Tibet some trouble arose with the machine guns owing to the extreme cold, which not only froze the water in the barrel-casing of the Maxims, but froze the lubricating oil in the lock and recoiling parts and thus rendered the gun useless. When very low temperatures are encountered in high latitudes, alcohol or spirit should be added to the water to prevent it freezing; and as potable spirit is liable to “evaporate,” a little paraffin oil should be added. Glycerine may be substituted for lubricating oil in temperatures where even Russian petroleum will freeze. CONVOYS Callwell defines a convoy as “a column of non-combatants guarded by a comparatively speaking small escort.” The object of this escort is to hold off all hostile parties and to get the convoy to its destination in safety. Escorts will generally have to be reduced to the smallest possible size in order not to deprive the fighting force of men. They will therefore always act on the defensive, while endeavouring to keep moving with the convoy, which will only be halted when compelled to do so for its own safety. When the escort consists of the three arms, machine guns may be used to reduce the number of infantry required very considerably, while adding to the defensive power of those necessary. In minor operations where every available rifle is required with the fighting force, machine guns will be found invaluable to replace infantry. Their exact position with the convoy must depend upon its composition and length and the number of guns available. The principle of having a machine gun, or where possible a section, at the head and another at the rear end of the line of wagons or pack animals is sound, as these are the vital points, and an attack on the centre can be met by a cross fire from these positions. If the convoy is unduly long, another gun or section may be placed in the centre. Should it be necessary to form laager, the machine guns in the front and rear enable this to be done under their converging fire. Where wagons or carts are used and the enemy is unprovided with arms of precision, machine guns may be mounted on the tops of wagons, so that they can open fire instantly and fire while moving forward with the convoy; this position not only provides them with a good field of fire, but also affords protection to the detachments from a sudden charge home of savages from an ambush. “The success of an attack upon a convoy usually depends upon the defeat of the protecting troops. This will involve a combat, which will be governed by the principles already laid down in this manual.... If fighting is inevitable, the enemy should be engaged as far from the convoy as possible.”[60]