When necessary for a man to fall out on account of sickness, he should be given a permit to do so. This is presented to the surgeon, who will admit him to the ambulance, have him wait for the trains, or follow and rejoin his company at the first halt. 633. Special attention should be paid to the rate of march. It is greater for trained than for untrained troops; for small commands than for large ones; for lightly burdened than for heavily burdened troops. It is greater during cool than during hot weather. With trained troops, in commands of a regiment or less, marching over average roads, the rate should be from 2-3/4 to 3 miles per hour. With larger commands carrying full equipment, the rate will be from 2 to 2-1/2 miles per hour. 634. The marching capacity of trained infantry in small commands is from 20 to 25 miles per day. This distance will decrease as the size of the command increases. For a complete division the distance can seldom exceed 12-1/2 miles per day unless the division camps in column. 635. In large commands the marching capacity of troops is greatly reduced by faulty march orders and poor march discipline. The march order should contain such instructions as will enable the troops to take their proper places in column promptly. Delay or confusion in doing so should be investigated. On the other hand, organization commanders should be required to time their movements so that the troops will not be formed sooner than necessary. The halts and starts of the units of a column should be regulated by the watch and be simultaneous. Closing up during a halt, or changing gait to gain or lose distance should be prohibited. (_C.I.D.R., Nos. 2 and 12._) _PROTECTION OF THE MARCH._ _General Considerations._ 636. A column on the march in the vicinity of the enemy is covered by detachments called "advance guards," "rear guards," or "flank guards." The object of these covering detachments is to facilitate the advance of the main body and to protect it from surprise or observation. They facilitate the advance of the main body by promptly driving off small bodies of the enemy who seek to harass or delay it; by removing obstacles from the line of advance, by repairing roads, bridges, etc., thus enabling the main body to advance uninterruptedly in convenient marching formations. They protect the main body by preventing the enemy from firing into it when in close formation; by holding the enemy and enabling the main body to deploy before coming under effective fire; by preventing its size and conditions from being observed by the enemy; and, in retreat, by gaining time for it to make its escape or to reorganize its forces. 637. Tactical units should not be broken in making details for covering detachments.