=365.= The sentinels and detachments of the old guard are at once relieved by members of the new guard, the two guards standing at ease or at rest while these changes are being made. The commander of the old transmits to the commander of the new guard all his orders, instructions, and information concerning the guard and its duties. The commander of the new guard then takes possession of the guardhouse and verifies the articles in charge of the guard. =366.= If considerable time is required to bring in that portion of the old guard still on post, the commanding officer may direct that as soon as the orders and property are turned over to the new guard the portion of the old guard at the guardhouse may be marched off and dismissed. In such a case the remaining detachment or detachments of the old guard will be inspected by the commander of the new guard when they reach the guardhouse. He will direct the senior noncommissioned officer present to march these detachments off and dismiss them in the prescribed manner. =367.= In bad weather, at night, after long marches, or when the guard is very small, the field music may be dispensed with. CHAPTER X. MAP READING AND SKETCHING. =Section 1. Military map reading.= When you pick up a map, the first question is, Where is the north? This can usually be told by an arrow (see fig. 1, p. 259) which will be found in one of the corners of the map, and which points to the true north--the north of the north star. On some maps no arrow is to be found. The chances are a hundred to one that the north is at the top of the map, as it is on almost all printed maps. But you can only assure yourself of that fact by checking the map with the ground it represents. For instance, if you ascertain that the city of Philadelphia is due east of the city of Columbus, then the Philadelphia-Columbus line on the map is a due east-and-west line, and establishes at once all the other map directions. Now, the map represents the ground as nearly as it can be represented on a flat piece of paper. If you are standing up, facing the north, your right hand will be in the east, your left in the west, and your back to the south. It is the same with a map; if you look across it in the direction of the arrow--that is, toward its north--your right hand will be toward what is east on the map; your left hand to the west; the south will be at the bottom of the map. [Illustration: Fig. 1, 2, 3, 4, 5 and scale.] There is another kind of an arrow that sometimes appears on a map. It is like the one in figure 2, page 259, and points not to the true north but to the magnetic north, which is the north of the compass. Though the compass needle, and therefore the arrow that represents it on the map, does not point exactly north, the deviation is, from a military point of view, slight, and appreciable error will rarely result through the use of the magnetic instead of the true north in the solution of any military problems.