[1869]. Effect of Slope on Movements
- 60 degrees or 7/4 inaccessible for infantry;
- 45 degrees or 1/1 difficult for infantry;
- 30 degrees or 4/7 inaccessible for cavalry;
- 15 degrees or 1/4 inaccessible for artillery;
- 5 degrees or 1/12 accessible for wagons.
The normal system of scales prescribed for U. S. Army field sketches is as follows: For road sketches, 3 inches = 1 mile, vertical interval between contours (V. I.) = 20 ft.; for position sketches, 6 inches = 1 mile, V. I. = 10 ft.; for fortification sketches, 12 inches = 1 mile, V. I. = 5 ft. On this system any given length of M. D. corresponds to the same slope on each of the scales. For instance, .15 inch between contours represents a 5° slope on the 3-inch, 6-inch and 12-inch maps of the normal system. [Figure 11], [Par. 1867a], gives the normal scale of M. D.'s for slopes up to 8 degrees. A scale of M. D.'s is usually printed on the margin of maps, near the geographical scale.
Fig. 14
[1870]. Meridians. If you look along the upper left hand border of the [Elementary Map] (back of Manual), you will see two arrows, as shown in [Fig. 14], pointing towards the top of the map.
They are pointing in the direction that is north on the map. The arrow with a full barb points toward the north pole (the True North Pole) of the earth, and is called the True Meridian.
The arrow with but half a barb points toward what is known as the Magnetic Pole of the earth, and is called the Magnetic Meridian.
The Magnetic Pole is a point up in the arctic regions, near the geographical or True North Pole, which, on account of its magnetic qualities, attracts one end of all compass needles and causes them to point towards it, and as it is near the True North Pole, this serves to indicate the direction of north to a person using a compass.