3. Estimate the distance along a parallel line, such as a telephone line or a railroad having on it a well-defined length with which you are familiar. 4. Take the mean of several estimates made by several well-instructed men. This method is used in battle, but is not applicable to instruction or during tests. 1. Preliminary Instruction To estimate distances by the eye with accuracy, it is first necessary that you become familiar with the appearance of the most convenient unit of length, namely 100 yards. Stake off a distance of 100 yards. Subdivide this 100 yards into four 25-yard divisions. Pace off the entire distance several times, and you will soon become familiar with the appearance of 100 yards. Next, take a distance more than 100 yards and compare it mentally with your unit of measure (100 yards) and make your estimate. Verify this estimate by pacing the distance. Do this once a day for several months, and you may become highly skilled in the art of estimating distances. 2. Preliminary Instruction If you know how a soldier, or group of soldiers, looks at the different ranges, it will often assist you in quickly making an accurate estimate of the distance. In order to acquire skill in estimating distances by this method one must have special exercises designated to demonstrate the clearness with which details of clothing, movement of the limbs, etc., can be observed at the different ranges. Have a squad march away from you to a distance of 1,200 yards. Then have it approach you and halt every 100 yards. Each time the squad halts make a mental note of the distance, and then observe carefully its appearance, the clearness with which you can see the clothing, movements of the limbs, etc. APPEARANCE OF OBJECTS Become familiar with the effect which the varying conditions of light, background, etc., have upon the apparent distance of the object. Don't be content to memorize the following data, but go after the underlying reason in each case. Objects seem nearer than they actually are: 1. When the object is seen in a bright light. 2. When the color of the object contrasts sharply with the color of the background. 3. When looking over water, snow, or a uniform surface like a wheat field. 4. When looking from a height downward. 5. In clear atmosphere of high altitudes, as in Arizona and New Mexico. Objects seem more distant than they actually are: 1. When looking over a depression in the ground (across a canyon). 2. When there is a poor light (very cloudy day) or a fog. 3. When only a part of the object can be seen. 4. When looking from low ground upward toward higher ground. ESTIMATING DISTANCES BY SOUND Sound travels at the rate of about 366 yards a second. Therefore, multiply the number of seconds intervening between the flash of the gun and the report of the same by 366, and the product will be the distance in yards to the gun. RANGE-FINDING INSTRUMENT