_Miscellaneous considerations._--For various reasons stations should not be located at or near camp grounds. These localities usually afford mixed backgrounds, and the presence of dust and smoke and the interference caused by moving bodies of troops and trains will militate against the efficient transmission of signals. Stations located in vicinities of this kind are also subject to annoyance from noise and visits of unauthorized persons. Signal stations should be convenient for messenger service and hence as near commonly traveled roads as the physical contour of the country will permit. Locations for signal stations should be so selected that the visual lines do not cross traveled roads, camps, etc., as dust and smoke in the daytime and lights at night are factors in determining the visibility of signals. Signal stations can if necessary be artificially concealed by erecting screens constructed of limbs of trees, etc., about the flanks and rear. Sheltered positions should be utilized in windy weather. _Intervisibility table._--The following table shows the extent of horizon for different heights above the sea level--that is, it shows how far one can see an object which is itself at the level of the sea: ----------------------------+----------- | Distance Height of the eye above sea |in statute level. | miles. ----------------------------+----------- 10 feet | 4 15 feet | 5 20 feet | 6 30 feet | 7 40 feet | 8 50 feet | 9 60 feet | 10 70 feet | 11 85 feet | 12 100 feet | 13 115 feet | 14 130 feet | 15 150 feet | 16 200 feet | 18 230 feet | 20 300 feet | 23 350 feet | 25 500 feet | 30 700 feet | 35 900 feet | 40 ----------------------------+----------- A formula to determine approximately the limits of visibility from a given height is as follows: The square root of the height of the station in feet multiplied by 1.26 equals the distance in miles at which the signal is visible. Hence, an observer whose eye is 30 feet above the sea can distinguish an object 7 miles distant, provided it is at the sea level; but if the object is itself 15 feet above the sea he can make it out 7 + 5 = 12 miles off. FINDING A STATION.