Choice of apparatus.—Sunlight conditions permitting, the heliograph will ordinarily be used for day signaling on account of the advantages of the great range and speed afforded by it. When its use is prohibited by weather conditions, the flag will be substituted for it. The white flag will be used against dark and the red against sky or broken backgrounds. The distant station is the better judge as to which color flag is best suited to given conditions and the color indicated by it should invariably be used. For night signaling, the acetylene lantern is usually employed. Long-range night signaling should be done with the searchlight if available. The employment of the semaphore, in daytime, and the Ardois system, at night, will be confined to more or less permanent stations. Rockets, shells, night fires, etc., are only employed for special or emergency signals.
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:
| Height of the eye above sea level. | Distance in statute 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.
To find a signalman near any known station, note with the unaided eye some prominent landmark near which the looked-for person or object is supposed to be, and direct the telescope upon the place, as sight is taken over a gun barrel, covering the object; if the eye is now placed at the eyeglass of the telescope, the prominent or directing landmark will be found in the field of view. It will be easy then to scale the country near the marker until the signalman is found. This method is often necessary at night, when only a point of light is seen far off through the darkness, and the telescope must be turned upon it. When the compass bearing of the object sought for is known, the telescope may be aligned by a line drawn with the proper compass bearing. Commencing then with the view at the horizon, the telescope is slowly moved from side to side, taking in fresh fields of view each time a little nearer to the observer, until the whole country shall have been observed from the horizon to quite near the station. When the general direction only of the object can be given and it is sought for, the whole landscape in that direction to the horizon should be divided into sections by imaginary lines, the limits of these sections being bounded between visible landmarks through which the bounding lines are supposed to pass. Each section should be scrutinized little by little until the glass has been passed over every spot. Such search will seldom fail to be successful.