2721 HYDROGRAPHIC MANUAL PaGE 154 



steel poles for use in liydrographic surveys off low islands in tropic areas. The poles 

 are in sections, each fitting into the other, and the height of a signal may be increased 

 as desired by adding additional bottom sections, the poles being held upright by guys 

 For oft'shore ship hydrography, a signal must have an elevation great enough to 

 overcome the curvature of the earth and be sufficiently large to be seen in a sextant at 

 the required distance. The elevation above the water of the tower to be constructed 

 can be computed from the formula: 



or 

 H=0.75 D' 



where H is the height of the tower in feet above the water, 1.15 is an approximate constant for curva- 

 ture and refraction, and D is the distance in nautical miles at which the signal must be seen from the 

 water level. Thus, the elevation of a signal to be just visible from the waterline 10 miles at sea 

 would be 75 feet. Sextant angles are usually observed from the ship's bridge and assuming the 

 height of the eye to be 23 feet above the water, then the same signal can be seen 5^^ miles farther, or 

 a maximum distance of 15^ miles. This extra distance of bji miles is computed from the same 

 formula by solving for D, the value of H being the height of the eye above the water. 



In perfectly clear weather a signal, especially if it shows agamst the skyline, can be 

 observed until it disappears below the horizon and often, just before disappearing, 

 refraction actually makes it appear larger than it appears at a slightly closer distance. 

 WTien the sea is rough, signals camiot be seen as far, by a considerable distance, as in 

 calm weather, because of the waves between the observer and the signal. 



Many different factors must be taken into consideration in determining the height 

 and size to build an object to be seen the required distance, some of wdiich are as 

 follows: The background, whether wooded, open sky, or intermittent trees and sky, 

 and whether the tree line is close to or far from the signal; the direction of the shoreline, 

 and whether a signal w^ill reflect the sunlight; and the prevalent weather, whether clear 

 or cloudy and overcast. A much larger target is required if cloudy or overcast weather 

 is prevalent or if the signals must be so placed that there is no reflection from the sun 

 during the greater part of the day. If the background behmd a white signal is inter- 

 mittent woods, a much larger and more distinct signal is necessary, for a small one 

 cannot be distinguished from an opening or break in the tree line. 



There is little use in making any part of the target black unless it projects well 

 above the trees or skyline, in which case the entire target should be black. 



2721. Ninety-Eight Foot Signal 



The construction of a wooden signal of this type is started by building, on the ground, a quadri- 

 lateral section 16 feet high and 4 feet square, which is raised into place in a hole about 8 feet square 

 and 6 feet deep, so that the entire base of the tower can be buried in the ground. The bottom braces 

 are doubled (one inside and one outside the legs) and are extended about 2 feet beyond the sides of the 

 structure. Scrap lumber or drift wood is laid across the bottom horizontal braces. It is important 

 that the first section be set exactly vertical before the hole is filled and the building continued. Its 

 verticality can be ensured by setting up and leveling a small theodolite and sighting up and down 

 the sides of the signal with the vertical cross hair or by sighting along the string of a plumb bob sus- 

 pended at arm's length from a position a little above the height of the eye. The structure is held in 

 this vertical position by temporary guy wires while earth is tamped firmly around the base of the signal 

 and the hole is filled. 



Construction is then continued, the verticality of successive sections being checked in the same 

 manner as the first section and more temporary guys used, if necessary, to pull the structure a little 

 one way or the other. The scaffold is 4 feet square in its horizontal section, the legs and sides being 

 parallel throughout its height. Horizontal braces are nailed to the tower at 4-foot intervals and 

 diagonal braces nailed between these 4-foot sections. Leg pieces, braces, and guy wires are added until 



