275 HYDROGRAPHIC MANUAL PaGE 160 



anchors if necessary. Observations with a repeating theodohte can be made from 

 the tower, if desired, when the sea is extraordinarily calm. 



A simple tripod signal that will stand in a moderate depth of water may be made 

 from lengths of iron pipe wired together through tee-fittings at their upper ends. Wlien 

 the exposed part is wrapped with cloth and flags are erected on top of it, it may be 

 seen at a considerable distance. Such signals may be made more secure by pumping 

 the legs into the bottom by means of a water jet, Wliere this is not possible, as in 

 rocky areas, weights with wire loops around the pipes may be slid down the legs to 

 secure them. Each pipe leg should be provided with a fitting near the bottom to 

 engage the wire loop and hold the weight in place. Long poles and saplings have 

 also been pumped into ocean bars by means of a water jet and have withstood moderate 

 storms. 



A type of water signal that is constructed on land and then towed out and sunk in 

 position is described in detail on pages 35-38 of Special Publication No. 234, Signal 

 Buildmg. This type of signal has been used to depths of 13 feet. 



The details of two wooden structures that have been used successfully for hydrographic control 

 on off-lying shoals are illustrated in figure 44. One is a simple tripod with centerpole, crossed banners, 

 and flag; the other is a simple four-sided tower with the top of the flag 42 feet above the surface of the 

 water. The legs are jetted into the bottom by means of a double-action hand pump with a 25'^-inch 

 suction and 2-inch discharge, mounted in a launch in such a manner that four men can handle the 

 brakes. A 2-inch fire hose is connected to the discharge end of the pump to which is added a length 

 of l}^-inch iron pipe to act as a reducer. A short piece of %-inch pipe forms the nozzle. 



When a signal is to be erected, the launch is moored in position by four anchors leading fore and 

 aft and from each side. One of the legs of the structure is then held upright and the pipe is placed 

 alongside it, with the nozzle held loosely alongside the bottom of the leg by a thin metal stop nailed 

 to the leg. A 4- by 6-inch leg can be jetted into 6 feet of mud or loose sand in 5 minutes. After the 

 legs are in place and the horizontal braces nailed to them, the men can mount the structure and 

 complete it as if it were on land. If the bottom material is very soft it is best to nail crosspieces on 

 the legs to prevent any of them from sinking deeper than desired. 



Water signals of the above types may be located by any of the conventional 

 methods. Theodolite observations from tliree triangulation stations are preferable, 

 but if impracticable, sextant fixes or cuts should be obtained. 



275. Signal Cloth 



Signal cloth is used to dress signals to make them conspicuous and easy to identify. 

 No color except white or black should be used on a signal which is to be observed from 

 any considerable distance. The choice of color depends almost entirely on the back- 

 ground and the greater the contrast, the more distinctive will be the signal. Unless 

 the signal is viewed against varying backgrounds from different directions, it is best to 

 use only one color for a target — i. e.., all black or all white rather than half black and 

 half white. White is best against a dark or green background and black best against 

 the skyline, water, or a light background. Red and orange may be useful as variations 

 on signals to be used at close range, being especially effective against a backgTound of 

 snow or a glaring white beach. Black is objectionable on signals in the snow as it is 

 difficult to distinguish from rocks or other black spots when viewed even at a short 

 distance. White is valueless against the sk3dine unless the sun's rays are reflected 

 from it. 



White signal cloth is a superior reflector to either whitewash or white paint. A 

 slanting surface is a better reflector than a vertical one, except in the early morning or 

 late afternoon. 



