Soundings. Even if objects cannot be seen, due to distance or thick weather, the chart furnishes a valuable aid when a vessel has approached within the limits where it is practicable to obtain soundings. Modern navigational sounding machines permit of obtaining soundings to depths of nearly one hundred fathoms without stopping the vessel. A rough check is at once obtained by comparing such soundings with those given on the chart for the position carried forward by dead reckoning. If a number of soundings are taken and plotted on a piece of tracing paper, spaced by the log readings to the scale of the chart, and this tracing paper is laid over the chart and shifted in the vicinity of the probable position until the soundings best agree with those on the chart, a valuable verification of position may be obtained. This is particularly the case if the area has been well surveyed, and the soundings taken on the vessel are accurate, and the configuration of the bottom has marked characteristics. For instance, in approaching New York the crossing of the 30, 20, and 10 fathom curves will give a fair warning of the distance off the Long Island and New Jersey coasts, and soundings across such a feature as the submerged Hudson gorge extending to the southeastward of Sandy Hook will give a valuable indication of position. The taking of soundings should be resorted to even in favorable conditions, in approaching shoal water, as a check on other means of locating the vessel. Many marine disasters are attributed to failure to make sufficient use of the lead, the simplest of navigational aids.
Vertical angles. The vertical angle of elevation of an object whose height is known will give the distance, and combined with a bearing or other information this permits of locating a vessel where better means cannot be used. Distance tables are published for this method. ([Fig. 46.]) The vertical angle is measured with a sextant and must be the angle at the ship between the top of the object and the sea level vertically beneath it; for a hill or mountain, therefore, the eye of the observer should be near the water. The object should not be so distant that curvature becomes appreciable. The "vertical danger angle" is a means of avoiding a known danger, on a principle similar to that of the horizontal danger angle; that is, the angle of elevation of a known object is not permitted to become greater than a fixed amount depending on the distance from the object to the danger to be avoided.
Positions by sound. In thick weather sound affords a valuable aid to the navigator. In narrow passages noting the echo of the whistle from a cliff is a method resorted to, as for instance in Puget Sound and along the Alaska coast. Fog whistles and bell buoys are maintained at many places. Submarine bells have recently been introduced at a number of points along the Atlantic coast, and vessels may be equipped to receive these submarine signals transmitted through the water, which indicate also the general direction from which the sound comes.
Need of vigilance. Too great importance cannot be attached to frequent verification of positions by the best available means, particularly when approaching the land. Neglect of this or overconfidence has caused many disasters. A notable instance was the loss of one of the largest Pacific steamers on the coast of Japan in March, 1907. In the afternoon of a clear day this vessel ran on to a well-known reef about a mile from a lighthouse, resulting in the total loss of vessel and cargo valued at three and a half million dollars. The captain was so confident of his position and that he was giving the reef a sufficient berth that he laid down no bearings on the chart and took no soundings.
FIG. 47. FIELD'S PROTRACTOR AND PARALLEL RULER IN USE ON A CHART, PLOTTING BEARING OF A LIGHTHOUSE.
Instruments. The principal instruments needed for use with charts are; dividers for taking off distances and latitudes and longitudes, parallel ruler for transferring directions to or from a compass rose and for taking off or plotting the latitude on a mercator chart, protractor of 180 degrees for reading the angle with the meridian of any direction or for laying off on the chart any given angle with the meridian, and three-arm or other full-circle protractor for plotting a position by the three-point problem.
Parallel rulers on the principle of Field's are strongly recommended for chart work, as they combine in a single instrument the advantages of a parallel ruler and a 180-degree protractor. Any direction can be read or laid off by simply moving the parallel ruler to the nearest projection line, which is a process not only more convenient than referring to the compass rose printed on the chart but also more accurate because of the longer radius. These instruments can also be used the same as a plain parallel ruler. Field's parallel rulers are made in two forms, one rolling and the other sliding. The former is a single ruler with edge graduated 90 degrees either way, and mounted on rollers; it is the most rapid instrument for reading or laying off a direction, but it requires a smooth surface. The latter is an ordinary two-bar parallel ruler with edge when closed graduated 90 degrees either way; it is a very serviceable instrument and probably more to be depended upon for ordinary use than the rolling form. Some form of combined protractor and parallel ruler should be in every navigational equipment, and it is unfortunate that these instruments are not better known in this country. There are other forms of half-circle protractors which are used on the same principle, that is, of bringing the center on to a projection line and reading where the line cuts the border graduation of the protractor. Thus a semicircular protractor is used with a separate straight edge, along which it is slid to the nearest meridian; another form is the simple circular protractor with a thread fastened at the center. All these forms of protractors, it will be noted, are intended to work from the true meridian, and they are usually graduated in degrees only; the use of degrees instead of points is becoming much more general in navigational work, and reference to the true meridian is also more common than formerly.