I shall add but one more thing before I end this brief and incomplete explanation of navigation and its related subjects. Navigation and dead reckoning we have touched upon. Piloting still remains untouched.
This branch of navigation, if branch it really is, shows the navigator the position of his ship by reference to objects ashore. Let us suppose that a ship has crossed the ocean and is approaching a harbour entrance. While at sea an error of half a mile or so meant little, but as he approaches shore he wants to know exactly where he is.
On each side of the harbour entrance let us suppose that there is a lighthouse. The navigator gets out his large-scale chart of the vicinity and lays it on his chart table. This chart shows the harbour entrance and shows the positions of the lighthouses. Then he determines the direction of these two lighthouses according to his compass. Let us suppose one lies exactly northwest and the other exactly southwest. On the chart, then, he draws two lines, one through the point marking each of the lighthouses. From the lighthouse to the northwest he draws a line extending southeast (the opposite direction) out to sea. From the lighthouse to the southwest he draws a line to the northeast. These two lines cross, and he knows that his ship was exactly at the intersection when he took his bearings. As this can be done in a minute or two the position is very accurate, unless his ship is sailing very rapidly, which it probably would not be. This is known as the “cross bearing” method of learning one’s position, and is one of the simplest problems in piloting.
Suppose, however, that a ship is sailing along the shore, and but one prominent object can be seen on the land. The navigator watches until the object (a lighthouse, perhaps) is “four points off his bow”—that is, until the angle between his course and the direction of the object is 45 degrees. From that moment the log is watched carefully, until the object is directly at right angles to the ship’s course. The distance sailed during that time is the same as the distance from the ship to the object ashore at the time the second bearing is secured, and if a compass bearing is taken when the ninety-degree bearing has been taken, a line drawn on the chart from the position of the object ashore can be marked with the distance in miles, and the navigator will know exactly the position of his ship at that moment. This is known as “bow and beam bearings.” There are other similar methods of obtaining the desired result.
In foggy weather when ships are “on soundings”—that is, where the water is shallow enough to permit of the easy use of a line with a weight attached for measuring its depth—careful navigators invariably use the “lead line” constantly.
SOUNDING BY MACHINE
A glass tube with the upper end closed and the lower end open is lowered in a special case to the sea bottom, and then brought to the surface. As the tube descends, the water compresses the air in the tube, and gradually creeps up inside. The inside of the tube being of ground glass the water leaves a mark showing how far it has entered the tube. By laying the tube on a special scale the depth to which the glass was carried can be gauged. There are other methods not greatly dissimilar from this.
This tells them not only how deep the water is, but by putting tallow or soap on the bottom of the lead weight they bring up sand or mud or shells from the bottom. With this and the depth, a line is drawn on tracing paper on the same scale as the chart. Along this line these soundings and the kind of mud or sand the lead brings up are marked, at intervals corresponding to the distance the ship has sailed between soundings. The chart is printed with the depth of the water in fathoms and with the kind of bottom that will be found. After the navigator has compiled his data for a few miles the tracing paper with the line on it can be moved about over the chart, and if care has been taken in sounding and watching the speed and direction of the ship, the navigator will find the place on the chart where his series of soundings will match the printed soundings. Then he will know very accurately where he is, even if it be a fog-enshrouded night.
Many, many important aspects of these three vital subjects have been completely passed over in this short chapter. If, however, I have been able to explain a little of the subjects, and if, particularly, I have quickened the interest of any of my readers in them, my purpose has been served. Going to sea is not so difficult as many people ashore are prone to think. But becoming a thorough seaman and a thorough navigator is not so simple, perhaps, as to become adept at much of the work that occupies men ashore.