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to a milky white (chloride of silver). The junction of these two colors, when applied to a graduated boxwood scale, tells the depth to which the lead descended.”
62. The Depth Recorder. – Instead of the arrangement with tube and chemicals, the depth recorder may be used. This instrument is shown in Fig. 23. As the sinker descends, the increased pressure of the water forces a piston up into the tube, while the spiral spring attached to the piston pulls it back. The amount that the piston is forced up against the action of the spring depends on the depth. To record the depth, a "marker" is used. As the recorder goes down, the marker c is pushed along the piston; when the recorder is brought up to the surface of the water, the piston comes back to the original position, but the marker remains at the place on the piston to which it was pushed and shows the depth reached by the instrument.
63. Of late, several improvements have added to the efficiency of this sounding machine, among them being a register on the side of the instrument, which indicates the number of fathoms of wire paid out.
Other sounding machines, somewhat varied in construction and type, have recently been put on the market; among them may be mentioned Dobbie's depth gauge, an instrument that, it is claimed, in some respects is superior to Lord Kelvin's sounding machine. A pamphlet accompanying each machine Fig. 23 gives full information and directions concerning its use. After seeing a sounding machine in operation once or twice, the mode of working it is so self-evident as to render the instructions unnecessary.
The advantages of the sounding machine over the lead are many, the main being that the speed of the ship need not be reduced, and the time required to sound great depths is minimized to a fraction of that required when the common lead is used.
64. Importance of Using the Lead. -A navigator should bear in mind that he fails to do his duty when he neglects to take soundings when approaching or navigating near land. While near a coast, the lead should be kept going frequently, no matter how fine and clear the weather may happen to be, or how confident he may feel as to the exact position of his vessel.
Too much attention cannot be given to the lead, the log, and the lookout. Frequent and careful soundings, correct logging, a good lookout, and a correct compass course should be maintained by all good seamen and careful navigators.
THEIR CONSTRUCTION AND USE 65. Charts
Charts are maps of the sea, representing the whole or part of the surface of the water and adjoining coasts, and are used for the purpose of navigating from one place to another. They contain various particulars that may serve to aid the mariner in successfully conducting a voyage and point out to him the dangers to be avoided; contours of the sea coast are shown on them, also the depth of water, nature of bottom, rocks, shoals, banks, variations of the compass, etc. Charts may be divided into two classes, viz., general charts and plans.
66. General charts are those that embrace a comparatively large part of an ocean, or the entire ocean, or a considerable extent of coast line with its contiguous waters. Thus, the general chart of the Great Lakes embraces the whole of the Lake region from Duluth to the easternmost part of Lake Ontario. This chart is officially known as the “Pilot Chart of the Great Lakes."
Four different general charts of the Great Lakes are issued by the Hydrographic Office, Navy Department, Washington, D.C. Of these chart, No. 1,415 sets forth information concerning Canadian and United States storm and weather signals, canals, docks, and the use of oil for calming stormy water; No. 1,675 gives information relating to the occurrence and duration of fog; No. 1,684 deals with the average times of the closing of navigation at places on the Great Lakes; and No. 1,695 deals with the average times of the opening of navigation for the same places.
On these charts are marked the positions of lighthouses and lightships; lines of magnetic variation; compass diagrams, showing true and magnetic north and other useful information.
67. ·A special chart, or a plan, is a chart that comprises a detached portion of a general chart on a large scale, such as a harbor, a small bay, the entrance to or part of a river, channels leading to a port, or a small part of the sea where navigation is difficult and dangerous. They generally contain the lights and buoys, the soundings and character of bottom, the leading marks, the courses through channels, dangers to be avoided, variation of the compass, and other information that will tend to facilitate the navigation of that locality. Such plans are often inserted for convenience in a corner of the general chart. For places along the shores of the Great Lakes over 150 special charts, or plans, have been prepared and published.
68. The charts now in general use on the Great Lakes are constructed by two different methods, or, to be more exact, on two different projections, viz., the polyconic and the mercatorial projection. In order to intelligently understand and use these charts, it is therefore necessary to know something about the peculiarity of each, due to the peculiarity of the different system, or projection, on which they are constructed.
69. The Polyconic Chart. – When the extent of surface to be charted is limited, such as the plan of a harbor, island, small section of the coast, etc., the polyconic projection is usually employed. Without touching on the mathematical principles underlying this projection, the characteristic features of the polyconic chart, briefly told, are as follows: The space to be laid down on the chart is considered as being divided by the latitude parallels into
narrow zones projected, according to certain rules, on a cone tangent to the zones' lower parallel (in north latitude the southernmost, in south latitude the northernmost). These zones a b b'a', c d d'c', etc., Fig. 24, are then placed so that the middle meridian m m', m' m'', etc. of each constitutes a common meridian M M' for the whole chart; the zones there
fore touch or are tangent to one another only at points on or near this line, and leave an
en space between their ends,
as shown in figure. d
Now, in order to fill these open spaces and to make the chart complete, the ends a b, cd, etc. of each zone are stretched, so to speak, so that the
lower edge b m' l' of Fig. 24
one zone will coincide with the upper edge c m' d' of the adjoining zone along the center line r m' r'. As a consequence of this stretching, it is evident that the parts of the chart near the vertical edges are somewhat distorted. The latitude parallels are not parallel with one another and the meridians ss', ss', etc. consist of curved lines converging toward the poles, with the exception of the middle meridian MM', which is a straight line.
This projection is therefore advantageous for the representation of a coast line that runs north and south, or in the direction of the meridian, and is for this reason extensively used by the United States Coast and Geodetic Survey in the preparation of working charts of the coast, and also by the United States Engineer's Office, War Department, in the preparation of charts used by mariners on the Great Lakes.
70. The course, or line of bearing, between any two places on such chart will necessarily be represented by a curved line if properly laid out. However, if the scale of the chart is large, the bearing may, without any practical error, be represented by a straight line, especially when the two places are situated in the central part of the chart.
71. If the places are widely separated in longitude, it should be borne in mind that, when using a straight edge for finding the course between them, the ship following this line will, at the end, be nearer the elevated pole by a fraction that amounts to about 14 miles for every hundred miles on charts including the region between latitudes 45° and 50° N. This error is greatly augmented on charts constructed on 'this projection where the scale is small.
72. Charts on the Mercatorial, or Mercator's, Projection. – The system on which this chart is constructed was invented in 1512 by Gerard Mercator, of Rupelmunde, Flanders. His system was not strictly a projection in the true sense of that word, but may be said to have resulted from operations illustrated in Fig. 25. By stripping off the globe - (a) representing the northern hemisphere - the gores formed by the meridians, and placing them in regular order beside one another on a flat surface, a chart is formed similar to the one shown in (6). Owing to the openings, or vacant spaces, between the meridians, this chart is very defective; and, in order to remedy this defect, the upper parts of the gores are stretched so as to form the chart represented in (c). A glance at this chart, however, wül reveal the fact that everything on it, except the equatorial parts, is distorted, and that this distortion increases in the higher latitudes. Now, in order to restore a balance of orientation, or the relative position and direction of spaces that are distributed horizontally (or in longitude), it is necessary to distort the chart in an equal proportion vertically (or in lat e). When this has been done, result as shown in (d), which represents a complete map, or chart, of half the northern hemisphere according to Mercator's system.