is thrown. Holding the line at about a fathom's length from the lead the operator swings it back and forth or around his head, as indicated in Fig. 5, so as to impart to the lead a certain velocity and then throws it as far forwards as deemed necessary, according to the speed of the vessel. The point a' where the lead touches the bottom will then, by the ship's forward motion, be directly underneath the leadsman and the sounding an up-and-down one, as indicated by the line a b. To make sure that the lead has reached the bottom, the leadsman usually pulls his line up and down in rapid succession, a foot or two each way, and then reads off the depth by the marks on the line near the water-line W W'. For sounding during the night when marks cannot be seen, the distance from the breast band to the water-line should be known, and at each cast this distance be deducted from the amount of line out.

23. To take soundings with the deep-sea lead, the ship's speed is reduced, if necessary. Three or four men are usually required for this operation, in case the depth is considerable. Generally it is carried out as follows: One man is stationed at a, Fig. 6, with the lead and several fathoms of line coiled up; at d is the operator (usually an officer), and

66

FIG. 6

between a and d is placed as many men as may be required, each man holding several fathoms of the lead line in his hand ready to let go. Everything ready, the operator at d gives the signal to heave," and the man at a throws the lead, whereupon the line is paid out freely by the assistants so that the lead is surely at the bottom before the operator is directly above it.

It should be remembered that the up-and-down sounding is the only true one, and that errors in sounding are generally in excess. When the line is run in, the cavity at the end

of the lead, termed arming, is examined in order to determine the nature of the bottom.

Kelvin's.

24. The Sounding Machine.-At the present time there are many sounding machines in general use. The best known among them are Massey's, Walker's, and Lord The last named is especially favored by navigators on account of its many excellent merits. This sounding machine, represented in Fig. 7, is described by Captain Lecky, in his "Wrinkles of Navigation," as follows:

FIG. 7

"Lord Kelvin's sounding machine consists of a drum D about 1 foot in diameter and 4 inches wide, upon which about 300 fathoms of steel pianoforte wire are tightly wound. To the wire is attached 9 feet of log line and to this is fastened a heavy

iron sinker about twice the length of the ordinary lead, but not so thick. On the log line, between the wire and the sinker, a small copper tube is securely fastened, the lower end of which is perforated; the upper end being opened and shut at pleasure by means of a close-fitting cap. When ready for sounding, the copper tube contains a smaller sized glass tube. This latter, also, is open at the bottom end and hermetically sealed at the other. The interior surface is coated with a chemical preparation of a light salmon color (chromate of silver). The drum is fitted with a brake

cord, which, on a cast being taken, controls its speed and ultimately arrests it when the lead touches the bottom.

"A pair of small winch handles h and h' wind up the wire again, and the depth is indicated by the height of the discoloration on the inside of the glass tube. The water is forced up the tube in its descent, in obedience to certain well-known laws, and the chemical action of the salt turns the salmon color to a milky white (chloride of silver). This junction of the two colors, when applied to a graduated boxwood scale, tells the depth to which the lead descended."

25. The Depth Recorder.-Instead of the arrangement with tubes and chemicals, a depth recorder may be used. This instrument is shown in Fig. 8. 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 pressure of the water, which increases with the depth. To record the pressure and the depth corresponding thereto, the "marker" is used. As the recorder goes down, the "marker" is pushed along the piston; when the recorder is brought up to the surface of the water the piston comes back to its original position, but the "marker" remains at the place on the piston to which it was pushed, and thus shows the depth reached by the instrument.

FIG. 8

26. Of late, several improvements have added to the efficiency of this sounding machine, among them being a register on the side of the instrument indicating 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 which, it is claimed, in some respects is superior to Lord Kelvin's sounding machine. A pamphlet accompanying each machine gives

full information and directions concerning its use; after seeing a sounding machine once or twice in operation, 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 advantage 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 when using the common deep-sea lead.

27. 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 off the 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, log, and 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.

THE CHARTS

THEIR CONSTRUCTION AND USE

28. 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 a ship 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 the bottom, rocks, shoals, banks, variation of the compass, etc. Charts may be divided into two classes, viz., general charts and plans.

29. 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 water. Meridian and latitude parallels, variation, currents, principal sailing routes, and other useful information are found on them. On general charts of certain parts of the coast are marked the position of lighthouses and light ships; the respective range of visibility of each is on most foreign charts indicated by circles. On American charts this useful arrangement is, as a general rule, absent, but should, when not indicated, be drawn in with red ink.

30. 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, 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 bottom, the leading marks, the courses through channels, the dangers to be avoided, variation of the compass, the tidal establishment of the port, and other information that will tend to facilitate the navigation of the locality. Plans of these kinds are often inserted, for convenience, in a corner of the general chart.

In order to understand and intelligently use charts, it is necessary to know something about the construction and peculiarities of those in most common use, and we will therefore present a few facts relative to this subject.

31. Principles of Construction.-Since the form of the earth is globular it is evident that any representation of its surface on a flat sheet of paper such as a chart must necessarily be an artificial construction, or, in other words, a projection of the globe upon the paper.

32. In order to insure a true representation of the original, there are certain conditions that must be fulfilled, viz., conformity, equivalence, and equidistance.

Conformity requires that each figure of the image shall possess a geometrical similarity to the original; that is all corresponding angles must be equal.