nautical miles. However, as the demand increased for an instrument denoting the distance run in statute miles, the manufacturers, in order to accommodate shipmasters desirous of using this unit in their reckoning, furnished a log of the same pattern but with a rotator having adjustable blades that, when given the proper pitch, will cause the register to indicate statute, instead of nautical, miles. With a patent log showing nautical miles, however, it is a simple matter to reduce the distance run into statute miles, or conversely, when the number of feet in both miles are known. This may be done either by an inspection of Tables IV and V or by the following rule: Rule. To convert nautical miles into statute miles, multiply the former by 1.15. To convert statute miles into nautical miles, divide the former by 1.15. 26.9 nautical miles and 15 statute miles = Thus, 26.9 X 1.15 30.9 statute miles, = 15 54. Running on Time. – The method of "running on time," as practiced on the Great Lakes, consists in noting the time by the clock when some well-known object on land bears in a certain direction and then, knowing the speed of the vessel and the distance to be run, to determine within reasonable limits the time of arrival to the next place. This method, which is frequently practiced in all kinds of weather without any check by patent log or otherwise, may, of course, be relied upon to a certain extent in fine, pleasant weather and smooth sea, but it should never be resorted to by a careful navigator in thick, hazy, or rough weather. At least, it should never be used without being checked by a log. Steamers with tows and others whose speed varies with the prevailing weather conditions should under any and all circumstances have the log for their guide. In Lake Navigation, Part 4, we shall consider the subjects of determining the distance run by the revolutions of the engine and of finding the number of revolutions per minute at which to run the engine in order to make a required speed. SOUNDING THE LEAD 55. The operation of measuring the depth of the water and investigating the character of the bottom. is called sounding. The instruments used for this purpose are the lead and the sounding machine. There are three kinds of leads, viz., the hand lead, the coasting lead, and the deep-sea lead. All of these leads are similar in form to the one shown in Fig. 20, being widest at the lower end a, which is hollowed out for the reception of a lump of tallow (see bottom view). The purpose of the tallow is to bring up a specimen of the bottom that it touches, so that the quality of the ground struck by the lead may be compared with the description of the bottom given on the chart or in sailing directions and the ship's position therefrom approximately determined. 56. The hand lead weighs from 7 to 14 pounds, and is therefore readily thrown by hand. It is used in shallow water, when in the vicinity of land, and for soundings in channels, rivers, and harbors where the depth is inconsiderable. Manner of Marking the Lead Line. The hand lead line, which usually has a length of 20 fathoms, is marked thus: 57. a FIG. 20 2 strips of leather. 2 fathoms In river and lake navigation the first four fathoms are usually subdivided into feet. The lines used for large hand leads and coasting leads, which are longer than 20 fathoms, are marked above the 20-fathoms mark with an additional knot at every 10-fathom point (at 30, 40, 50, etc.) and with a single knot at each intervening 5-fathom point (at 25, 35, 45, etc.). It should be noticed that by this marking of the lead line the intervening fathoms of 4, 6, 8, 9, 11, etc. are without any markings, and consequently the leadsman has to depend on his own judgment concerning the depth between the marked fathoms. In order to obviate the uncertainty of guesswork, however, a lead line should be marked at every fathom and half fathom up to 12 or 15 fathoms, and the first 4 or 5 fathoms should always be subdivided into feet by means of suitable markings. 58. The deep-sea lead is of much larger size, weighing from 80 to 150 pounds, and is attached to a much longer line, in order to take soundings in depths of 100 or more fathoms. The line used for this lead is marked in the same manner as the coasting-lead line; at 100 fathoms is a piece of bunting and then the knots recommence. 59. Methods of Sounding. – The operation of sounding, or measuring the depth of water, is always performed on the windward side of the vessel. Usually the leadsman selects a place that will insure a free use of his arms without the danger of falling, and which, at the same time, is within hearing distance of the officer in charge of the deck. Before sounding, the lead line should be coiled up near by in such manner as to insure its running out freely and without a hitch when the lead is thrown. Holding the line a few feet from the lead, the operator swings it back and forth, or around his head, so as to impart to the lead a certain velocity, and then throws it as far forward as is deemed necessary, according to the speed of the vessel. The point a', Fig. 21, 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 ab. To make sure that the lead has reached the bottom, the leadsman pulls his line up and down in rapid succession, a foot or two each way, and then reads off the depth by the mark on the line near the water-line W W'. For sounding during the night when marks cannot be seen, the distance W... FIG. 21 from the breast band to the water-line should be known, and at each cast this distance should be deducted from the amount of line out. 60. It should be remembered that the up-and-down sounding is the only true one, and that errors in soundings are generally in excess. When the line is run in, the tallow in cavity at the bottom of the lead, termed arming, is examined in order to determine the nature of the bottom. 61. The Sounding Machine. - An excellent substitute for the lead is the sounding machine. At present, there are many sounding machines in general use. Among them, Massey's, Walkers's, and Lord Kelvin's are the best known. The last named is especially favored by navigators on account of its many excellent merits. This instrument, represented in Fig. 22, is described by Captain Lecky in his "Wrinkles of Navigation" as follows: "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 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 one. 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, 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 FIG. 22 |
