A general description of the patent log is as follows: A rotator r (Figs. 2 and 3), attached to a line, is thrown overboard and towed by the ship, thus causing it to revolve more or less rapidly, according to the rate of speed at which it is drawn through the water; this rotary motion is then transmitted to a register R, which, by means of dials, records the number and fractions of miles run. The register is connected either directly with the rota tor, as in Fig. 2, or it is fastened to the rail or any other suitable place on board, and connected to the rotator by a line, as illustrated in Fig. 3. By the former arrangement it is necessary to pull in the log in order to read off the distance run by the ship; but, in the latter form, this inconvenience is done away with, and the distance covered can be ascertained at any moment. The first of these instruments, represented in Fig. 2, is known as the harpoon log; the second, as the taffrail log, the latter being the one most extensively used. 12. It is needless here to dwell on the patent log, since there are many different kinds on the market, and directions for the use and care of each particular instrument are given by the manufacturers. Attention is called, however, to the fact that when the speed of the ship is small (2 to 3 knots), the distance as registered by a patent log is not always reliable; in some logs it is greater, in others less than the actual distance. Again, at a very high rate of speed the rotator sometimes has a tendency to jump above the surface of the water, causing the register to record a smaller distance than the actual distance covered by the ship. This jumping can be easily overcome by "service," or wrapping the towing line with a strip of sheet lead at a suitable distance from the rotator. 13. Before setting out a patent log to work, the number of miles indicated by the register must be noted and recorded; this should also be done when any change of the ship's course occurs. Under any circumstances, the distance should be noted at least once every hour, either by patent or the common chip log. Like any other mechanical device degree of accuracy is expected, the given an appropriate amount of care. be oiled daily, and when not in use it should be kept clean and dry. from which a certain patent log should be Its mechanism should In steamships the revolutions of the screw afford a valuable means of checking the indications of the log and also of controlling the speed. This subject, however, does not come within the scope of this Paper. 14. The Ground Log.-In shallow water, where the set of the tide or current is much affected by the irregularity of the channel, and in cases where the shore presents no distinct objects whereby the position of a drifting vessel can be ascertained, a ground log may be used with advantage. This consists of a common log line with a small lead attached to it; the lead remaining fixed at the bottom, the line shows the actual speed of the ship over the ground, and by taking a bearing of the direction of the line, the compass course may also be obtained. 15. The Dutchman's log is an old method of determining the speed of a ship. On the ship's rail is marked off a certain distance. When about to observe the rate of speed, an observer is stationed at each end of the marked-off distance. Some floating object is then thrown overboard from the forward station, and, as far as possible, in a forward direction. The time is then noted when the object (an empty bottle or a log of wood) passes the two marks on the rail. By this means a distance and an interval of time is obtained whence the speed is determined by the following formula: where d denotes the distance between the marks, and s the number of seconds of the elapsed time. EXAMPLE. The distance between the marks is 150 feet; the time required for the floating object to pass is 15 seconds. What is the speed of the ship? SOLUTION. The speed is .592X150 =5.92 mi. per hr. Ans. a FIG. 4. THE LEAD 16. 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. 17. There are three kinds of leads, 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. 4, being widest at the lower end a, which is hollowed out for the reception of a lump of tallow (see dotted lines). The purpose of this 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 it found on the chart or in the sailing directions, and the ship's position therefrom approximately determined. 18. 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 sounding in channels, rivers, and harbors, where the depth is inconsiderable. 19. Manner of Marking the Lead Line.—The hand lead line, which has a length of 20 fathoms, is usually marked thus: 2 fathoms . 3 fathoms . 5 fathoms . 7 fathoms . 10 fathoms . 13 fathoms . 15 fathoms . 17 fathoms . 20 fathoms . a piece of white bunting. a piece of red bunting. a strand with two knots in it. In river navigation the first 4 fathoms are sometimes subdivided into feet. The lines used for large hand leads and for coasting leads, which are longer than 20 fathoms, are marked above the 20-fathom mark with an additional knot at every 10-fathom point (30, 40, 50, etc.), and a single knot at each intervening 5-fathom point (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 upon 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. 20. The coasting lead weighs from 25 to 50 pounds, its line being marked up to 100 fathoms. As its name implies, it is used when navigating along the coast. 21. The deep-sea lead is of a larger size, weighing from 80 to 150 pounds, and is attached to a much longer line to find bottom in 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 placed a piece of bunting, and then the knots recommence. 22. Methods of Sounding. The operation of sounding, or measuring the depth of the water, is always performed on the windward side of the ship. Usually the leadsman stands near or between the foreshrouds, or at any other convenient place that will insure a free use of the arms without the danger of falling. In using the hand lead the line should be coiled up near by in such manner as to insure of its running out freely and without a hitch when the lead |