Two kites are supplied; the black kite for depths not exceeding 30 fathoms; the red kite not exceeding 40 fathoms depth. To use the machine slow the vessel to a speed not exceeding 10 knots; lower the sentry to the depth you expect to run into, then go ahead at any speed not exceeding 13 knots, with the black kite; or 10 knots with the red kite. When the trigger A B strikes the bottom, the catch C is released, throwing the whole strain on D, thus upsetting the kite and causing it to rise to the surface. At the same time that the tension on the wire is reduced, one end of a crank of the machine on deck is freed, allowing the other end to fall back, and strike a gong; the signal that bottom has been struck. The depth can be verified by getting a cast of the lead. This machine should be frequently overhauled to prevent rusting and should be carefully examined before being used. THE LOG. Various methods have been proposed for measuring the rate at which a ship sails; but that most in use is by the Log and Glass. The Log is a flat piece of thin board, of a sectoral or quandrantal form, Figs. 6a and b, Plate 6, loaded, on the circular side, with lead sufficient to make it swim upright in the water. To this is fastened a line, about 150 fathoms long, called the log-line, which is divided into certain spaces called knots, and is wound on a reel, Fig. 7, which turns very easily. The Glass is of the same form as an Hour-Glass, Fig. 8, and contains such a quantity of sand as will run through the hole in its neck in twenty-eight seconds. Marking the Log-Line. Previous to marking a new Log-line, it is soaked in water for a few days, in order to get it in the condition it will be when in use. From 15 to 20 fathoms is allowed for "stray-line," to carry the chip out of the eddy of the ship's wake. The length of a knot is determined (for the 28-second glass) by the following proportion, viz. As the number of seconds in an hour is to the number of feet in a sea mile (one-sixtieth of a degree of a great circle of the earth.) so is the length of the glass to the length of a knot, or, : 3,600 s : 6,080 ft.—28 s : 47.29 ft. : 47 feet 3 inches; therefore the length of the knot is 47 feet 3 inches for the 28-second glass.* A statute mile is 5,280 feet. To convert sea miles into statute miles, multiply the former by 1.153. To convert statute miles into sea miles, multiply by the decimal .868. The velocity of the ship is estimated in knots and tenths of a knot. The limit of stray-line is marked by a piece of red bunting about six inches long, and each length of 47 feet 3 inches after that, by a piece of fish-line with one, two, three, etc., knots in it, according to its number from the stray-line. Each length of 47 feet 3 inches (the knot) is subdivided into five equal parts, and a small piece of white bunting about two inches long is turned into the line at every twotenths division thus formed. Always, before leaving port, the Navigator has the line thoroughly soaked for a few days, and then all the marks placed at their proper distances. He also compares all the sand-glasses with a watch, and if any should be incorrect, he makes them run the proper time by taking out or putting in sand, as the case requires. During daylight, especially in very damp weather, it is preferable to use a watch rather than a sand-glass for noting the time. Errors of the glass due to moisture are commonly corrected by drying it. Heaving the Log.-To find the ship's speed is called heaving the log, and is thus performed: One man holds the reel, and another the glass; an officer of the watch throws the log over the ship's stern, on the lee side; or, on the side opposite to the patent log if it be out. When he observes the stray line is run off, and the red rag is gone, he cries, Turn; the glass-holder answers, Turn. Watching the glass, the moment it is run out, he says, Up! The reel being immediately stopped, the last mark run off shows the number of knots, and the distance of that mark from the rail is estimated in tenths. Then the knots and tenths together show the distance the ship has run the preceding hour, if the wind or motive power has been constant. But if the wind has not been the same during the whole hour, or interval of time between heaving the log, or if there has been more sail set or handed, a proper allowance must be made. Sometimes, when the ship is before the wind, and a great sea setting after her, it will bring home" the log. In such cases, it is customary to allow one mile in ten, and less in proportion if the sea be not so great. Allowance ought also to be made, if there be a head sea. In heaving the log, you must be careful to veer out the line as fast as the chip will take it; for if it be left to turn the reel itself, it will come home and deceive you in your reckoning. You must also be careful to measure the logline pretty often, lest it stretch and deceive you in the distance. Like regard must be had that the glass be just 28 seconds; otherwise no accurate account of the ship's way can be kept. The glass is much influenced by the weather, running slower in damp weather than in dry. The glass may be examined by a watch, as above stated, or by the following method :-Fasten a plummet on a line, and hang |