moving the index bar forwards, the instrument being held face up. Again, if conditions are reversed, x, Fig. 3 (b), being a fixed light and y a flash light, y for similar reasons is used as a base and x is brought in contact with it, the instrument being held horizontally, face down. Should both lights be flash, the one having the longest or most frequent interval. of display should be brought in contact with the other. 10. Cautionary Remarks. It must be remembered that before using the vertical or the horizontal danger angle in actual practice of navigating a ship past a danger, the student should be efficient in its use. Do not try to experiment with it in places where actual danger exists. Practice it at first with imaginary danger until sure of being able to use it with confidence. 11. To Find the Distance From the Shore by a Celestial Body Visible Above It.-In both of the foregoing methods, as well as in all methods relating to the determination of a ship's position when in sight of land, it is necessary that one or two known objects on shore should be in sight before the distance and position of the ship can be determined. But suppose it is desired, when sailing along a coast, or at anchor near a coast on which no known objects happen to be in sight, to determine the distance of the ship from land, the shore of which lies between the ship and the sea horizon. Under such circumstances the distance may be found very nearly by measuring, at the same instant, the altitude of a celestial body visible above the land, from two points of different height on the vessel. For example, let 7, Fig. 4, be the shore line of the intervening land and o and o' the position of two observers, one stationed on deck, the other at the masthead, the height of both positions being known. Let o s' and o' s represent the direction of the sun or a star visible above the land, these lines of course being parallel owing to the great distance of the celestial body. Then, by measuring the angular distance between s and 7, the observer at the masthead will get Or, the angle at a,) and the observer on deck will get the Now, the angle subtended by o and o' is equal to the difference between the observed angles a, and a,. Let the height of o' be denoted by H, and the height of o be denoted by h; then, the distance ol of the ship from the shore is found by the formula where H and h are expressed in feet and a, and a, in minutes, whence the resulting distance will be expressed in nautical miles. It is evident that the observer at o should be as near to the surface of the water as possible, so that the angle loo' will be practically a right angle. EXAMPLE. From a position on deck 13.1 feet above the water-line, the measured angular distance between the shore line and the sun's center is 29° 11.5'. At the same instant another observation taken at a height of 65.6 feet above the water-line gives the angular distance between the same points as 29° 21.5'. Find the distance of the ship from the shore. Inserting these values in the given formula, we get 12. The hydrometer is an instrument for measuring density or specific gravity. 13. Specific gravity of a solid is its weight compared with the weight of an equal volume of distilled water at the temperature of 39.2° F. Water is taken as the standard for solids and liquids, and air for gases. A cubic inch of sulphur weighs twice as much as a cubic inch of water; hence, its specific gravity is equal to 2. 14. The hydrometer used on board ship for obtaining the density of sea-water and the water in docks and rivers is shown in Fig. 5. It consists of a glass tube, near the bottom of which are two bulbs. The lower and smaller bulb is loaded with mercury or shot, so as to cause the instrument to remain in a vertical position when placed in water. The upper bulb is filled with air, and its volume is such that the whole instrument is lighter than an equal volume of water. The scale on the tube reads from 0 downwards. FIG. 5 15. When placed in distilled water, the instrument will sink to the division marked 0, but in seawater it will sink to about the division marked 26. In brackish water it will sink to some point between these marks, according to the amount of salt the water contains. When finding the specific gravity of water by this instrument, the figure read off on the scale should be added to 1.000. The specific gravity, therefore, of ordinary seawater is 1.026. In docks and rivers where fresh water enters, the specific gravity will vary between the limits of 1.000 and 1.026. Now, since a body floats higher in salt water than in fresh water, it is evident that the saltness of the water in which a ship floats will have an important bearing on the draft of the ship. For instance, a ship may be loaded deeper in fresh or brackish water because she will "rise" a certain amount when she goes to sea. In England and a few other countries the law determines the load line for seagoing vessels. 16. How to Find the Sea Draft When Draft of Ship in Harbor Is Known, and Vice Versa.-If the specific gravity of the water at the loading place is obtained. by a hydrometer and the draft (or depth of ship) when loaded is known, a simple proportion will give the sea draft very nearly; also, when the sea draft is known the draft in. a dock or river can be found by the same proportion, which is as follows: where W: w = d: D, W specific gravity of sea-water (= 1.026); w= specific gravity of water in harbor. EXAMPLE 1.—The specific gravity of the water in a harbor is 1.015 and the ship's draft is 23.5 feet. Find her draft in sea-water. SOLUTION. In this case D is the quantity sought; hence, w Xd 1.015 × 23.5 D= W = 1.026 = 23.2 ft., nearly. Ans. EXAMPLE 2.-At sea a ship draws 26.75 feet of water. What will be her draft in the dock at her destination, where the specific gravity of the water is 1.01? SOLUTION.-In this case d is the quantity sought; hence, THE SHIP'S LOG BOOK 17. The official log book of a ship should contain a carefully prepared record of the day's work, or the details affecting the navigation of the ship. In it should be entered the courses and distances run, with the amounts of leeway, variation, and deviation applicable to each, together with other data that have an important bearing on the safe navigation of the vessel. The position of the ship as determined by dead reckoning and astronomical observations are entered. in separate columns. The log book may be made very simple or very elaborate; usually each nation has its prescribed form of log book. At the end of each watch the officer in charge of the deck inserts in the rough log, or scrap log, book the compass courses, the distance run, and other noteworthy data, all of which are subsequently transferred to the official log book. A simple and quite satisfactory form of log book for merchant ships is shown on the following page. 18. The entries to be made in the different columns are perfectly apparent. The variation allowed is that taken from the chart. The column headed "Deviation" should be filled in from the table of deviation, previous to correcting the courses. The column headed "True Bearing and Distance" is filled in by computing (usually by Mercator's sailing) the course and distance between the position at noon, found by astronomical observations, and the place of destination or some point or danger lying near the intended track of the ship. INTERNATIONAL CODE OF SIGNALS 19. Object of the Code.-The object of the international code of signals is to supply a means of intercourse between ships meeting at sea as well as between ships and established signal stations on land. It has been adopted by all the important maritime powers of the world, and the interpretation of the several thousand different signals composing the system are translated into the language of each separate |