The lead lines are marked as follows: At 2 fathoms from the lead with 2 strips of leather. At 7 fathoms from the lead with a red rag. At 10 fathoms from the lead with leather having a hole in it. At 13 fathoms from the lead with same as at 3. These are known as the marks. The numbers omitted, as 1, 4, 6, 8, etc., are called the deeps, and they are spoken of together as the marks and deeps of the lead line. Soundings by the hand lead are taken while the vessel has weigh on, the leadsman throwing the lead forward and getting the depth as the vessel passes, while the line is nearly perpendicular. He communicates the soundings obtained thus: 66 If the depth corresponds with either of the above marks, he says, "By the mark 5" or "7." If the depth is greater, or one-half more than any of the marks, he says, "and a quarter," or and a half 5" or "7." If the depth is a quarter less, he says, "quarter less 5" or "7." If he judges by the distance between any two of the marks that the depth of water is 4, 6, 9, 11, 12, 14, 16, 18, or 19 fathoms, he says, "by the deep 4, 6," etc. On the hand lead there are 9 marks and 11 deeps. On going into the chains for the purpose of sounding, the leadsman should see that the breast rope is properly secured, his line clear, and the end made fast. If at night, he should take the distance from the breast rope to the water's edge; then at each cast deduct this distance from the mark at hand and give it as the true sounding. CHAPTER 13 THE COMPASS A simple straight bar magnet suspended by a string and allowed to move freely will, when it settles down, point to the north magnetic pole. This is a simple magnetic compass. Of course this type could not be used aboard ship, but the principle is the same. On board ship we have a circular card which is balanced on a needle point at its center, so that it can revolve freely. Attached to this card are four magnets, two on each side of the center. The needle is mounted on a pivot in a bowl and the bowl filled with alcohol, which will not freeze. This alcohol keeps the card from wobbling or moving too quickly. On top of the card is a little air chamber to help take the weight of the card and its magnets off of the needle point. The face of the card is marked with "points" and degrees. The main or cardinal points are north, or 0°; east, or 90°; south, or 180°; and west, or 270°. Between these main or cardinal points are four intercardinal points. To box the compass is to name the points in regular succession, beginning at one point and ending with the same. Below will be given the boxing of the compass by intercardinal points, beginning with north and going around to the right back to north with the corresponding degrees. The lubbers' point is a vertical line drawn on the inside of the bowl of the compass to correspond to the ship's head. The point on the compass card coinciding with this lubbers' line gives the heading of the ship, or the course being steered. as All courses steered by our ships are given in degrees, course 45°, course 196°, etc. You should learn the four cardinal points with their degrees and also the four intercardinal points with their degrees. The main thing is to know the markings in degrees. The Navy.standard compass card, showing the four cardinal and four intercardinal points and degrees, is illustrated in Figure 63. The earth is a big magnet with a north magnetic pole and a south magnetic pole. Any magnet on this earth, as the magnets in the compass, will have their north seeking pole attracted by the north pole of the earth; and if the magnet can swing around, as our compass magnets can, the north seeking pole of the magnet will point to the north magnetic pole on the earth. Now all magnets work on other magnets near them and attract or repel each other. Besides, all iron is magnetic and acts like magnets. Our poor compass, then, unless it happens to be aboard a ship that hasn't a bit of iron in her, is being pulled and pushed by many other magnets, due to the iron of the ship. The compass, with its bowl, is placed in a binnacle, or a little compass house. In this binnacle are magnets and two M 0 210 220 230 240 250 260 270 280 290 300 310 320 330 S MN سلسل السلسا FIG. 63.-Standard Navy compass card iron balls which are used to neutralize the magnets of the ship so that the compass will point to the earth's magnetic pole. Now the iron on the ship is riveted in 48406-28-12 |