staves one inch square, two feet long, pointed at one end. Unship the staff at station A and drive in a peg; slip the ring at one end of the line over it, take the other end towards the flagstaff and station B, haul it well taut, drive in a peg, and ship the other ring over it. The line should be sighted to see that it is in the right direction, then measured to see that it is right in length. The first end is then slid off the first peg and carried forward and the station staff replaced. When the line is again taut another peg is driven in and the ring shipped on to it. This process is repeated until station B is reached. The number of pegs represent the number of hundreds of feet or any other agreed unit of measurement. The line should occasionally be remeasured with the tape. Measuring on the water between two buoys, boats, or rocks the wire should be attached to floats to keep it from sinking, and as long a stretch as possible made. A boat or buoy moored takes the end. Such a measurement should be made when there is no tide or currents. Such measurements are only possible for short bases. When the base is a mile or more in length and cannot be measured as above the distance can be measured by sound. Sound Measurements. A small gun is mounted at each end of the base line. When all is ready the gun is fired and the interval between the flash and the report is accurately noted by hack watch. A gun is then fired from the other end and the operation repeated several times. The mean of all the times is taken as the true time. The distance is then found from the formula. Distance = Time x velocity of sound at the given temperature. Sound travels at the rate of 1,091 feet per second at the temperature of 32° F. and increases 10 feet per second for an increase of 9° in temperature. Example. The mean time of observations was 11-5 seconds, temperature 60° F. What is the distance? As an error of I second causes an error in the measurement of 100 feet this method is not suitable for short base lines. Masthead Angles. When from any reason a base cannot be fixed on shore, then a base must be measured on the water. The vessel forms one station and the other station may be fixed on shore preferably, or by a boat moored at a suitable distance and direction. The distance is obtained by measuring the angle made by the masthead and the waterline. The height of the masthead being known and the angle measured, the distance is found by the fórmula D=M cot. A. Where D=distance, M=height, and A=the angle subtended. The angle subtended from the shore station was 5°. What is the distance from the ship? The two positions being determined and marked by boats, buoys, or in any other suitable way, take up a position some distance from the first buoy ; when well under weigh stream the log. Note the log when the buoy is a beam and note again when the second buoy is abeam, also note the time elapsed. If possible this should be repeated, steering the opposite course, and the mean taken as the distance. It should be done at slack water where there is a tide. In all cases the true bearing of the base line should be found by the sun's bearing in transit across the two stations, or if a light is fixed at each end of the base, bearings can be taken frequently by stars, or the bearing may be found by azimuth and sextant angle combined. Compasses cannot be depended upon when they are landed on strange ground; there may be magnetic ore in the neighbourhood. Tidal Observations, Soundings, etc. The most important information on the chart, to the navigator, are the soundings. There are therefore two prime objects to be kept in view— the depth of water and the nature of the bottom. The latter information is indispensable to the seaman in foggy weather. He should also gather all the information he can as to the set and drift of the tide at flood and ebb and at spring and neap tides, and carefully note any local peculiarities. While operations on shore are going forward a “tide gauge" should be set up, fixed in a well-sheltered place easy of access and in such a manner that the zero of the gauge is below the lowest low-water mark. The gauge should be firmly fixed, able to withstand any weather. Observations should be continued through a lunar month, if possible. The mean of all the high and low-water readings will give the half-mean spring range or what would be the mean level of the sea if there were no tidal rise and fall. The halfmean spring range is the true scientific level of reference in all matters relative to the tides. This data should be marked in a permanent manner by being cut in the rock or on a spot or in some other suitable way that would ensure permanency. Soundings. Determine upon what plan and in what direction it is intended to run out lines of soundings. Note any objects in transit, especially if well distant from one another. If there are no natural objects in line use flag staffs, keeping them one behind the other as far apart as possible, shifting them to suit each new line. Fix the boat's position at starting by sextant angles and station pointer, and fix anywhere along the line of soundings when suitable objects present themselves. Make a good fix at the end of the line. The boat should be pulled on these lines at a uniform speed and casts of the lead made at regular intervals; the time, the depth, and the nature of the bottom should be carefully noted. In a harbour where the tidal range is not known an observer should be stationed at the tide gauge to note the height every half-hour, but at more frequent intervals at about the time of high and low water. The watches used at the tide gauge and by the boat parties should be compared before and after operations. The lead line should be wetted, stretched, and marked in fathoms and feet. For shallow water a staff or pole is used, marked in feet and tenths of a foot; the pole is shod with a square piece of wood to prevent the pole sinking into the mud. All soundings must be reduced to mean low-water spring tides before they are inserted on a chart. This information is obtained by means of the tide gauge, and is the result of the observations carried through a full month. To fix the Position of a Rock or Shoal when out of sight of land. Required Position, extent, depth of water on the rock or shoal. In bad weather no fixing of any value can be made. In fine weather and smooth water the fix will depend on the accuracy of the vessel's position. Assuming that to be correct, and a shoal patch discovered, send a boat away to anchor if possible in the least water. The observer in the boat should be provided with a lead line, a sextant, a reliable watch, notebook and pencil, and Morse or semaphore signalling flags. When the boat is in position the vessel steams or sails round the boat, sounding at irtervals and asking the boat to take masthead angles as they are required, at the same instant compass bearings of the boat are taken from the ship. This is repeated until the circuit of the shoal is made. With the data obtained the boat's position can be plotted with reference to the ship, and ship's position with reference to the boat, by working out the masthead angles for the distance and the direction by the compass bearing corrected for compass error. At one, or more than one, position the ship is fixed by observation, from which the position of the shoal can be worked out. It is hoped that the following examples will enable the student to grasp the principles involved in carrying out a survey and inculcate in him a desire to master this much-neglected branch of useful knowledge to the mariner. EXAMPLE I. Let it be required to survey the harbour shown in Plate I. by observations made on the water. Having sailed round the harbour and fixed upon the several stations on the coast, let the two buoys, A and B, be moored so that all the points or stations selected may be seen from both A and B ; the bearing of B from A is 45°, that is, N. 45° E. true, the variation 23° W., and the distance one mile; then, having taken the boat to the stations A and B, assume the following bearings to have been taken. The bearings are given both true and magnetic, but it is advisable to construct plans from true bearings, as they will not be affected by lapse of time. These bearings were checked by sextant angles from A and B, and the positions of A and B as found by the station pointer were in agreement with the positions of A and B as laid down on the plan. Sextant angles from A I 79° H 44° G Sextant angles from B G 47° F 87° D EXAMPLE II. Wanting to survey a coast whilst sailing along it, I ran from A to B (see Plate II.), 248°, 6 miles; from B to C, 270°, 4 miles; from C to D, 331°, 3.5 miles, taking the following bearings and angles at each station. All courses and bearings are true. Let it be required to make an accurate survey of the harbour and adjacent island and make a plan (see Plate III.). |