delineation is got over, and the relation between the different parts preserved by a distortion polarwise, that is, if we also widen the distance between the parallels of latitude to the same proportionate extent that we have widened the meridians. And such is the case in Mercator's Projection (See Chart.) In other words, all the meridians are parallel right lines, and the degrees of longitude are all equidistant: the parallels of latitude are everywhere at right angles to the meridians, and the degrees of latitude are increased in length from the equator to the pole in the same proportion as the degrees of longitude decrease on the globe; and a chart on this projection is constructed by the aid of a Table of Meridional Parts. It matters not that the extent of land and water in the higher latitudes is out of proportion when compared with the equatorial regions, the shape is still approximately preserved, and what is of most importance in navigation-the relative direction from one part to another, so that the track of a ship always steering the same course is drawn as a straight line. Mercator's projection has been compared to a cylinder unrolled-the cylinder of indefinite length being such an one as we might suppose to circumscribe the globe at the equator. A GENERAL CHART takes in a large expanse of the ocean and bounding land, an entire sea, or a considerable extent of coastline with its contiguous waters. The parallels and meridians are drawn on it, the lights are marked, the soundings, the variation of the compass, and such other details as the scale of projection will admit of. The compasses (in different parts of the chart) are often true, but sometimes correct magnetic. It will be a large scale, or small scale, chart according to the extent of coast and sea delineated. A PLAN is a chart that comprises a detached portion of a general chart, on a large scale, as a harbour, roadstead, small bay, the entrance to a river, channels leading to a port, a small part of a sea where the navigation is intricate, an island, &c. On it are given the lights and buoys, the soundings and bottom, the leading marks, the courses through channels, the dangers to be avoided, the variation of the compass, the tidal establishment of the port, the rise of tide at springs and neaps, and every thing that tends to facilitate the navigation of the locality; the compass is correct magnetic; and the scale on which the chart is drawn is noted. Such a chart only occasionally gives the parallels and meridians; when these are absent, the scale in miles is given. (1.) When a chart is properly spread out before you, so that you can read it like the page of a book, the top is the North, the bottom is the South, the side to the right is the East, and the side to the left is the West. The only exception to this arrangement is when, for some special purpose, the chart is drawn on what is called the diagonal scale; but reference to one of the compasses, the North point of which is invariably denoted by a device, as an arrow, star, or fleur-de-lis, will at once show the north and south, and consequently the east and west. (2.) The PARALLELS are represented by straight parallel lines drawn across a chart from east to west; their width apart is unequal, and the inequality increases more and more in proceeding from the equator. If the increase is upwards in the direction of the North part of the chart, then the chart represents some portion of the northern hemisphere; if the increase is downwards, towards the South, it embraces some part of the southern hemisphere. The parallel of 60° N. passes near Cape Farewell. (3.) The MERIDIANS are represented by straight parallel lines drawn from the top to the bottom of a chart, that is from North to South; and the space between them is everywhere equal. Parallels and meridians are at right angles to each other. The meridian of 20° W. passes through Iceland. (4.) THE SCALE OF LATITUDE AND DISTANCE.-The two meridians which bound a chart on the right and left (as a c and в D) are called graduated meridians, because they are marked to degrees,—and also to minutes if the width between the parallels is sufficiently great. On these meridians latitude is measured from the equator, polarwise, in degrees and minutes according to the scale of the subdivisions. The DISTANCE of two places is also measured on the graduated meridian. (5.) THE SCALE OF LONGITUDE.-The two parallels which bound a chart at the top and bottom (as A B and C D) are called graduated parallels,-being marked to degrees (and minutes if sufficiently large); on these parallels longitude is measured, and nothing more. Distance cannot be taken from them. (6.) To find the Latitude of a Place on the Chart.-With the dividers take the distance of the place from the nearest parallel; refer this distance to one of the graduated meridians, placing one point of the dividers on the same parallel, and the other point upwards or downwards as required; read off the latitude at the position of the latter point. (7.) To find the Longitude of a Place on the Chart.-With the dividers take the distance of the place from the nearest meridian; refer this distance to one of the graduated parallels, placing one point of the dividers on the same meridian, and the other point to the right or left of it as required; read off the longitude at the position of the latter point. (8.) To mark the Ship's place on a Chart.-With the dividers take from the graduated parallel the given latitude; mark this on the meridian nearest meridian to the given longitude; lay the edge of a pair of parallel rules on a near parallel and work one side of them to the exact latitude you have marked on the meridian: then, with the dividers, take the given longitude from the graduated parallel: lay this down along the edge of the parallel rules which already marks the latitude, and you have the ship's place. (See Track from Lizard to Madeira, on chart.) NOTE;-to find the Courses and Distance between two places on the Chart, see Q. 3, 4 and 5 p. 50-51 (Examination in Chart). (9.) To lay down a ship's place by cross-bearings when near the land has already been described on p. 39; remember, that having taken the bearings, they must be corrected for the deviation due to the direction of the ship's head, if an iron ship or a steamer; also, the nearer the angle between the two bearings approaches 8 points or 90° the better will be the determination of the position; under 45° is much too small an angle, and over 12 points or 135° is much too large an angle, for accuracy. Having the ship's place on the chart by cross-bearings, you can find the Lat. and Long. by par. 6 and 7 above, if you desire it. (10.) Mercator Course (rhumb line) and Great Circle Course.-You have already been told that on Mercator's Chart the course between any two places is a straight line; I have shown the course between St. John's, Newfoundland, and the Scilly Islands (English Channel); as here projected it is N. 854° E. or E. & N. (easterly), and the distance is 1847 miles; but this rhumb line (and, indeed, any course on Mercator's projection, except a true north, south, east, or west one), when placed on a globe, cutting every meridian it passes at the same angle, is a spiral, which, if pursued, and no land intervened, would be always tending polarwise, without reaching the pole. The arc of a Great Circle is the true course between two places, and the distance along that track is the shortest possible; it does not seem to be such when drawn on Mercator's chart, but it is so nevertheless: the Great Circle course and distance between St. John's and the Scilly Islands is represented on the chart (as a curve) for comparison with the rhumb line: a ship on such a course would be constantly changing the direction of her head, as every meridian is not cut at the same angle; in this case, at starting the course would be N. 68° E. or E.N.E., gradually change towards East as the highest latitude (Latitude of Vertex) 51° 14′ N. in Long. 24° 10′ W. was reached, the direction would then trend to southward of east, and the last course would be S. 761° E. or E. by S. southerly. The distance is 1807 miles by the Great Circle. It is always useful to know the position of the Great Circle with respect to the Mercator track, because, when winds head a vessel, that tack must be best which leads the nearest to the great circle: on the great circle a ship's head always points directly to her port; on Mercator's track she only appears to point to it, although on the given course she must eventually reach the port. (See Chart.) The remaining instructions on the use of the chart are given in the answers to, and notes on, the questions set in the Board of Trade paper— EXAMINATION IN CHART. The Applicant for the Yachting Certificate of Competency will be required to answer in writing, on a sheet of paper which will be given him by the Examiner, all the following questions, numbering his answers with the numbers corresponding with those on the question paper. Q. 1. A strange chart being placed before you, what should be your special care to determine, before you answer any questions concerning it, or attempt to make use of it? A. Determine, from the compass, which is the North part of the chart. Denmark, Holland, Norway, Germany, Russia, Sweden, and the U.S. of North France, Austria, and Italy adopt Paris, 2° 20' 9" 4 E. of Greenwich. Q. 2. How do you ascertain that in our British charts? Q. 3. Describe how you would find the course by the chart between any two places, A and B. A. Lay the edge of the parallel rules over the two places, then slide them (preserving the direction) till the edge of one rule is on the centre of the nearest compass, and read off the course indicated by the direction of the rule. Note. Thus, (see chart) laying the parallel rules over Scilly Islands and St. Michael's (Azores), as indicated by the dotted line, and traversing them to the compass, the course from Scilly to St. Michael is S.W. W. (true), from St. Michael's to Scilly N.E. E. (true.) Q. 4. Supposing there to be. points of . . . . variation at the first named place, what would the course be magnetic,-the true course being . . .? In answering this question merely write down the magnetic course corresponding to the true course given. Explanation:-Allow easterly variation to the left, westerly variation to the right :-thus, Note.-From Scilly to St. Michael's, the variation at starting being 2 points W., true course S.W. W. gives correct magnetic W.S.W. W.; but the compass course would have to be changed on the voyage, owing to change in the variation, Q. 5. How would you measure the distance between those two, or any other two places on the chart? A. Measure off with a pair of dividers half the distance between them : opposite the middle point, on the graduated meridian, place one leg of the dividers, and with the other leg measure upwards and downwards: the number of degrees (in miles) between the two extreme points gives the distance nearly. Note.-From Scilly to St. Michael's, half the distance (at) is opposite middle latitude 44° N.; one point of dividers in this latitude, and the other extended upwards reaches Lat. 52° 20′ N., then extended downwards reaches Lat. 34° 20′ N.; difference between 52° 20′ and 34° 20′ is 18°, which multiplied by 60 gives 1080 miles Q. 6. Why would you measure it in that particular manner? A. Because the degrees of latitude on Mercator's chart increase in length as the latitude increases. Q. 7. What do you understand those small numbers to indicate that you see placed about the chart? A. Soundings in fathoms or feet, as specified on the chart. Note.-In a corner of charts and plans, these particulars are given in full. Q. 8. At what time of the tide ? A. Low water ordinary spring tides. Q. 9. What are the requisites you should know in order that you may compare the depth obtained by your lead-line on board with the depths marked on the chart? A. The rise and fall (or range) of tide, and its state as regards low water. Note.-The rise of tide at springs and neaps, as well as the range, is given on a chart to facilitate finding the height of tide at different hours between high and low water. Q. 10. What do the Roman numerals indicate that are occasionally seen near the coast, and in harbours ? A. The time of high water at full and change of the moon. Note.-It is generally expressed thus-H.W. at F. and C. VIh. 23m., that is, high water at full and change of the moon occurs at 6 hours 23 min. Q. 11. How would you find the time of high water at any place, the Admiralty Tide Tables not being at hand, nor any other special tables available? A. To the time of high water at full and change (by chart) add 48 minutes for every day elapsed since full or new moon. Or, to the civil time of the moon's meridian passage at place, add the time of high water at full and change. All the above questions should be answered, but this does not preclude the Examiner from putting any other questions of a practical character, or which the local circumstances of the port may require. BUOYS are marked :—B. (black); Cheq. (chequered); H.S. (horizontal stripes); R. (red); W. (white); B.W. (black and white); B.R. (black and red); R.W. (red and white); V.S. (vertical stripes). A green buoy indicates the position of a wreck. = = = = LIGHTS are indicated by a dot, coloured yellow with a red spot in the middle; if there is any uncertainty about its character, it is simply marked Lt.; Lt. F. light fixed; Lt. Fl. light flashing; Lt. Int. = light intermittent; Lt. Rev. light revolving; Lt. F. and Fl. = light fixed and flashing; Flg. Lt. floating light; Lt. Ves. light vessel; the colour of the light is expressed in full,—as red; blue; green ; white; red and white; white, red, and green; when no colour is expressed it is taken to be white. = CURRENTS are indicated by a feathered arrow, and the direction of the arrow shows the direction of current. FLOOD tide stream is shown by an arrow feathered on one side; EBB tide stream by unfeathered arrow. ROCKS just under water are indicated by a small dotted circle with cross in centre; rocks awash or just above water are indicated by dotted circle with a dot or dots in centre; a dotted circle with a numeral in it signifies a shoal with the feet or fathoms of water over it. Rock, island, or shoal with E.D. signifies existence doubtful, P.D. position doubtful though known to exist. COMPASS.-If the N. and S. line of the compass is not on or parallel with a meridian, it is magnetic, according to the variation in the locality; if it coincides with the meridian it is true. Fms. = = high water fathoms; Ft. foot or feet; H.W. = high water; H.W. F. & C. |
