30° 20° 10° Plot buoys and give their bearings and distances from A. Draw a horizontal line 7" long at bottom of paper, and at each end erect perpendiculars as in Art. 114, and make them 5" long; measure to see if their extreme ends are 7" apart, if so join up and frame is complete. Inside the frame near the bottom draw scale of cables and 10ths as in ટ 6 6 10 cables I mile FIG. 177. diagram, and put in a compass with magnetic north and south inclined 15° to sides of frame. Graduate compass. Plot in 1" from bottom and left-hand side; and from it set off bearings and distances from your scale and compass to plot positions of A, B, and C. Join AB and BČ. For buoy "a" it is noticed that A is on the left, as seen from the buoy, and the angle between A and B is 103°, the arc is less than a semicircle, and the complement of 103°, that is 13°, is laid off at each end of A and B, on the side away from where the fix is to be, the centre "1" in Fig. is the intersection of these lines, and the arc is drawn. Again the angle between B and C is 43°, so its complement 47° is laid off at each end of BC, on the same side as the fix is to be, and intersect in the centre "2"; where the former arc cuts this is buoy a. 66 66 99 At buoy "b" A is again on the left, since the angle between A and B is 47°, set off lines making 43° with AB at each end to intersect in centre "3" on same side as fix, describe arc; then set off lines making 18° with BC at each end to intersect in centre "4," on opposite side to fix, since angle between B and C is 108°; describe arc: these arcs cut at position of buoy "b." For buoy "e" it will be noticed that both angles being less than 90°, the centres "5" and " 6," determined as before, are both on the same side as the buoy. Now take off the bearings and measure distances. 1. Construct a Mercator's chart on a scale 2"-4 to a degree of longitude, extending from lat. 56° N. to 57° 30′ N., and from long. 25° W. to 28° W. A ship from lat. 57° 8' N., long. 25° 10′ W., sailed as follows by compass: S.W. by S. 50 miles, dev. 6° 10′ W.; W.N.W. 53 miles, dev. 6° 50′ W.; N.E. 40 miles, dev. 10° E.; N. by W. 52 miles, dev. 1° 10′ E. The variation is two points W. Find latitude and longitude in. 2. Construct a Mercator's chart, scale 0.8" to 1° long., extending from 63° 30′ S. to 65° 30′ S., and from 120° to 123° E., putting in parallels at each 10. A ship leaves A, lat. 65° 7′ S., long. 120° 52′ E., and sails the following true courses and distances: N.E. 47 miles; N.N.W. 63 miles; S.W. by S. 42 miles; E. 27 miles. Find latitude and longitude in. The variation being 8° 30′ W., what is the magnetic bearing of A from ship? 3. Construct a Mercator's chart to extend from lat. 54° 30' N. to 56° N., and long. 30° W. to 33° W., scale 1′′-3 to 1° long. A point of land in lat. 54° 32′ Ñ., long. 32° 56′ W., bears S.W. from the ship by compass, distant 7 miles; ship's head, E. & N.; dev. 9° 55′ E.; variation, 18° W. Ship sailed as follows by compass: E. N. 90 miles, dev. 9° 55' E.; N.N.W. W. 40 miles, dev. 2° 20′ W.; W.S.W. 45 miles, dev. 7° 50′ W.; N.E. † E. 75 miles, dev. 10° 35′ E. Find latitude and longitude in, true course, and distance made good. 4. Construct a Mercator's chart to extend from lat. 54° N. to 55° 30' N., and long. 3° to 6° W., scale 1"75 1° long. Insert following positions: Abbey Head, lat. 54° 47′ N., long. 3° 58' W.; Little Ross, 54° 46' N., 4° 5' W.; Burial Island, 54° 29′ N., 5° 25′ W.; South Rock, 54° 22′ N., 5° 26' W. At the ship, Abbey Head bore N. 46° E., and Little Ross, N. 37° W.; she then steamed S. 63° W. 12 knots for 4 hours, when her position was fixed by cross-bearings of Burial Island W. 8° N., and South Rock S. 23° W. Find true position of ship, set and drift of current. 5. Construct a Mercator's chart, scale 2" = 1° long., extending from lat. 68° to 71° N., and 6° to 10° E. long. A ship leaves 69° 38′ N., 8° 20′ E., and sails as follows by compass: N.E. E., 30', dev. 12° E.; N.W. by W. W., 55', dev. 8° W.; South, 80', dev. 40° W.; variation, 20° W. Find her latitude and longitude in, course, and distance made good. 6. Construct a Mercator's chart on a scale of 1.3 inch to 1° long., extending from lat. 48° 45' S. to 52° S., and long. 20° E. to 26° E. Position of lighthouse, lat. 51° S., long. 25° E. A peak lies 15 miles true North from Lt. Ho. Variation 20° E. A ship steamed from 50° S., 21° E., as follows: When the Lt. Ho. bore S. 45° E. true with an angle of 75° between it and the peak, give the lat. and long. of the position, amount of current experienced, and assuming the same rate of speed to have been maintained, plot the soundings in their correct positions. 7. Draw a plan on a scale 0.85 inch = 1 mile; and insert following: B, C, D are three objects, the true bearings and distances of which from A are: B, S. 81° E., 3 miles; C, S. 85° E., 6 miles; D, S.S.E. 1 E., 3 miles. Fix the following soundings: AD 65° 20' C (5 feet, mud). A 41° B 65° C (31 fathoms, broken coral). A 116° B 123° D (7 fathoms, sand and shells. 8. Construct a plan on a scale 0.8 inch to 1 mile lat., to extend from 52° 3' N. to 52° 18′ N., 6° to 6° 25′ W. Insert a compass showing 21° W. variation and graduate. A ship steering east magnetic, observed a lighthouse lat. 52° 12′ N., long. 6° 7' W., in transit with a buoy when the horizontal angle between light vessel, lat. 52° 7′ N., long. 6° 24′ W., and Lt. Ho. was 81°. After steering 1.5 miles on above course, the horizontal angle was observed to be 50° between Lt. V. and Lt. Ho., and 22° between buoy and Lt. Ho. Find position of buoy with reference to its charted position, which was at first S. 31° W. of Lt. Ho. and 1.6 miles from it. 9. On a scale of 14 inch to 10 miles long, construct a Mercator's chart to extend from 55° 15′ N., to 55° 45' N., and from 6° to 7° 30′ W., insert a magnetic compass variation 19° 15′ W., graduate margins. At 7 30 a.m. a point (P) bore 130° from a ship proceeding on a magnetic course S. 80° E., 10 knots, in a current setting N. 34° E. magnetic. At 9 45 a.m. it bore 206°, and at 10h 30m a.m. it bore 225°. P. lat. 55° 14' N., long. 6° 39′ W. Find the ship's position and distance from P when the last bearing was taken, average drift of the current, course and distance made good. CHAPTER XXVII. LAWS OF STORMS. ART. 117.-The facts gathered from observations, and known as the Laws of Storms, apply more particularly to the violent Hurricanes, Cyclones, and Typhoons (which may be all classed under the name "Cyclone ") met with between the parallels of 10° and 35° of latitude; but also in a modified degree to the strong winds, gales, and storms of higher latitudes. These facts may be summarized as follows: 1. The tropical Cyclones are never met with in the belt between 10° N. and 10° S. lat., but outside of 10° they occur in the following regions: N. Atlantic: The western part near the West Indies. N. Indian Ocean: The Bay of Bengal and Arabian Sea. N. Pacific: The China and Java Seas. S. Indian Ocean: The Western part, passing near Mauritius, and hence called "Mauritius Hurricanes.' S. Pacific: Eastern part. 2. Cyclones (as the name implies) have a rotatory motion. round a centre, and a progressive motion, varying from three or four miles an hour to twelve or more. Near their origin they cover only a small area, but afterwards expand to as much as a thousand or more miles in diameter. 3. The barometer stands lowest at the centre (often 28 inches or less), and gradually increases in height towards the circumference, whilst the wind force is generally greater the nearer the centre. 4. The bearing of the centre from the ship may be inferred approximately by counting from the wind direction 8 or 10 points to the Right in the N. Hemisphere, and the same number of points to the Left in the S. Hemisphere. 5. The path of the centre, or Line of Progression, divides the Storm Area into two semicircles, and by observing the change of wind it may be determined whether the ship is on the right or left-hand side of the Line of Progression-a matter of great importance in the handling of the ship. Thus, if the wind changes to the Right, the ship is on the Right-hand side, but if to the left, she is in the Left-hand semicircle. If the wind is "steady," i.e. changes very slightly in direction, it indicates that the Ship is on the Line of Progression. 6. The Direction of Rotation in the N. Hemisphere is to the Left, and in the S. Hemisphere to the Right. 7. The approach of a Cyclone is often indicated by well-marked signs, such as FIG. 178. (a) An oppressive stillness, with threatening sky and much lightning. (b) A long "ground swell." (c) A dense cloud bank on the horizon. (d) A rapid motion of the upper clouds. 8. When within the Storm Area, the approach of the centre is shown by (a) A rapidly falling barometer. (b) Increase of wind, with heavy squalls, much lightning and rain, and general murkiness. (c) Heavy and confused sea. (d) Continuous "veering" or " backing" of the wind, except The receding of the centre would be indicated by- (b) Wind becoming more "steady" and decreasing in force. (c) Weather clearing, but the sea still confused and dangerous. 9. Vessels, especially steamships, sometimes overtake Hurricanes, because their speed is greater than the rate of progression of the Storm Centre. In this case, if the wind changes to the Left, the ship is on the Right-hand side of the storm, but if to the Right, she is in the Left-hand semicircle; contrary to par. 5. 10. In entering the Central Area, which may be up to a hundred miles in diameter, the wind suddenly ceases, with glimpses of a clear sky and a general calm of perhaps hours' duration, interrupted by puffy squalls, whilst the sea is particularly confused and dangerous. After passing through the centre, the wind strikes the ship from the opposite point of the compass with renewed hurricane force. 11. The paths of Storm Centres follow the same general directions for the same regions of the Globe. The following are the usual tracks: (a) N. Atlantic: Having their origin near the Windward Islands, they first travel about W.N.W., then more northerly, and about 25° N. to 30° N. they recurve to the N.E. towards Mid-Atlantic. (b) N. Pacific: The same general direction as in the N. Atlantic. (c) N. Indian Ocean, including Bay of Bengal and Arabian Seas: A general N. and W. direction. |