페이지 이미지
PDF

(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 Mantis 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. 1 W. (true), from St. Michael's to Scilly N.E.iTS. (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,

True course N.N.E., with 1 point E. var. gives N. by E. magnetic.

1 point W. var. gives N.E. by N. „

True course S.S.E., with 1 point E. var. gives 8.E. by 8. „

1 point W. var. gives S. by E. „

True course S.S.W., with 1 point E. var. gives 8. by W. „

1 point W. var. gives S.W. by S. „

True course N.N.W., with 1 point E. var. gives N.W. by N. „ 1 point W. var. gives N. by W.

Note.—From Scilly to St. Michael's, the variation at starting being 2 points W., true course S.W. J W. gives correct magnetic W.S.W. i 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. Mote.— From Scilly to St. Michael's, half the distance (at J) 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° 2(X 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 comer 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 1

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. Vlh. 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 1

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.

Signs And Abbreviations Used On Chaets.
Quality of Bottom.

[table]

ground

hard

mud

ooze

oysters

pebbles

[ocr errors]

red

rock

rotten

sand

shells

soft

[ocr errors]

speckled

stiff

stones

weed

white

yellow

[ocr errors]

peb.

Buoys are marked:—B. (black); Cheq. (chequered); H.S. (horizontal stripes); B. (red); W. (white); B.W. (black and white); B.B. (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 yeUorv with a red spot in the middle; if there is any uncertainty about its character, it is simply marked Lt.; Lt. P. = light fixed; Lt. Fl. = light flashing; Lt. Int. = light intermittent; Lt. Rev. = light revolving; Lt. F. and Fl. = light fixed and flashing; Fig. Lt. = floating light; Lt. Ves. = light vessel; the colour of the light is expressed in full,—as red; bine; green; white; red and white; white, red, and green; when no colour is expressed it is taken to be white.

Curbentb are indicated by & 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 nnder 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. = fathoms; Ft. = foot or feet; H.W. = high water; H.W. F. & C. = high water at full and change of moon; L.W. = low water; Np. = neaps; Rf. = reef; Ek. — rock; Sh. = shoal; Sp. = springs; Vis. = visible; + Obs. Spot = Place where observation was made. Anchorage is indicated by an anchor, and the depth is generally close by.

Soundings are reduced to mean Low Water of ordinary Spring Tides, and are expressed in Fathoms and fractions of a fathom, or in Feet and fractions of a foot.

The Velocity of Tide is expressed in Knots and fractions of a knot.

A number with a line and dot above signifies no bottom at the depth given, thus T^j = no bottom at 123 fathoms.

All Heights are in feet above High Water ordinary Springs, and where there is no tide, above the mean level of the sea.

The Rise of Tide is measured from the mean Low Water level of ordinary Springs; the Bange of Tide from the Low Water of one tide to the High Water of the following tide.

All distances are in Nautical Miles; a Cable's length is the tenth part of the nautical mile.

A fathom = 6 feet. A nautical mile = 1-1528 statute mile. A statute mile = '8674 nautical mile. 13 nautical miles == 15 statute miles, very nearly.

Soundings on Foreign- Charts.

Danish and Norwegian Favn 6-175 Eng. Feet

Dutch Vadem 5575

Dutch (recent) Elle 3281

French Metre 3-281

French (old) Brasse 6329

Portuguese Braca 6-004

Prussian Faden 5-906

Bussian, the Eng. Fathom & Foot

Spanish Metro 3-281

Spanish (old) Braza 5-492

Swedish Famn 5-843

THE LEAD LINE.

Sounding is the operation of trying the depth of the water, and the quality of the sea bottom, by means of a plummet (lead) sunk from a ship.

There are two plummets used for this purpose—the hand-lead and the deepsea lead—which are attached, by means of a stop, to a long line called the

LEAD-LINE.

Both leads are cylindrical in form, and tapered, being widest across at the bottom end (the heel), which is hollowed out for the reception of a lump of tallow; this tallow is called the arming, and its purpose is to bring up some of the bottom it touches, so that the quality or nature of the ground you strike soundings on may be compared with the instructions on the chart or in the sailing directions, and the ship's position approximately known thereby. The narrow upper end of the lead has a hole in it, through which is rove a gromet well served over to keep it from chafing. In the end of the lead-line there is a long eye spliced, which is also served over. The eye is rove through the gromet and taken over the lead, being thus secured.

[table]

The Hand-lead, of which there are two—one 7 pounds weight, and the other 9 pounds, hence readily thrown by the hand—is used in shallow water, when in the vicinity of land, and for sounding in narrow channels, rivers, and harbours, to show how much water the ship is in, as well as the bottom if necessary.;

The Hand-lb Ad-line, which is usually 25 fathoms in length, is marked at every 2 or 3 fathoms, so that the depth of water may be ascertained either in the day or night; it is said to have nine marks and eleven deeps—the latter not being indicated by marks; they are as follows :—

Fathom Deeps. Fathom Marks.
1

2 Leather, with 2 ends.

3 Leather, with 3 ends. ,

4

6 White calico.

6

7 Red bunting.

8

9

10 Leather, with a hole in it.

11

12

13 "Blue serge.

14

15 White calico.

16

17 Red bunting.

18

19

20 Strand, with 2 knots in it.

Calico, bunting, and serge are preferred as distinctive marks, because-a man can tell the difference in the dark by the feel.

The line is always marked from the heel of the lead, and should be well stretched before marking.

Sounding with the hand-lead, which is called talcing a cast of the lead, as well as heaving the lead, the leadsman stands in the chains to windward. Having the line all ready to run out, without interruption, he holds it nearly at the distance of a fathom from the lead, and having swung the latter backwards and forwards three or four times, in order to acquire the greater velocity, he swings it round his head, and thence, as far forward as is necessary; so that, by the lead's sinking while the ship advances, the line may be almost perpendicular when it reaches the bottom. The leadsman then makes known the depth of water in a kind of song:—thus, if the mark of 5 fathoms is close to the surface of the water, he calls, "By the mark five;" as there are no marks at 4, 6, 8, &c., he estimates those numbers, and calls, "By the deep four," &c.; if he judges it to be a quarter or a half more than any particular number, he calls, "and a quarter live," "and a half four," and so forth: if he conceives the depth to be three-quarters more than a particular number, he calls it a quarter less than the next, thus at 4| fathoms he calls, "a quarter less five;" thus the only fractions of a fathom spoken of are a quarter and a half.

A/ote.—An tip and down cast is the only true one; and the error in soundings is generally in excess. Deep is a corruption of dip, the leadsman having to haul up and then dip the lead line when ascertaining the unmarked fathoms.

The Deep Sea Lead is of larger size than the hand lead, varying from 28 to

85 pounds in weight, and is attached to a much longer line, to find bottom in 100 or more fathoms.:

The Deep-sea Lead-line is a strong and water-laid line, marked as the hand lead-line to 20 fathoms; and then a strand with three knots indicates 30, four knots 40 fathoms, and so on, with an additional knot for every ten; the intermediate fives are marked by small strands with a single knot, or by a piece of leather-. At 100 fathoms is a piece of bunting, and then re-commence the knots. To use this lead effectively it is usual to bring-to the ship ; the lead is then thrown as far as possible from the ship on the line of her drift, so that, as it sinks, the ship drives more perpendicularly over it. There is a suitable reel for this line.

Nate.—To heave the lead properly requires practice, and nerve; there is a knack and sensitiveness to the lead taking the ground which can only be acquired early in life.

The principal Patents are—Massey's Frictionless Shield Sounder, and Walker's Harpoon Sounding Machine.

THE SEXTANT AND QUADRANT.

The Sextant derives its name from the extent of its limb, which is the sixth part of a circle, or 60°, but being an instrument of double reflexion it is divided into 120°. It is used for measuring angles,—as the altitudes of, and distance between, heavenly bodies,—as well as angles between terrestrial objects.

The Quadrant is properly an octant, as the limb is only the eighth part of a circle, or 45°, but, like the sextant, being an instrument of double reflexion, it is divided into 90°.

Both instruments are constructed on the same principles, and have similar parts; but the Quadrant is generally the rougher and commoner instrument of the two, sufficient however for taking altitudes at sea. From a mechanical point, however, both instruments can be made equally accurate and perfect. It will be sufficient to describe the Sextant.

Accuracy of the Instrument.—When the joints of the framework are close and tight, and the various screws fit closely and act well,—when the centering is perfect,—when the graduation of the limb and the vernier is accurate in every part,—when each of the reflectors or mirrors has its two faces parallel, and the glass perfectly clear,—and when the shades have clear glasses, the two faces of each glass parallel, and the set work with all their faces parallel,—the instrument may be considered perfect, as regards the optical and mechanical principles of its construction, and without any sensible error, but what the adjusting screws can rectify.

Description Op The Parts Of The Sextant.—The form of the Sextant is that of the figure, p. 56, and the flat upper surface is the plane of the instrument; the following are the principal parts:—

A A is the Arc (or limb) which is graduated from right to left, from O (the zero point) to 120° or 150°; this is the arc proper, and the subdivisions are by 10', 15', or 20', according to the size of the instrument. To the right of O, from left to right, there is also a small space of graduation which is called the arc of excess.

« 이전계속 »