Imágenes de páginas
PDF
EPUB

To find the Correction by Calculation

in. Half range 29:5

Log. 1.469822 19°

Cos. 9.975670 27.89 inches

Log. I'445492

As the interval between H.W. and time of cast is greater than half the duration of the tide, the quantity found by calculation is to be subtracted from M.T.L. to find the required correction.

ft. in.

M.T.L. 7 5
Amount tide has fallen below M.T.L. 2 4

Correction 5 I to subtract.

Examples for Practice I. On Aug. 26th, 1915, at 5h. 32m. a.m., mean time at ship, being off Barrow Docks. Required the correction to be applied to the depth obtained by lead line before comparing it with the chart.

M.

Ans.

H.
Duration of fall

7 II
Time of cast after H.W. 6

Angle 150°

ft.

in.

14 61 M.T.L.

15 53 Correction to subtract 2 103

Half range

2. On April 14th, 1915, at 11h. 36m. p.m., standard time at ship, being off Thurso. Required the correction to be applied to the depth obtained by lead line before comparing it with the chart.

M.

Ans.

H.
Duration of fall 6 0
Time of cast after H.W. 3

Angle 90°

ft. in.
Half
range

6 5
M.T.L. 6 7
Correction 6 7 to subtract.

3. On March 3rd, 1915, at 1h. 14m. p.m., M.T.G., being off Devonport. Required the correction to be applied to the depth obtained by lead line before comparing it with the chart.

[blocks in formation]

A Line of Soundings The following method is much used at sea

Draw in pencil upon the chart a line from A to B upon which it is desired to keep the ship; this line will cover a line of soundings on the chart. Procure a long strip of paper an inch wide upon which the soundings are recorded to scale, using the graduated meridian for that purpose.

The distance the ship has run between the casts with the reduced depth and nature of the bottom all being placed upon the strip of paper, a comparison is made with the line upon the chart. If the vessel is on the line the soundings upon the paper will agree with the soundings upon the chart. If they do not the slip of paper should be moved up or down, backward or forward, until they do agree. For if the soundings are carefully taken and correctly reduced the line must be found on the chart.

If it is desired to alter the course, place the paper upon a compass on the chart and give the slip of paper a half-turn equal to the angle of the new

Done with care, soundings can be recorded continuously for long distances.

course,

The Lead There are two kinds of leads, the hand and the deep-sea lead. The hand lead is used in shallow waters, chiefly going in or out of port. The depth is obtained by actual measurement.

The weight of the hand lead is from io to. 14 pounds; it is bent on to a water-laid line iš inch in circumference and is 25 fathoms in length.

The lead line is marked as follows

[merged small][merged small][merged small][ocr errors][merged small][merged small]

There are nine marks and eleven deeps.

The deep-sea lead used with a hemp line weighs from 28 to 30 pounds. The line is 100 fathoms long; it is marked the same as the hand lead up to 20 fathoms and has an additional knot for each 10 fathoms, that is, three knots at 30 fathoms, four knots at 40 fathoms, etc., and every five fathoms is marked by a single knot.

SOUNDING MACHINES

The Sounding Machine invented by Sir William Thomson has been the type of other designs. Three hundred fathoms of thin pianoforte wire are wound round a drum, which has a V-shaped groove around its rim. When running out the drum runs freely round a spindle and is stopped by a frictional brake consisting of wooden chocks on each side. To the end of the wire is secured, by a hemp line, a specially constructed lead with a long iron shank weighing about 24 pounds.

On the hemp line is lashed a brass guard tube in which is placed a glass tube 24 inches long; the tube is placed open end down inside the guard tube. The glass tube is coloured inside with chloride of silver, which makes the glass appear red. The salt water penetrating turns the colouring matter white.

The tube is withdrawn after a cast, placed upon a graduated scale, and read off.

The principle is an application of Boyle's law of the compressibility of E gases.

The temperature remaining the same, the volume of a given quantity of gas is inversely as the pressure to which it is subjected.

Thus, in a tube 24 inches long when it has descended –

[ocr errors][merged small][ocr errors]

These soundings are affected by the atmospheric pressure. The following corrections must be applied if the barometer is above 29.50–

Barometer 29.75 add 1 fathom in 40
30.00

30
30-50

20 31.00

15

LOG-SHIP AND LOG-LINE,

WITH THE

LOG-GLASS AND PATENT LOG.

THE LOG AND LOG-GLASS

The common LOG consists of the LOG-SHIP and LOG-LINE.

The Log-ship.The log-ship is a quadrantal-shaped piece of hard wood, about 5 or 6 inches radius, and a quarter of an inch thick ; the circular part is loaded with lead to make it swim perpendicularly, and just sufficient to immerse it.

The outer extremity of the log-line terminates in two or three ends,-for as many holes as are in the flat of the log-ship, and which, when fitted, forms a sling or bridle; the end of one part of the sling being a leather peg, this is fitted into one of the holes near the circular edge of the log-ship, and draws upon being checked, after the operation of heaving the log has been effected, and thus the log-ship is more easily hauled in : the other (or inner) end of the log-line is attached to a REEL, around which is wound 120 fathoms of line.

The Log-line.—The log-ship end of the line is marked off, to the length of 10 fathoms or more, according to the size of the ship, by a bit of white rag or bunting; and this length is called stray-line,—its use being to carry the log-ship out of the eddies of the ship's wake before counting commences. The line from the stray part inwards is divided into equal lengths—called knots—by pieces of cord let into the strands of the log-line,-each piece of cord carrying the requisite number of knots to distinguish it. The subdivisions of a knot are called fathoms. Each knot is the representative of a nautical mile, and its length is proportionate to the seconds of the log-glass that is used when heaving the log. The log-line is subject to variation in length, due to wetting and strain.

The hand Log-line is usually marked as follows; first half knot, one end of leather; first knot 2 ends of leather; I knots, 3 ends of leather ; 2 knots of cord with two knots; 2knots apiece cord with one knot; 3 knots apiece cord with 3 knots, and so on; an additional knot for each knot, and a single knot at each half-knot.

Heaving the Log.When the log is hove, a seaman holds the reel by the two ends, another seaman takes the log-glass, and an officer of the watch asks, “ All clear ? On receiving a reply, “ All clear,” he throws the logship well out to leeward from the lee quarter; as soon as the whole of the stray line has gone, the officer calls “ turn”; the seaman then turns the sand end of the glass uppermost; the log-ship, being perpendicular in the water, and presenting a face toward the ship, by its resistance to the ship's progress draws the marked part of the line off the reel; when the sand has run out, the seaman calls "stop"; the officer at that instant clutches tight the line, and the number of knots which have passed out indicates how many nautical miles per hour the ship is moving through the water. In a heavy sea the line requires to be paid out rapidly at the time the stern is rising, and slightly retarded as the stern is falling. In heaving the log, great care should be taken to veer out the line as fast as the log takes it; for if the log be left to turn the reel itself, it will come home, and give an erroneous distance.

The Log-glass.—The log-glasses, of the same shape as an hour-glass, are glasses filled with sand or metal filings, and run to seconds; the long glass runs out in 30 seconds, or in 28 seconds; and the short glass in half the time, viz., in 15 seconds, or in 14 seconds. When the ship's rate is more than 5 knots, the short glass is used, and the number of knots shown by the log-line is doubled.

As the glasses may be affected by variation in the temperature, and the sand certainly by damp weather, it is necessary to examine their accuracy, from time to time, by comparing them with a seconds’ watch; they can be made true, by drying the sand, or changing its quantity, to do which it will be necessary to remove the cork at the stoppered end of the glass.

The Nautical or Sea Mile.-In Navigation, distance is invariably measured in Nautical Miles; and one such mile is considered to be the 21600 part of the earth's circumference (360° x 60 = 21600).

The geographical mile of the trigonometrical survey is 6087•23 feet, which corresponds to the admeasurement obtained from the data of Sir G. B. Airy, Bessel, and Col. A. R. Clarke, using the earth's equatorial radius as the basis of computation; whence we get respectively 6086.5 ft., 6086.5 ft., and 6087•1 ft.; and the geographical mile, as 1' (minute) of the Equator = 2029.1 yards = 1•1529 Eng. statute miles.

The geographical mile is generally defined to be the length of a minute of arc of the earth's equator ; but the nautical or sea mile as defined by hydrographers is the length of a minute of arc of the meridian, and is different for every latitude. It is equal to a minute of arc of a circle whose radius is the radius of the curvature of the meridian at the latitude of the place. ThusLength of a nautical mile in lat. 0° 6045.93 feet.

45° = 6076.82

6107.98

[ocr errors]

90°

And the length of a mean nautical or sea mile = 6076-91 feet, or 2025.63 yards, or 1•1509 statute miles. This admeasurement agrees with those obtained from the data of Sir G. B. Airy, Bessel, and Col. A. R. Clarke, using the mean of the equatorial and polar radii of the earth, and whence are obtained respectively 6076-3 ft., 6076-3 ft., and 6076-7 ft., as the length of a mile.

The Admiralty knot, however, is = 6080 feet, or 2026j yards; and this is the admeasurement usually adopted for the nautical or sea mile.

Length of a Knot.— Coming to the length of a knot on the log-line, you can at once understand that the length between two adjacent knots should be the same part of a nautical mile, that the seconds of the glass are of an hour.

Before, however, commencing to determine this length, you must, in the first place, know that it is safest to have the reckoning ahead of the ship (to apprise the seaman the sooner to look out when approaching the land), an

« AnteriorContinuar »