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TO FIND THE COURSE TO STEER IN ORDER TO MAKE GOOD ANY COURSE IN A KNOWN CURRENT,
AND ALSO THE DISTANCE MADE GOOD. Draw a line on a chart to represent the course to be made good ; from the ship's place on the chart lay off a line in the direction of the set of the current, on which mark off from the ship's place the rate of the current per hour; then take in the compasses the distance the ship sails in an hour by log, and put one foot on the last-named mark, and from the point where the other foot reaches the first line draw a line to the mark on the line representing the direction of the current. The course to be steered is represented by the line last drawn, and the parallel ruler being placed to it, and moved to the centre of the compass on the chart, will give the course of the ship; and that portion of the first line drawn, intersected by the last line drawn, will be the distance the ship will make good per hour.
On a chart, suppose A to be the place of the ship, B the port of destination; also A C the set of the current, the rate per hour being taken from the scale of miles and laid off in the direction of the line. Take the distance sailed by the ship per hour from the scale of miles, and with one foot of the dividers at C, make an arc cutting A at D. Join C D, and move the parallel ruler from C D to A, drawing A E parallel to C D: then A E will be the direction of the ship's head. And the parallel ruler being moved to the centre of the compass on the chart, will give the course of the ship on the chart; and A D will be the distance the ship will make good.
The length of the stray-line should be sufficient to allow the log-chip to be clear of the eddies of the vessel's wake.
The distance between the knots should bear the same proportion to the number of seconds run by the glass intended to be used, as the number of feet in a nautical mile bears to the number of seconds in an hour.
The number of feet in a nautical mile is 6080.
Therefore, to find the length of a knot corresponding to a 28 seconds glass, we proceed as follows:
3600 : 6080 :: 28
ft. in. 360,0) 17024,0(47 34.
We have for glasses running 30 seconds and 32 seconds the following proportions :
3600 : 6080 :: 30 : 50 feet 8 inches.
MARKING THE LEAD LINE.
In nautical phrase the lead line has "nine marks and eleven deeps."
At two fathoms, the mark is leather; at three fathoms, leather; at five, white rag; at seven, red rag; at ten, a piece of leather with a hole in it; at thirteen, blue rag; fifteen, white rag; seventeen, the same as at seven; at twenty fathoms, a piece of cord with two knots.
Deep-sea lead lines are marked the same as far as twenty fathoms; then add a piece of cord with an additional knot for every ten fathoms, and a strip of leather for every five fathoms.
In the open sea, the tides require about six hours and a quarter to rise from low to high water, and an equal interval to fall from high to low water. If the rise or fall was an uniform quantity throughout, by simply taking a proportionate part of the rise or fall due to the time of tide, we should at once obtain the quantity required to reduce the soundings to the low water of that day. But the water does not rise in equal proportions, the rise during the first and last hours being very small (about one-sixteenth of the whole range): in the second hour there is a considerable increase of rise; in the third and fourth hours a still greater increase of rise; and then the rise begins to take off in the same proportions as it increased.*
The correct amount for every half-hour, and for various ranges, is given in the "Tide Tables for the English and Irish Ports for 1869," (p. 98, Table B), published by the Hydrographic Office, Admiralty.t
As the soundings upon the charts are all referred to low water of ordinary spring tides, casts of the lead taken at any other time of the tide, or any other day than full and change, will exceed the depth marked on the chart (except when it happens to be low water of greatest spring tides). It is necessary for the seaman to be able to calculate the difference between the actual depth obtained by means of his lead, and that marked on his chart, in order to the identification of his ship's
The reader may obtain an idea of this law, sufficiently exact for practical purposes, in the following manner:-Describe a circle, and divide the circumference into six equal parts on each side, corresponding to the hours of the tide; then divide the diameter into proportional parts, corresponding to a given (assumed) range of tide. Connect the segments of the circle by straight lines drawn across the figure, when it will be perceived that they intersect the diameter at certain divisions of the range. These are the correct quantities respectively due to each hour's rise or fall of such a tide from low to high water, and rice versa. An examination of these quantities will show, that in the first hour of the tide the rise equal to one-sixteenth of the whole range; at two hours from low or high water, the tide has risen or fallen one-fourth of the whole range; at three hours, it has risen just half its range; at four hours, it has risen three-fourths of the whole range; at five hours, to within a sixteenth-of the whole range. The above method, which is constructed upon principles theoretically correct, will represent with sufficient exactness all that is necessary for practical purposes.
+ Table XIX, Raper, which the author, in 1847, computed for Raper's work, also shows the space through which the surface of the water rises and falls at given intervals from high or low water.
On most charts the soundings expressed are reduced to low water of ordinary spring tides ; but in some charts, however, the soundings are reduced to the low water of extraordinary spring tides-such, for example, is the case on the chart of Liverpool, surveyed by Captain Denham, R.N., the soundings on which are reduced to a spring range of thirty feet, while the mean spring range for that place, as deduced from observations made for two years at the Tide Gauge, St. George's Pier, is 26 feet.
place, more especially when the range of tide is considerable, and the depth not great. Also, when about to enter a port in a vessel whose draught of water is nearly equal to the depth, it is necessary to find the height of tide as exactly as circumstances will permit.
Two classes of questions may be proposed in reference to this subject -- firstly, to find the depth of water at a given place and time; secondly, having obtained the actual depth by a cast of the lead, to find the sounding on the chart corresponding thereto, and thence to identify the ship's place. Both these classes of questions require us to know the time of high water and the range of the tide on the given day; and for this purpose almanacs are published. The most correct, and by far the most useful of all these, are the “Tide Tables” published by the Admiralty, and to which we have already referred. In this book are given the times of high water and the height of the tide for every day in the year, at each of the principle ports in Great Britain.
To find how much we must subtract from casts of the lead, in order to a comparison with the soundings marked on the chart, proceed by
RULE LXXV. © Open the Admiralty Tide Tables at the proper month; and in the column under the head of the place near your position, and opposite the day of the month, take out the "time" of high water in the morning or afternoon, as the case requires, and write it down.
2° Next place underneath the time at ship, and take the difference and call it "time from high water.”
3°. Enter the Admiralty Tide Tables at the proper month; and in the column under the head of the place, and under height, take out the figures which stand opposite the day.
4o. From this subtract the half mean spring range, which stands at tho foot of the column.
The remainder is the half-range of the day.
5°. Enter Table B, page 98, Admiralty Tide Tables; and under the time from high water, and opposite the half-range for the given day, take out the correction corresponding thereto, observing whether it is to be added or subtracted.
6o. Add or subtract the correction, as directed, to the half spring range marked on the chart. The result is the correction to be made to the sounding.
EXAMPLES. Ex. 1. 1869, September 5th, at 8h 39 P.M., a ship off Liverpool strikes soundings in 8 fathoms : required the corrected sounding to compare with the chart. (The half spring range by Captain Denham's chart is 15 feet.)
Admiralty Tide Tables (page 70): time of high water at
ft. in. Height at Liverpool
26 6 Half mean spring range
13 o Half-range of the day
13 6 In Table B, page 98, under 2h, opposite 131 feet, stands add 6 9 Half spring range by chart
15 Correction 31 fathoms, or
9 Depth by lead
8 fathoms Correction ..
3) Showing the depth by comparison 4, Whence the depth to compare with the chart is only 4) fathoms, instead of 8 fathoms.
1869, Oct. 7th, at 7h 20m A.M., a vessel anchored off Weston-super-mare, in 7 fathoms : at low water the vessel was “high and dry :” required the cause of this. (Half spring range by chart 23 feet.)
By Table : October 7th, the time of high water at Weston-
7644m A.M. Time of anchoring
3 Water below the sounding; or, the ship is found to be 3 ft. r in. dry at low water.
3 Ex. 3. 1869, September 8th, at 10h 28m A.M., a vessel has to cross the Victoria Bar, Liverpool: it is required to know what water she will have over the Bar, (Depth at low water springs on chart, 11 feet.)
By Admiralty Tide Table: September 8th, time from
13 6 By Table B: with these quantities the correction is add 6 6 Half spring range by chart
13 Add for Liverpool chart