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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.f
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 vice versa. An examination of these quantities will show, that in the first hour of the tide the rise is 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
1o. 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.
4. From this subtract the half mean spring range, which stands at the 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.
6°. 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.
Ex. 1. 1869, September 5th, at 8h 39m 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
Time from high water
Height at Liverpool
Half-range of the day
In Table B, page 98, under 2h, opposite 13 feet, stands add 6 9
By Table B, 24m and half range 22 feet give
Showing the depth by comparison Whence the depth to compare with the chart is only 4 fathoms, instead of 8 fathoms. Ex. 2. 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
Correction to low water
Correction By chart: depth on Victoria Bar at low water springs Depth on Bar at 2b 7m from high water, September 8th,
Water below the sounding; or, the ship is found to be 3 ft. 1 in. dry at low water. 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
Half range for the day (see Ex. 1 and 2.)
By Table B: with these quantities the correction is add
10h 39m P.M.
EXAMPLES FOR PRACTICE.
Ex. 1. 1869, June 22nd, at 6h 48m P.M.: required the depth of water on the "four-fathom Ledge" off Weston-super-mare.
Ex. 2. 1869, August 14th, at 5h 6m A.M.: required the depth of water at the Fairway Buoy in the Old Formby Channel (Admiralty Soundings, 2} fathoms.) Ex. 3. 1869, June 13th, at 10h 26m A.M., a vessel having to cross the Victoria Bar, Liverpool: required the depth of high water at that time.
Ex. 4. 1869, March 7th, at 7h 46m P.M., a vessel anchored off Weston-super-mare, in 6 fathoms required the depth at low water.
FOR THE STOWAGE OF MIXED CARGOES. Prepared by HENRY C. CHAPMAN & Co., Agents for Lloyd's, Liverpool.
1. Owners, Commanders, and Mates of ships, are considered in law in the same situation as common carriers, it is therefore necessary that all due precautions be taken to receive and stow cargoes in good order, and deliver the same in like good order. The law holds the shipowner liable for the safe custody of the goods when properly and legally received on board in good order, and for the "delivery" to partios producing the bill of lading. The captain's blank bill of lading should be receipted by the warehouse keeper, or person authorised to receive the contents. Goods are not unfrequently sent alongside in a damaged state, and letters of indemnity given to the captain by the shippers for signing in good order and condition; this is nothing more or less than conniving at fraud; fine goods are also often damaged in the ship's hold by lumpers, if permitted to use cotton hooks in handling bales. All goods must be received on board according to the custom of the port where the cargo is to be taken in; and the same custom will regulate the commencement of the responsibility of the master and owners.
2. Hemp, flax, wool, and cotton, should be dunnaged 9 inches on the floors, and the upper part of the bilge, the wing bales of the second tier kept 6 inches off the side at the lower corner, and 2 inches at the sides. Sand or damp gravel ballast to be covered with boards. Pumps to be frequently sounded and attended to. Sharp bottomed ships one-third less dunnage in floor and bilges. Avoid horn shavings as dunnage from Calcutta.
3. Oil, wines, spirits, beer, molasses, tar, &c., to be stowed bung up; to have good cross beds at the quarters (and not to trust to hanging beds);
to be well chocked with wood, and allowed to stow 3 heights of pipes or butts, 4 heights of puncheons, and 6 heights of hogsheads or halfpuncheons. All moist goods and liquids, such as salted hides, bales of bacon, butter, lard, grease, castor-oil, &c., should not be stowed too near, "dry goods," whose nature is to absorb moisture. Shipowners have often to pay heavy damages for leakage in casks of molasses, arising from stowing too many heights without an intervening platform or 'twixt decks. From Bengal, goods also are frequently damaged by castor-oil.
4. Tea and flour, in barrels; flax, clover, and linseed, or rice, in tierces; coffee and cocoa, in bags; should always have 9 inches, at least, of good dunnage in the bottom, and 14 to the upper part of the bilges, with 24 inches at the sides; allowed to stow 6 heights of tierces, and 8 heights of barrels. All ships above 600 tons should have 'twixt decks or platforms laid for these cargoes to ease the pressure-caulked 'twixt decks should have scuppers in the sides, and 24 inches of dunnage laid athwartship, and not fore-and-aft ways, when in bags or sacks; and when in boxes or casks not less than 1 inch. Rice, from Calcutta, is not unfrequently damaged by indigo, for want of care in stowing.
5. Entire cargoes of sugar, saltpetre, and guano, in bags, must have the dunnage carefully attended to, as laid down for other goods. Timber ships are better without 'twixt decks if loading all timber or deals. Brown sugar to be kept separate from white sugar, and both kept from direct contact with saltpetre.
6. Pot and pearl-ashes, tobacco, bark, indigo, madders, gum, &c., whether in casks, cases, or bales, to be dunnaged in the bottom, and to the upper part of the bilges, at least 9 inches, and 24 inches at the sides.
7. Miscellaneous goods, such as boxes of cheese, kegs and tubs of lard, or other small or slight-made packages, not intended for broken stowage, should be stowed by themselves, and dunnaged as other goods.
8. Barrels of provisions and tallow casks, allowed to stow 6 heights. All metals should be stowed under, and separated from, goods liable. to be damaged by contact.
9. All manufactured goods, also dry hides, bales of silk, or other valuable articles, should have 2 inches of dunnage against the side, to preserve a water-course. Bundles of sheet-iron, rods, pigs of copper or iron, or any rough hard substance, should not be allowed to come in contact with bales or bags, or any soft packages liable to be chafed. When mats can be procured, they should be used at the sides for silk, tea, &c.