CHAPTER VII.

TIDES.

ART. 29.-Observation reveals the fact that the waters around our coast are not at the same height at all hours of the day. At times great tracts are uncovered which at other times have several feet of water over them. This is a consequence of what are known as the "Tides," and is due to the attraction by the moon and sun of the waters of the ocean, combined with the rotation of the earth on its axis. The changes in the distances and relative positions of these bodies account for the general variations in the character of the tides, but local influences modify their action, and make the determination of high and low water at any place a matter of extreme difficulty. The rise and fall occurs twice in about 24 hours, which is the mean value of the lunar day, and tends to show that the moon's influence plays the most important part, being more than twice that of the sun's.

Let the figure represent the earth surrounded by an envelope of water, shown by the dotted circle.

Under the action of the moon M, the waters assume an elliptical form, being piled up on the sides nearest to and furthest from the moon, and correspondingly reduced on the sides at right angles to this direction, thus causing the waters to be higher at A and B, in the direction of the line of centres EM; and lower

at C and D at right angles to this direction. As the earth rotates from west to east, this crest moves from east to west. The action of the sun is similar to the moon, and causes two solar tides in a day.

When the centres of the sun and moon are in a line, that is, at new moon, the bodies being in conjunction, or at full moon, when they are said to be in opposition, the solar and lunar effects correspond, and we have the highest or Spring tides.

When the bodies are 90° apart or in quadrature, the solar low water corresponds with the lunar high water, and the lowest or Neap tides occur.

As the moon moves eastward faster than the sun, the combined effect will be to produce a crest to the westward of the moon during the first and third quarters, and thus the time of high water will be hastened; also an opposite effect during the second and fourth quarters, when the time of high water will be retarded. This is called the priming and lagging of the

tides.

The middle between the levels of high water and low water at Springs is called the mean level of the sea (M.T.L.).

The datum line is the level of low water of ordinary Springs, (M.L.W.S.) and the height of a tide is measured above this line unless it is stated otherwise in the Admiralty Tide Tables, where full information is given at the foot of the page for each port showing what the zero of predictions is.

The interval which elapses between the moon's transit each day and the time of H.W. next following is the Lunitidal Interval; the mean of the lunitidal intervals for all days between new and full moon is the Corrected establishment of the Port; the lunitidal interval on the days of new and full moon is the Vulgar establishment, and is registered in Roman figures on Admiralty charts as H.W.F. and C.

ART. 31.-In the Admiralty Tide Tables are given the times of High and Low Water and Heights above the zero of predictions selected for the various ports, around the British Isles and at other places chosen through the world, called "standard" ports. The Tables are in two parts, at the end of Part I. is an index of all standard ports, and of ports referred to a particular standard, and full information is tabulated for each day of the month throughout the year on the pages mentioned. At the end of Part II. is a general index, and on referring to the proper pages the lat., long., H.W.F. and C., and rise at Springs and Neaps are found for each place. Part II. is used when insufficient, or no information is given in Part I. for the required place.

The tides in Part I. are entered in order of occurrence, P.M. times and heights in block type.

The times given are STANDARD or mean time of certain meridians. Local and Greenwich times may be obtained by applying D. Long. in time. The "time differences" show how much earlier (if marked) later (if marked +) high and low water occur at the required port than at the standard.

When no time difference is entered, or place not found in Part I., turn up index Part II. and on the page or pages will be found the Greenwich times of H.W.F. and C. at the required port and at the selected standard. The difference between these is the time difference, and is added (+) when full and change at place is greater than that at standard, subtracted (-)

when less.

The port to be selected as standard is the nearest one geographically to the ship.

REMARK. When the difference is +, and the sum amounts to more than 12", reject 12h and change from A.M. to P.M. on the same day, or from P.M. to A.M. on the day after: it may be necessary to take the P.M. time of tide for the day before, in order to obtain the A.M. time of tide on the required day.

When the difference is -, and the time entered in the Tables is less than the difference, borrow 12", and change from P.M. to A.M. on the same day, or from A.M. on the day after to P.M. on the required day.

Examples.

1. Find the M.T.G. and M.T.S. of H. W. on September 20th off Pembroke Dock.

2. Find the M.T.G. and M.T.S. of H.W. on August 22nd off Llanelly.

M.T.G. H.W. Pemb. Dk. 0h 1TM A.M.

Oh 36m P.M.
+1

0 37 P.M.
- 17

0 20 P.M.

.. No A.M.

3. Find the M.T.G. of H.W. on July 22nd off Limerick.

Dublin T. H.W. Galway 10 1m A.M.

Diff. +1 34

10h 30m P.M. +1 34

4. Find the M.T.G. of H.W. on Oct. 21st off St. Mary's.

M.T.G. H.W. Devonport Oh 31 A.M.

Diff.

1 17

1h 9 P.M. - 1 17

11 52 A.M.

on 20th.

No P.M.

M.T.G. H.W. St. Mary's 11 14 P.M.

5. Find the M.T.G. and M.T.S. of H.W. on Sept. 21st off Lizard Point. H.W. F. & C. Devonport 6h 0m

Lizard Point 5 21

2. Find the M.T.S. of high water—

at Rotterdam on Jan. 19th and 20th.
,, Ayr on Nov. 9th and 10th.

Crinan on Dec. 21st and 22nd

Port Carlisle on Sept. 28th and 29th.
Wicklow on Sept. 30th and Oct. 1st.

3. Find the M.T.G. and M.T.S. of high water

CHAPTER VIII.

REDUCTION OF SOUNDINGS.

ART. 32-Near to land in foggy weather, the greatest security for the navigator is the frequent use of the "lead," and feeling his way by "ground navigation." A line of soundings and specimens of the bottom (brought up by the "arming" of the lead) will generally enable him to fix his position on the chart, or give him due warning of danger.

In Board of Trade inquiries into cases of stranding, the first importance is properly attached to the taking of soundings, and woe to the captain who may be found to have neglected this precaution.

The soundings marked on charts are from the Mean Low-water Level of spring tides, but the actual depth at a place varies with Springs and Neaps and the interval from high water. The difference between the sea-level at the "time of cast" and the Mean Low water of Springs is the Reduction of Soundings.

The Heights of Tides in the Admiralty Tables are measured from the same zero or datum line as used for the chart soundings, but some local tables, e.g. the Liverpool or Holden's Tables, give the heights from a datum line five feet below the Mean Sea-level.

In the diagram, let O (zero) be the mean level of low water spring tides, H the mean high water springs, and M the mean level of the sea or half-tide level; also let h1, ha, hз, hị be the high-water marks of various tides, and l1, la, la, l the corresponding low-water marks, and B the sea-bottom.

Now, if the lead were cast when the sealevel is at O, it would give the same sounding as the chart, but at other times there would be

H

B

FIG. 93.

a difference depending on the height of the tide in the tables and the interval from high water. This difference is the Reduction of Soundings.

H