TIDES AND THE TIME OF HIGH WATER.

When about to enter a tidal harbour it is often an absolute necessity to know the time of high water. The Admiralty Tide Tables are the best source for obtaining the required information respecting the time of high water at any place, and very full instructions as to their use are given in "Brief Rules in Navigation," pp. 6-8. Next to them stand the Nautical Almanac.

But the calculation of the true Tidal Hour day by day for any given place is a tedious operation, requiring many more elements than the time of the moon's meridian passage, so that the use of the latter will only give an approximation (to half an hour perhaps) at best.

The Tidal Establishment of the Port is the time of the first High Water on the afternoon of the day of full and change of the moon.

A constant is the difference in time between the tidal establishment of a standard port and that of some other port; so that knowing the constant and also the tidal hour on any day at the standard port, the time of high water at the other port is found by adding or subtracting the constant.

The following are the principal Rules for finding the time of high water at any place.

1. By the Moon's Age. This is a very old method, and, at best, only a rough estimate; it may be in error from one to two hours.

RULE. To the epact of the year add the epact of the month, and the day of the month; the result if less than 29 days 13h. is the moon's age at noon of the day; if the result exceeds 29d. 13h., subtract 29d. 13h. for the moon's age. In leap years subtract one day in January and February.

Note.-Every almanac in the present day gives the moon's age, or at least the time of new moon, from which the moon's age can be reckoned, therefore the above process is unnecessary.

Multiply the moon's age by 4 and divide the product by 5; the result is the time of the moon's meridian passage.

For the Time of High-Water.-To the time of the moon's meridian passage add the tidal establishment of the port.

If the sum exceeds 12h. 24m., or 24h. 48m., it gives, in the former case next morning's tide, in the latter case, the afternoon tide of the next day: hence, by subtracting those quantities as required you get the P.M. time of High Water for the given day.

2. By the Admiralty Tide Tables:-see

"Brief Rules," p. 6—8.

These Tables also admit of the finding (approximately) of the time of high

P.M.

43

water at many foreign ports in addition to those for which the "constants given.

are

At the end of the "Tide Tables" are to be found the times of High Water at Full and Change of all the chief Ports in the world, and taking Brest as the Standard Port, the constant will be the difference between the Establishment of the Port (Time of H.W. at F. and C.) at the given place and at Brest (3h. 47m.), which constant is to be used as explained in the "Brief Rules," p. 6—8.

3. By the Nautical Almanac, and the Difference (CONSTANT) of the Tidal Establishments.

In the Nautical Almanac the Mean Time of High Water at London Bridge is given for every day in the year, on the assumption that the Time of High Water on full and change days, or the Establishment of the Port, is 1h. 58m. The first high tide which happens after Mean Noon of any day is inserted in the 1st. column, and the second in the 2nd. column; the day used being the astronomical one.

Where a line (-) is inserted, it indicates that there is only one high tide on that astronomical day. Thus, on Jan. 9, 1880, there is only one high tide; it occurs at 11h. 44m., but the succeeding high tide does not take place until 20m. after mean noon of Jan. 10.

The Table which follows the Tidal Table for London Bridge, gives the times of High Water on full and change days (reckoned from Apparent noon) at numerous places on and near the coast of Great Britain (Naut. Alm. p. 478-9).

These are the Tidal Establishment for the given Ports; and the difference between any one and the Establishment for London Bridge (which is 1h. 58m.) gives a constant, which is to be applied to the Tidal Hour at London Bridge for any given day.

The constant for the Port is additive if its Tidal Establishment exceeds that of London Bridge, otherwise subtractive.

Ex.—Find the A.M. and P.M. tides at Port Glasgow, July 11th, 1880.

h. m.

London Bridge Estab. 1 58 Port Glasgow 18

h. m. London Bridge, July 10d. 16

h. m.

11th 4 9 A.M.
1 40

Constant 1 40

Port Glasgow, July 11th. 2 29 A.M.

Ex.-Find the A.M. and P.M. tides at Sunderland, Sept. 25th, 1880. h. m. h. m. London Bridge Estab. 1 58 London Bridge, Sept. 24d. 17 33 Sunderland 3 22

Constant + 1 24

h. m.

25th 5 33 A.M. Constant + 1 24

Sept.25th 5 53 P.M. +1 24

4. By the Moon's Meridian Passage and the Establishment of the Port:-Find the time of the moon's Mer. Pass. at place, from Naut. Alm. (Green. Mer. Pass.), and applying thereto the correction for Long. + if West, but if East (Table XVI., Norie's Epitome); to this time apply the correction with its proper sign, from Table p. 65 (entering Table with D's Mer. Pass. at side, and D's Semidiam. at top); to the result add the Establishment of the Port, for the time of high water on the afternoon of the given day, unless such sum exceeds 12h. 24m., or 24h. 48m., in which case subtract one

or other of these times, as necessary, from it, and the remainder will be the approximate time of high water on the afternoon of the given day. For the A.M. tide subtract 24m. from the P.M. tide.

Ex.-Required the time of High Water at Reikiavik, Iceland, on June 15th, 1880..

Fig. 7 is intended to explain the terms Spring Rise, Neap Rise, and Neap Range as used on Charts and in Sailing Directions :

Fig. 8 represents the average rate of rise and fall of tide from low water to high water, and vice versa.

Thus, if the total rise be 16 feet one-sixteenth of the
whole in the 1st hour, will give a rise of 1 foot.

An additional three-sixteenths in the 2nd hour, will be
3 feet, and the tide will have risen 4 feet.
An additional four-sixteenths in the 3rd hour, will be
4 feet, and the tide will have risen 8 feet.
An additional four-sixteenths in the 4th hour, will be
4 feet, and the tide will have risen 12 feet.

An additional three-sixteenths in the 5th hour, will be
3 feet, and the tide will have risen 15 feet.

An additional one-sixteenth in the 6th hour, will be 1 foot, and the tide will have reached 16 feet, its total rise:

Its fall to low water will be at the same rate.

Similarly, if the total rise and fall be 12 feet :

1-sixteenth in the 1st hour will be 9 inches.

3-sixteenths in the 2nd hour will be 2 ft. 2 in. and give the rise 3 ft. 4-sixteenths in the 3rd hour will be 3 ft. and give the rise 6 ft. 4-sixteenths in the 4th hour will be 3 ft. and give the rise 9 ft. 3-sixteenths in the 5th hour will be 2 ft. 3 in. and give the rise 11 ft. 3 in. 1-sixteenth in the 6th hour will be 9 in. and give the total rise of 12 ft. From high water to low water the fall will occur in the reverse order,-thus, successively to 11 ft. 3 in., to 9 ft., to 6 ft., to 3 ft., to 9 in., to low water.

For places given in the Admiralty Tide Tables there are special rules for finding the depth at different hours, and to these you can refer.

NOTES ON THE MAGNETISM OF IRON SHIPS.

In an iron ship, the hull and fittings of which consist partly of hard iron and partly of soft iron, magnetism (more or less intense) is developed by the process of hammering while she is on the stocks. N. polarity is developed in the part of the ship which is below and towards the North, and S. polarity in the part which is above and towards the South. Thus, an iron ship partaking of the character of a large magnet, affects the compass on various azimuths much as a magnet would, besides which there is the effect of the soft iron which, from position, becomes more or less magnetic,

As the usual position of the steering compass is near the stern, it follows that if the ship has been built head North, that compass is in the part of the ship where S. polarity attracts the North end of the compass-needle strongly to the stern and downwards, the effect of which is a large deviation when the ship is on an even beam, besides a large heeling error; in this case, therefore, it is of importance to have the standard-compass well forward. Near to the bow N. polarity repels the North end of the compass-needle.

In a ship built head South the magnetic conditions just mentioned are reversed. The N. polarity of the stern repels the North end of the compass

needle, and the S. polarity of the bow attracts it. On the whole there is generally less deviation, and little heeling error in the usual position of the steering compass. The standard compass should also be aft.

The place of little or no deviation in a ship built head North is towards the bow, but in a ship built head South, towards the stern.

In a ship built head East, or head West, the whole of the lower part has N. polarity, and the whole of the upper part S. polarity. But S. polarity predominates on the starboard side in a ship built head East, attracting the North point of the compass-needle to starboard: and S. polarity predominates on the port side in a ship built head West, attracting the North point of the compassneedle to port-for compasses on the upper deck. The deviation in both cases is rarely large, but less regular than in ships built head South.

Theoretically there should be no spot of no deviation on the deck of ships built East or West.

From the special magnetic property developed in a ship according to her position when building, it follows that for a compass aft, in the usual place of the steering compass, the character of the deviation may be approximately represented in a tabular form, as follows:

Besides the Deviation that an iron ship has when on an even beam, it is subject to be altered in character and amount by the vessel's heeling to starboard or port; and the maximum heeling errors are found on northerly and southerly courses, none on east and west courses (by compass). The amount of heeling error would, under ordinary circumstances, have to be found by heeling in the dock or river, or, when at sea, by observing the error for 10° of heel to starboard or port, when on or near North and South by compass. As for the character of this deviation (E. or W.) in the northern hemisphere, in the majority of iron ships, the North end of the compass needle is drawn to windward (or to the weather side), when the binnacles are above the upper deck, and the following results would arise from a disregard of this heeling error :—

"If the ship be kept steady on one compass course, she will be found to windward of her supposed position when on northerly courses, and to leeward on southerly courses. If she be steered steadily for a fixed point on the horizon, she will appear to fall off as she heels on northerly courses, and to come up on southerly courses." Therefore, as a general rule, it should be borne in mind "that in steering by compass and wishing to make a straight course, we must keep away, by compass, on either tack, as the ship heels, when on northerly courses; and keep closer to the wind, by compass on either tack, when on southerly courses." Captain F. J. Evans, R N., F.R.S.