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TIME OF HIGH WATER BY THE ADMIRALTY
Commencing at about page 104 or 106 of these Tables is a list of ports in the British Islands, and a few on the Continent, from Gibraltar to Heligoland. Seek among these ports the port at which the times of High Water are required: take out the Constant with its sign + or -, and the port for reference.
Next turn to the Index to the Tables at the commencement of the book, and opposite the port for reference, and under the given month is the page of the book you must turn to. Having found the page, proceed by the following rules:
WHEN THE CONSTANT IS +: Write down the morning tide at port for reference, and add the constant. The result is the required A.M. tide if less than 12 hours; but it is the P.M. if more than 12 hours; in which case reject 12 hours.
WHEN THE CONSTANT IS : Write down the afternoon tide at the port for reference, and subtract the constant: the result is the required P.M. tide; but it is the A.M. if 12 hours are borrowed.
WHEN A BLANK (THUS- -) OCCURS IN EITHER OF THE COLUMNS.-Write down the tide from the other column, and apply the constant by its sign, changing A.M. into P.M., or P.M. into A.M., if 12 hours are borrowed or thrown away. The tide thus found is always one of the two required.
TO FIND THE OTHER TIDE.-When the first tide found is the A.M., take out the next following tide; when the first tide found is P.M., take out the previous tide; apply the constant. Take care to change A.M. into P.M., or P.M. into A.M., if 12 hours are borrowed or thrown away.
The result is the other tide, if of a different name to that first found, but if of the same name there is no other tide on the day.
1884.-November 11th.--Find the times of High Water at Lundy Island, A.M. and P.M.
By entering page 105 of the "Tide Tables," we find Lundy Island, and alongside of it the Tidal Constant-1h 39m and the Port of Reference, Westonsuper-Mare.
Turn to page 87, under Weston-super-Mare, and opposite 11th day of November, we find
oh 24m morning or A.M., and 1h 2m afternoon or P.M.
The Constant being — or subtractive, we take the P.M. first, and work it thus:
The A.M. tide being produced first, we take out the following tide to 1h 2m P.M., which will be the following morning's tide, or 1h 41m A.M., on the 12th, and work thus :-
I 41 A.M. on the 12th.
O 2 A.M. on the 12th.
Now, as we are asked for the times of High Water on the 11th, we see from the above results that there is no High Water at Lundy Island between the A.M. tide on the 11th and the A.M. tide on the 12th, that is, there is no P.M. The answer is therefore 11h 23m A.M. and No P.M.
EXAMPLES FOR PRACTICE.
Required the times of High Water at the following places, on the days of the month opposite them for the year 1884.
1.-On November 11th, at Guernsey, Spurn Point, and Swansea.
2.-On December 11th, at Limerick, Westport, and Exmouth.
3.-On October 31st, at Aberdeen, Oban, and Holy Island.
4.-On September 21st, at Strangford Quay, Workington, and Deal.
6.-On December 25th, at Portrush, Ballycastle Bay, and Dieppe.
TO FIND THE GREENWICH APPARENT TIME. Write down the year, the month, the day, hours, minutes, and seconds, and name it A.M. or P.M., as stated in the question. If A.M., add 12 hours and take off one day; then turn the Long. into time, and place it below the hours, minutes, and seconds.
WEST Long., add
EAST Long., subtract.
The result is the Greenwich Apparent Time or G. A. T.
TO FIND THE DECLINATION. Meridian Altitude of the Sun.
Proceed as in the
TO FIND THE TRUE AMPLITUDE.-To the log. sine of the corrected Dec., add the log. secant of the Latitude; the sum (rejecting 10 from the Index) is the] log. sine of the true amplitude.
If A.M., name it E.; if P.M., name it W.;
If Dec. is N., name it N.; if Dec. is S., name it S. Note.-The True Amplitude must always have the E. or W. on its lefthand side, and the N. and S. on its right-hand side.
TO FIND THE ERROR OF THE COMPASS.-Below the true amplitude place the observed amplitude, being very careful to reckon it from the E. or from the W., according as the true amplitude is E. or W. If true and observed amplitudes are both N. or S., subtract; if one is N. and the other S., add. The result is the error of the Compass.
If true is to the right of observed, the error is E.
TO FIND THE DEVIATION.--Below the error of the Compass place the variation which is given in the question.
If error and variation are both E. or both W.,
If error and variation are one E. and the other
The result is the Deviation.
TO NAME THE DEVIATION.-Draw a vertical line N. and S., lay off from North, the degrees in the error to the right or left depending upon the name of the error. Also lay off in the same manner the variation.
Then if error is to the right of variation, deviation is E. But if error is to the left of variation, deviation is W. The following rule is perhaps more simple :—
The deviation is the same name as the error with one exception, that is when the error has been subtracted from the variation.
1885-July 15th, at 7h om A.M., Apparent Time at Ship; in Latitude 34° 5' S.; Longitude 159° 6′ W. The sun's observed amplitude was E. by S. Required the error of the compass, and supposing the variation to be 20° W., required the deviation of the compass.
EXAMPLES FOR PRACTICE.
1885.-February 17th, at 6h 54m P.M., Apparent time at Ship; in Latitude 50° S.; Longitude 29° E. The sun's observed amplitude was W. by N. & N. Required the true amplitude and error of the compass, and, supposing the variation to be 26° W., required the deviation.
2. 1885. April 21st, at 6h 41m A.M., Apparent Time at Ship; in Latitude 40° S.; Longitude 61° 10' W. The sun's observed amplitude was E.N.E. E. Required the true amplitude and error of the compass, and, supposing the variation to be 19° W., required the deviation.
3. 1885.-June 20th, at 5h 2m P.M,, Apparent Time at Ship; in Latitude 30° 10' S.; Longitude 71° 10' E. The sun's observed amplitude was W. by N. Required the true amplitude and error of the compass and, supposing the variation to be 4° W., required the deviation.