« ForrigeFortsett »
other Places, and these Times are commonly put in a Table against the Names of the Places, in an Alphabetical &. for which Reafon, it is called the Tide Table; but in this Treatise, the Times of high Water at Full and Change Days, are set down after the Latitude and Longitude of each Place, in the Table of Latitudes and Longitudes of Places.
RULE. To the Time of the Moon's Southing on the given Day, add the Time of High Water at Full and Change at the given Place, taken from the Table; the Sum is the Hour, past Noon, on the given Day, when it is High Water at that Place; and if this Hour exceeds 12, subtract 12 from it, and the Remainder shews the Time of High Water in the Morning; but if it exceeds 24, subtract 24 from it, and the Remainder shews the Time of High Water in the Afternoon.
E x A M P L E III. -Required the Time of Hig Water at the Start Point, July 10, 1796? - aćt
Hence it appears that it is High Water at the Start at ten o’Clock in the Afternoon. This is the Method of finding the Time of High Water inserted in common Books of Navigation, which, at Times, will give the Moon's Age whole Days, and the Time of her Southing, and of High Water, Hours wide of the Truth; and even if the Moon's Southing be exactly found, yet the Tides may differ less or more from the com. puted Time; for the Floods do not always happen at the same Distance of Time from each other, but at different Distances, according to the Times of the Moon's Age, and her Aspect with Respect to the Sun, or as the Waters are acted upon by the Sum or Difference of the attractive Forces of the Sun and Moon, and also on Account of Winds and Storms, even when out of Hearing, which reatly affected the Tides; the real Time of High Water at any lace will often differ from the computed Times; therefore Pilots and all concerned, would do well to use the following Method, which will, in general, give the Time of High Water within Half an Hour of the #. when the Tides are not greatly influenced by the Wind. - -
A TABLE for the ready finding the Day of the Month the New
Moon will fall on till the Year 1900, and consequently the Moon's Age, for any Day, by knowing the Golden Number, according to the Method commonly used for finding, the Moon's Age.—Add 1 to the given Year, and divide the Sum by 19, the Remainder will be the Golden Number, which, being marked with Pencil at the
top of the Table, will shew the Golden Number d
uring that Year.
The Use of the foregoing Tables.
To find the Moon's Age, on any given Day, look in the first Column (marked Gn. No.) for the Golden Number, and under the Month, on the same Line, stands the Day of the New Moon; then count the Pays which have completely passed fince the last Change, and they will be her Age on the given Day.
To find the Time of High Water.
Look for the Moon's Age in the Table of Times, and the Hours and Minutes opposite to which being added to the Time of High Water, on the Change and Full Days, at any Place, will, if it does not exceed 12 Hours, give the Time of High Water there in the Afternoon of the given Day; but if it does exceed that Number, take 12 from it, and the Remainder will shew the Time of High
Water in the Morning.
E X A M P L E I. At what Time will it be High Water at London, April 29, 1796?
Opposite 11 the Golden Number, and under April, I find it was New Moon the 7th Day; and reckoning forward to April 29, gives 22 Days, for the Moon's Age.
Against 22, in the Table of Times, stand 4 Hours 36 Minutes, to which add 3 Hours, the Time of High Water at London on the Full and Change Lays, and that gives 7 Hours 36 Minutes, the Time of High Water at London, in the Morning ; differing one Hour from that found in the first Example by the common Method.
E X A M P L 'E II.
Opposite to 11 the Golden Number, and under Oétober, I find it was New Moon the 1st Day; reckoning forward to O&tober 12, I find the Moon's Age is 11 Days ; against II in the Table of Times stand 8 Hours and 8 Minutes. This added to Io Hours 30 Minutes, the Time of High Water on Full and Change Days at Dover, gives 18 Hours 38 Minutes; from which I take 12, and the Remainder 6 Hours 38 Minutes is the Time of High Water in the Morning at Dover on the given Day; differing from that found in the second Example by the common Method, 41 Minutes.
- E X A M P I, E III. What Time will it be High Water at the Start, july 10, 1796 ° By the Tables it was New Moon the 4th Day, and reckoning forward to the 10th, I find there are 5 Days completely past. Against 5 in the Table of Times, stand 3 Hours 2 Minutes, which added to 6 Hours the Time of High Water at the Start, on Full and Change Days, gives 9 Hours 2 Minutes that Time of High Water in the Afterroon, on the above Day; differing from that given by the former Method, 58 Minutes. 2 In
In like Manner may the Time of High Water be found at any other Place. Whoever will compare these Examples with the Methods used in other Books of this Kind, will find a considerable Difference, as they frequently give the Moon's Age several Days, and of High Water, Hours wide of the Truth; indeed, on Account of the Irregularities in the Moon's Motion, the Times of her Change may differ half a Day from the Truth; and the Time of High Water 30 Minutes, but seldom more, if the Sea is not greatly influenced by the Wind. -
The Tides do not always answer to the same Distance of the Moon from the Meridian, at the same Places, but are variously affected by the Aétion of the Sun, which brings them on sooner when the Moon is in her first and third Quarters; and keeps them back later when she is in her second and fourth Quarters; because, in the förmer Case, the Tides raised by the Sun alone would be earlier than the Tide raised by the Moon; and in the latter Case later,” as may be seen in the Table of the Shifting of the Tides. As the Nautical Almanack is become now of general Use in long Voyages, the Time of High Water at any Part of the World may be readily found, if the Time making High Water at Full and Change be known; for in the sixth Page of each Month is given the Time of the Moon's Passage over the Meridian of Greenwich every Day: This Time may be reduced to the Meridian of any other Place, by allowing 1 Hour for every 15 Degrees of Longitude. To this Time add the Time making High Water there, on Full and Change Days, which gives the Time of High Water nearly on that Day, if the Sum be less than 12 Hours; but if above, subtract 12 Hours, or 24 Hours from it; observing, that the Days in this Almanack begin 12 Hours later than the common Day.
Among Pilots it is customary to reckon the Time of Flood, or High Water, by the Point of the Compass the Moon bears on at that Time, allowing three Quarters of an Hour for each Point. Thus, in Places where it is Flood at Noon on the Days of Full and Change; the Tide is said to flow North and South, or at 12 o’Clock. In Places where the Moon bears 1, 2, 3, 4, or more Points to the East or West of the Meridian, when it is High Water on the same Days, the Tide is said to flow on such a Point; so if the Moon bears S. E. at Flood, it is said to flow S. E. and N. W. or 3 Hours before the Meridian, that is, 9 o'Clock; if she bears S. W. it flows S. W. and N. E. or at 3 Hours after the Meridian; and in like Manner for other Points of the Moon's bearing, as in the following Table: