change places, the winter morning high-tides becoming the fame as the fummer evening high-tides. Some of thefe effects arife from the different diftances of the moon from the earth after a period of fix months, when he is in the fame fituation with respect to the fun; for, if she be in perigee at the time of the new moon, the will, in about fix months after, be in perigee about the time of full moon. Thefe particulars being well known, a pilot may chufe that time which will prove most convenient for conducting a fhip out of any port, where there is not a fufficient depth of water on common fpring tides. Small inland feas, fuch as the Mediterranean and Baltic, are little fubject to tides; because the action of the fun and moon is always nearly equal to the extremities of fuch feas. The tides, in very high latitudes alfo, are very inconfiderable; for the fun and moon acting towards the equator, and always raifing the way ter towards the middle of the torrid zone, the neighbourhood of the poles muft confequently be deprived of the waters, and the fea within the frigid zones must be low in comparison to the other parts. All the things hitherto explained would be exactly obtained, were the whole furface of the earth covered with fea, But fince there are a multitude of iflands, befides continents, lying in the way of the tide which interrupt its courfe; therefore there arife, in many places near the fhores, a great variety of other appearances, befides the foregoing ones, which require particular folutions, in which the fituations of the fhores, ftraits, thoals, winds, and other things, muft neceffarily be confidered. For inftance; as the fea has no vifible paffage between Europe and Africa, let them be fuppofed one continent, extending from 79° north, to 34° fouth: the middle of those two would be in latitude 19° north, near Cape Blanco, on the west coast of Africa. But it is impoffible the flood tide fhould fet to the westward, upon the western coaft of Africa (for the general tide, following the course of the moon, must set from eaft to weft), because the continent, for above 60°, both northward and fouthward, bounds that fea on the east; and therefore, if any regular tide, proceeding from the motion of the sea, from caft to weft, fhould reach this place, it must be either from the North of Europe fouthward, or from the South of Africa northward. This opinion is further corroborated, or rather fully confirmed, by common experience, which fhews that the flood-tide fets to the fouthward along the weft coaft of Norway from the North Cape to the Naze, or entrance of the Baltic Sea, and so proceeds to the fouthward along the east coast of Great Britain, and in its paffage fupplies all thofe ports which lie in its way, one after another. The coaft of Scotland has the tide firft, because it comes from the northward to the fouthward. On the full and change days, it is high-water at Aberdeen at 12h. 45m. but at Tinmouth-bar not till 3h. Rolling thence to the fouthward, it makes high-water at the the Spurn a little after 5h. at Yarmouth Roads a little after 8h at Harwich at 10h. 30m. at the Nore 12h. and at London 2h. 30m. all in the fame day. And although this may feem to contradict the hypothefis of the natural motion of the tides being from eaft to weft, yet as no tide can come weft from the main continent of Norway or Holland, it is evident that the tide we have been tracing, by its feveral ftages from Scotland to London, is fupplied by that tide, the original motion of which is from east to west. water always inclines to the level, it will in its paffage fall to any other point of the compafs, to fill up vacancies where it finds them; and yet not contradict, but rather confirm, the hypothefis. As While the flood tide is thus gliding to the fouthward along the eaft coaft of England, it alfo fets to the fouthward along the weft coafts of Scotland and Ireland; one branch of it falls back northeaft into St. George's Channel; and another runs between Ufhant and the Lizard, into the British Channel. Some may object that this course of the flood-tide, eaft up the Channel, is quite contrary to the hypothefis of the general motions of the tides being from eaft to weft; and confequently of its being high-water where the moon is vertical, or any where else on the meridian. But it may be answered, that this particular direction of the tides does not contradict the general direction of the whole. A river with a western courfe may fupply canals which wind north, fouth, or even eaft, and yet the river keep its natural courfe; and if the river ebbs and flows, the canals fupplied by it would alfo do the fame, although they did not keep exact time with the river; because it would be flood, and the water advanced to fome height in the river, before it reached the farthest part of the canals; and the more remote the extremity of the canals are, the longer time it would require; it may alfo be added, that if it were high-water in the river juft when the moon was on the meridian, he would be far paft it before it could be high-water in the remoteft part of those canals; and the flood would fet according to the courfe of the canals that received in, and could not fet welt upon a canal of a different pofition. As St. George's Channel, the British Channel, &c. are no more in proportion to the vaft ocean, than fuch canals would be to a large navigable river; it will evidently follow that the flood-tide may, among thofe obftructions and confinements, fet upon any other point of the compafs, as well as weft; and may make high-water at any other time, as well as when the moon is upon the meridian, without any wife contradicting the general theory of the tides. Among pilots it is cuftomary to reckon the time of high-water by the point of the compafs the moon bears on at that time, allowing three quarters of an hour for each point. Thus, in places! where it is high-water at noon, on the full and change days, the tide is faid to flow north and fouth, or 12 o'clock. In places where the moon bears 1, 2, 3, 4, or more points to the eastward or Q 2 weftward westward of the meridian, when it is high water on fuch days, the tide is faid to flow on fuch a point; fo, if the moon bear foutheaft, at high-water, it is faid to flow fouth-eaft and north-weft, or 9 o'clock; if the bears fouth-weft, it flows fouth-weft and north-east, or 3 o'clock; and in like manner for every other point of the moon's bearing. From the obfervations of many perfons, the time of high-water on the days of the new and full moon on most of the coafts of Europe, and feveral other places, have been collected; and thofe are generally put in a table, against the names of their refpective places, in an alphabetical order; hence it is called the Tide Table. which is at the end of the Book. The method generally prefcribed for finding the time of highwater at any place, is contained in the following particulars: To find the Leap Year. Divide the given year by 4, if nothing remains, it is leap-year, but if 1, 2, or 3 remains, they fhew that it is fo many years after. Biffextile or Leap-year, as the remainder is: thus, in the year 1806, divided by 4, gives 451, and the remainder [2] fhews it is the fecond year after Biffextile, or Leap-year. To find the Golden Number for any Year. RULE. Add one to the given year, and divide the fum by 19, the remainder will be the Golden Number. EXAMPLE. Required the Golden Number of 1806? By adding one to that year, it gives 1807; this divided by 19 gives 95 for the quotient, and the remainder is 2, the Golden Number for 1806. To find the Epact for any Year. NOTE. The Epact is the moon's age at the beginning of the year, or rather the ift of March. The Epact advances II every year to 30, because the folar year is 11 days longer than the lunar year, and as the Epact increases, it fhews the moon's age at the beginning of the year; it is here fuppofed that at the end of 19 years, the fun and moon make all the variety of fituations they poffibly can with one another, and thence begin, and go over the fame again. The Golden Number at the birth of Chrift was 1, which is the reafon that one is added to the given year, to find the Golden Number. RULE. Divide the given year by 19, the remainder multiply by 1, and the product will be the Epact, if it does not exceed 29; but if it does, fubtract 30 from it as often as you can, and the remainder will be the Epact, for it never exceeds 29. EXAMPLE.' EXAMPLE. What is the Epact of the Year 1806? 1806 divided by 19, gives 95 for the quotient, and £ remaining thews the Epact is (11) for 1806. To find the Moon's Age. To the Epact add the day of the month, and the Epact or number for the month; the fum, if it does not exceed 30, is her age; but if it does, fubtract 30 from it as often as you can, and the remainder is her age. NOTE. The Epact, or number for each month, is found thus: divide the number of days contained between the 1ft of January and the ift day of any month, by 29, the remainder will be the number for that month. Required the Number or Epact for Sept. 1806? The number of days contained between the 1ft of January, 1806, and the ift of Sept. are 243 days, divided by 291, gives 8 for the quotient, and 7 for the remainder, which is the number fought; and fo for any other month. Numbers for the months are nearly as follow: Jan. Feb. Mar. Apr. May June July Aug. Sept. O&. Nov. Dec. In com. years O 2 O 2 2 4 4 6 7 8 9 10 In leap years 2 3 3 5.5 7 8 9 10 II To find the Moon's Southing on any Day of her Age. Since the fun returns to the meridian he has left in the space of 24 hours, and the moon in about 24 hours 49 minutes; therefore, if the moon leaves the meridian at the fame time that the fun does, on any day, the next day fhe will come to the meridian 49 minutes after him, falling back about 49 minutes every day; whence, to find the time of the moon's fouthing, or coming to the meridian on any day, we have this eafy RULE: Multiply the day of her age by 49, and divide the product by 60, the quotient is the hours, and the remainder the minutes afternoon when the fouths. Or, which is rather eafier, and in many respects fufficiently exact for the mariner's purpose; multiply the moon's moon's age by 4, and divide the product by 5, the quotient is the hours, and the remainder multiplied by 12, gives the minutes afternoon when she is upon the Meridian; but if this time exceeds 12, fubtract 12 hours from it, and the remainder is the time of her fouthing in the morning. N. B. From the full moon to the change fhe comes to the meridian, or fouths, in the morning; but from the change to the full, in the afternoon. EXAMPLE. Required the Moon's Southing, Aug. 14, 1806? Number for the month is Day of the month II 6 14 Hence it appears that the moon comes to the south at 49 minutes afternoon. To find the Time of High Water on any Day of the Moon's Age at any Place. RULE. To the time of the moon's fouthing on the given day, add the time of high-water at the full and change, at the given place, taken from the Table; the fum is the hour past noon on the given day when it is high-water at that place; and if this hour exceeds 12, fubtract 12 from it, and the remainder fhews the time of high water in the morning; but if it exceeds 24, fubtract 24 from it, and the remainder fhews the time of high-water in the afternoon. Required the Time of High Water at Milford on the 29th Jan. 1806. EXAMPLE I. |