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to which is a number of minutes to be applied to the time of passing the meridian at Greenwich, by adding when in west longitude, but subtracting when in east longitude; the sum or difference will be nearly the time that the moon passes the meridian of the given place. With this time enter Table B, and take out the corresponding correction, which is to be applied to the time of passing the meridian of the place of observation, by adding or subtracting, according to the direction of the table.

To this corrected time add the time of full sea on the full and change days; the sum will be the time of high water at the given place, reckoning from the noon of the given day. If this sum be greater than 12h. 24m, you must subtract 12h. 24m. from it, and the remainder will be the time of high water nearly, reckoning from the same noon; or if it exceed 24h. 48m. you must subtract 24h. 48m. from that sum, and the remainder will be the time of high water, reckoning from the same noon nearly.

EXAMPLE I.

Required the time of high water at Charleston (S. C.) March 17, 1820, in the afternoon, civil account?

By the Nautical Almanac I find that the moon passed the meridian of Greenwich at 2h. 31m.: to this I add 11m. taken from Table A, corresponding to the longitude of Charleston. With the sum 2h. 42m. I enter Table B, and find (by taking proportional parts) that the correction is 45m. which is to be subtracted from 2h. 42m. (because immediately over it in the table it is marked Sub.); to the remainder 1h. 57m. I add the time of high water on the full and change days 7h. 15m. (which is found in the tide table following;) the sum 9h. 12m. is the time of high water on the afternoon of March 17, 1820, civil account.

EXAMPLE II.

Required the time of high water at Portland, (Mass.) May 25, 1820, in the afternoon, civil account?

By the Nautical Almanac the moon will pass the meridian of Greenwich at 8 hours 49 minutes. The correction from Table A, corresponding to 70° the longitude of Portland is 9m. which added to 8h. 49m. gives the time of the moon's southing at Portland 8h. 58m. nearly. The number in Table B corresponding to 8h. 58m. is 23m. which is to be added to 8h. 58m. (because immediately over it, in the table, is marked Add.) To the sum 9h. 21m. I add the time of high water, on the full and change days, 10h. 45m. and the sum is 20h. 6m. consequently the high water is at 20h. 6m. past noon of May 23, that is, at 8h. 6m. A. M. of May 24. And by subtracting 12h. 24m, from 20h. 6m. we have 7h. 42m. which will be nearly the time of high water on the afternoon of May 23, 1820.

In this manner we may obtain the time of high water at any place, to a considerable degree of accuracy. But the tides are so much influenced by the winds, freshets, &c. that the calculated times will sometimes differ a little from the truth.

Many pilots reckon the time of high-water by the point of the compass the moon is upon at that time, allowing 45 minutes for each point. Thus on the full and change days, if it is high water at noon, they say a north and south moon makes full sea; and if at 11h. 15m. they say a S. by E. or N. by W. moon makes full sea; and in like manner for any other time. But it is a very inaccurate way of finding the time of full sea by the bearing of the moon, except in places where it is high-water about noon on the full and change days.

When you have not a Nautical Almanac, you may find the time of high water by means of the following tables C and D; and although the former method is the most accurate, yet the latter may be useful in many cases. To calculate the time of full sea by this method, observe the following rule.

RULE.

Enter Table C, and take out the number which stands opposite to the year, and under the month for which the tide is to be calculated; this number, added to the day of the month, will give the moon's age, rejecting 30 when the sum exceeds that number. Against her age found in the left hand column of Table D, is a number of hours and minutes in the adjoined column, which being added to the time of high water at the given place on the full and change days, will give the time of high water required, observing to reject 12h. 24m. or 24h. 43m. when the sum exceeds either of those times.

By this rule I shall work the two preceding examples.

EXAMPLE III.

Required the time of high water at Charleston (S. C.) March 17, 1820, in the afternoon, civil account?

In the table C, opposite 1820, and under March, stand 16, which, added to the day of the month 17, gives 33, and by subtracting 30, leaves S, the moon's age: opposite 3 in Table D, is 1h. 46m. which added to 7h, 15m. the time of high water on the full and change days, gives 9h. 1m.for the time of high water; differing eleven minutes from the former method.

EXAMPLE IV..

Required the time of high water at Portland, (Mass.) May 23, 1320, in the afternoon, civil account?

In the Table C, opposite 1820, and under May, stand 18, which added to the day of the month 23, gives (by neglecting 50) the moon's age 11; opposite to this, in Table D, is 9h. 19m. which added to 10h. 45m. the time of high water on the full and change days, gives 20h. 4m. from which subtracting 12h. 24m. there remains 7h. 40m. for the time of full sea May 25, 1820; this differs 2 minutes from the former method.

In the third column of Table D is given the time of the moon's coming to the meridian, for every day of her age: thus, opposite 11 days stand 8h. 57m. which is the time of her coming to the meridian on that day. This table may be of some use when a Nautical Almanac cannot be procured; but being calculated upon the supposition that the moon moves uniformly in the equator, the table cannot be very accurate. The numbers in this Table are reckoned from noon to noon; thus, 1h. A. M. is denoted by 13h.; 2h. A. M. by 14h. &c.

The time of new moon is easily found, by subtracting the number taken from Table C from 30. Ex. Suppose it was required to find the time of new moon for May, 1820? By examining the table, we find the number corresponding to that time is 18; this subtracted from 30 leaves 12; therefore it will be new moon the 12th. May, 1820.

When the time of high water is known for any day of the moon's age, we may from thence find the time of high water on the full and change days, by the following

RULE.

Find the time of the moon's coming to the meridian of Greenwich, in the 6th. page of the Nautical Almanac; to this time apply the corrections taken from the tables A and B, (in the same manner as directed in the preceding rule for finding the time of high water) subtract this corrected time from the observed time of high water, and the remainder will be the time of high water, on the change and full days.

NOTE. If the time to be subtracted be greater than the observed time of full sea, you must increase the latter by 12h. 24m, or by 24h. 48m. nearly.

EXAMPLE.

ton, (S. C.) was found to be at 9h. 12m. P. M. required the time of high water on the full and change days?

I find, as in example 1st. preceding, that the number to be subtracted is 1h. 57m.-taking this from 9h. 12m. leaves 7h, 15m. which is the time of high water on the full and change days.

When you have not a Nautical Almanac, you may find the time of high water on the full and change by means of the Tables C and D. For in the present example, I find by Table C, that the moon's age was 3, corresponding to which, in the second column of Table D, is 1h. 46m. this subtracted from 9h. 7m. leaves 7h. 21m. for the time of high water on the full and change days.

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In all the preceding calculations of the time of high water, we have neglected the correction arising from the variation of the distances of the sun and moon from the earth, and from the different declinations of those objects. These causes might produce a correction of 10 or 12′ in the time of high water, but in general will be much less, and may therefore be ne glected,

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CURRENTS.

A CURRENT is a progressive motion of the water, causing all floating bodies to move that way towards which the stream is directed. The set of a current, is that point of the compass towards which the waters run, and its drift is the rate it runs per hour. The most usual way of discovering the set and drift of an unknown current, is thus:

Let three or four men take a boat a little way from the ship: and by a rope fastened to the boat's stern, let down a heavy iron pot or loaded kettle to the depth of 80 or 100 fathoms; then heave the log, and the number of knots run out in half a minute will be the miles the current sets per hour, and the bearing of the log will show the set of it.

There is a very remarkable current, called the GULF STREAM, which sets in a north-east direction along the coast of America, from Cape Florida towards the Isle of Sables, at unequal distances from the land, being about 75 miles from the shore of the southern states, but more distant from the shore of the northern states; the width of the stream is about 40 or 50 miles, widening towards the north; the velocity is various from one to three knots per hour, or more, being greatest in the channel between Florida and the Bahamas, and gradually decreasing in passing to the northward; but is greatly influenced by the winds both in drift and set.

We are chiefly indebted to Doctor Franklin, Commodore Truxton, and Mr. Jonathan Williams, for the knowledge we possess of the direction and velocity of this stream; its general course, as given by them, is marked on the chart affixed to this work. They all concur in recommending the use of the thermometer, as the best means of discovering when in, or near the stream. For, it appears by their observations, that the water is warmer than the air when in the stream; and that at leaving it, and approaching towards the land, the water will be found six or eight degrees colder than in the stream, and six or eight degrees colder still, when on soundings. Vessels coming from Europe to America, by the northern passage, should keep a little to the northward of the stream, where they may probably be assisted by a counter current, as is observed by Commodore Truxton. When bound from America to Europe, a ship may generally shorten her passage by keeping in the gulf. By steering N. W. you will generally cross the gulf in the shortest time, as the direction of the stream is nearly N. E. Those who wish for further information on this subject, may consult an ingenious treatise on "Thermometrical Navigation," published by Mr. Jonathan Williams, at Philadelphia, in 1799.

In the other parts of the Atlantic ocean the currents are variable, but are generally south-easterly, along the coast of Spain, Portugal and Africa, from the bay of Biscay towards Madeira and the Cape de Verds. Between the tropics there is generally a current setting to the westward.

There is also a remarkable current which sets through the Mozambique channel, between the Island of Madagascar and the main continent of Africa, in a south-westerly direction: in proceeding towards Cape Lagullas the current takes a more westerly course, and then tends round the Cape towards St. Helena. Ships bound to the westward from India, may generally shorten their passage, by taking advantage of this current. On the contrary, when bound to the eastward, round the Cape of Good Hope, they ought to keep far to the southward of it. However there appears to be a great difference in the velocity of this current at different times; for some ships have been off this Cape several days endeavouring to get to the westward, and have found no current; others have experienced it setting constantly to the westward during their passage from the Cape towards St. Helena, Ascension and

All cases of sailing in a current are calculated upon the principle, that the ship is affected by it in the same manner as if she had sailed in still water, with an additional course and distance exactly equal its set and drift; on this principle the projection and calculation of any problem of this kind may be easily made.

EXAMPLE.

If a ship sails 98 miles N. E. by N. in a current which sets S. by W. 27 miles in the same time; required her true course and distance?

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A SHIP'S RECKONING is that account, by which it can be known at any time where the ship is, and on what course or courses she must steer to gain her port. DEAD RECKONING is that account deduced from the ship's run from the last observation.

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