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. Add the number taken from this Ta-Day H. M. H. M. March. Feb. April. May. June. Aug. July Sept. Oct. Nov. Dec. 0000 0 35 049 1 10 1 38 146 226 2 22 3 15 3 144 10 20 20 34 30 50 3 44 453 41 31820 15 1716 18 18 20 51821 26 28 26 28 29 1 50 7 70 35 10 815 8 8 110 14182429 1 0 1 2 4 4 6 8 8101615 12 91212 120 0 110 15 1825121311121314151618191921 16 12 44 13 1 17 13 19 13 50 150 171826 22 24 23 25 24 25 25 27 29 29 1 218 13 5414 38 140 34 01332 28 29 28 0 1 3 3 5 7 7 8 929124 024 0 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 ng glected. Ff 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, and re-published by Edm. M. Blunt, to accompany his Chart of the Western Ocean, in 1819. 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 the West-India Islands. 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? BY CALCULATION. The shortest method of calculating this problem is by means of Table I. as in the adjoined Traverse Table; putting in it the course sailed by the ship, and the set of the current, and finding the difference of latitude and departure by that Table, then find the course and distance made good, as in Case VI. Plane Sailing. In the present example the course is N. E. & N. and the distance 74 miles nearly. TRAVERSE TABLE. E. W. 54.4 26.5 5.3 81.5 26.5 54.4 5.3 Diff. Lat. 55.0 Dep. 49.1 METHOD OF KEEPING A SHIP'S RECKONING OR JOURNAL AT SEA. 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. THE LOG-BOARD.. H. K. F. Courses. Winds. Leeway Transactions. 2 61 S. W. N. E. 10 12 E. N. E. N.W.by W. 2 4 Moderate gales & fair weather; at 8 A. M. saw a ship to the northward. 8 S. W. W. N. W. 1 4 5 No observation. sert. 4 6 The daily occurrences on board a ship are marked on a board or slate, called the log-board or log-slate, kept in the steerage for that purpose, being usually divided into seven columns; the first contains the hours from noon to noon, being marked by some for every two hours, but by others for every single hour; in the second and third columns 10 are the knots and fathoms 12 the ship is found to run per hour, set against the hours when the log was hove. Some navigators do not divide the knot into ten fathoms, but into half knots only, marking the third column H. K. The fourth column contains the courses steered by compass; the fifth, the winds; the sixth, the lee-way,* and the seventh, the alteration of the sails, the business done aboard, and what other remarks the officer of the watch thinks proper to inFor it should be observed, that it is usual to divide a ship's company into two parts, called the starboard and larboard watches, who do the duty of the ship for four hours and four hours, alternately, except from 4 to 8 P. M. which is divided into two watches. The remarks made on the log-board are daily copied into a book called the LOG-BOOK, which is ruled like the logboard. This book contains an authentic record of the ship's transactions, and the persons who keep a reckoning, transcribe them into their journals, and from thence make the necessary deductions relative to the ship's place, every day at noon, which operation is called working a day's work. While a ship is in port, the remarks entered in the log-book are called harbour work, or harbour journals, and the day is then estimated according to the civil computation as on shore, that is from mid-night to mid-night; but at sea the day's work ending at noon is dated the same as the civil day, so that the day's work marked Monday began on Sunday noon, and ended on Monday at noon; the day thus marked is called a nautical day; the first 12 hours being marked P. M. the latter A. M. There are various ways of keeping journals at sea, according to the different tastes of navigators. Some keep only an abstract of each day's transactions, specifying the weather, what ships or lands were seen, accidents on board, the latitude, longitude, course, and run these particulars being drawn from the ship's log-book. Others keep a full copy of the log-book, and the deductions drawn therefrom, arranged in proper columns:-this is the most satisfactory method to those who may have occasion to inspect the journal; and we have adopted it In the following, but shall give an abstract at the end conformable to the other method. When a ship is about losing sight of the land, the bearing of some noted place (whose latitude and longitude are known) must be observed, and its distance estimated and marked on the log-book: this is called taking a departure. In working this first day's work, the calculation is to be made in the same manner as if the ship had sailed that distance from that place upon a course opposite to that bearing, and that course and distance are to be entered accordingly into the traverse table, after allowing for the variation. To allow for the Variation. We have already taught the methods of finding the variation, which must be allowed on all courses steered, and on all bearings taken with the compass; to the right hand. if the variation be east; but to the left hand, if west; the observer being supposed to be placed in the centre of the compass, looking towards the point from which the variation is to be allowed. The cause of the lee-way and manner of allowing for it, are explained in the following paget. To find the lee-way and allow for it. E. S. S. W. S. N. E.E. The courses must likewise be corrected for lee-way, the nature of which may be thus explained. When a ship sails upon a wind, in a fresh gale, that part of the wind which acts upon the hull and rigging, together with a considerable part of the force exerted on the sails, tend to drive her immediately from the direction of the wind, or, as it is termed, to leeward. But since the bow of a ship exposes less surface to the water than the side, the resistance will be less in the first case than in the second; the velocity therefore in the direction of her head will, in most cases, be greater than the velocity in the direction of her side, and the ship's course will be between the two directions, and the angle contained between the course towards which the ship's head is directed, and the course she really describes through the water, is termed her lee-way. The quantity of lee-way to be allowed will depend upon a variety of circumstances; as the mould and trim of the ship; the quantity of sail she carries; her velocity through the water, &c. hence no general rules can be laid down with accuracy that will determine the quantity of lee-way in all cases. The following have, however, been usually given by most writers on navigation. 1. When a ship is close hauled with all her sails set, the water smooth, and a light breeze of wind, she is then supposed to make little or no leeway. 2. When the top-gallant sails are handed, allow 1 point. 5. When under close reefed topsails, allow 2 points. As these allowances depend entirely on the quantity of sail set, without regard to any other circumstance, it is evident that they can be considered only as probable conjectures, and may indeed serve to work up the day's work of a journal that has been neglected. But since the computation of a ship's way depends much upon the accuracy of this allowance, it would be proper for the officer of the watch to mark the lee-way on the log-board, in the column reserved for that purpose. The lee-way may be estimated by observing the angle which the wake of the ship makes with the point right astern, by means of a semi-circle marked on the taffrail, and divided into points and quarters; by means of which the angle contained between the direction of the wake and the point of the compass directly astern, may be easily ascertained. The lee-way thus determined is to be allowed on all courses steered, to the right hand of the course steered, when the larboard tacks are aboard,* but on the left hand, when the starboard tacks are aboard; the person making the allowance being supposed to be looking towards the point of the compass the ship is sailing upon. |