NAVIGATION

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NAVIGATION ; whatever relates to traversing the sea in ships; the art of ascertaining the geographical position of a ship, and directing her course.Horace has well said, that his heart must needs have been bound with oak and triple brass, who first committed his frail bark to the tempestuous sea. Nothing, indeed, conveys a higher idea of human daring than the boldness with which man rushes forth to encounter the elements: nothing speaks louder in praise of hu man ingenuity than that wonderful art by which he is enabled to forsake the land, stretching forth until it fades from the horizon, and nothing visible remains but the hollow heavens above, and a trackless waste below; driven from his course by adverse winds, yet, by dint of perseverance, wearying out the elements; and, at length, arriving, with unerring certainty, at the haven where he would be. And if the daring and ingenuity of the navigator deserve our admiration, the result of his efforts will not appear unworthy of the means. It is to the exercise of his wonderful art, that we are indebted for the improvement of our condition, which arises from the exchange of the superfluity of one country for that of another, the whole world being penetrated, and every clime made tributary to every other, until the whole globe is reduced to one common country. Above all, to navigation are we indebted for that higher and nobler advantage,the interchange of sense and sentiment, which makes wisdom common to the world, and urges man onward to perfection. Yet it has not always been so. Time was when the canoe, or the raft, constituted the only ship of the sailor, and when the narrow precincts of a lake or river set bounds to his roving disposition, and confined him within view of familiar objects. Advancing a step farther, we find him venturing from headland to headland, or from island to island, with a view of gratifying his curiosity, or bettering his condition, until a gale, driving him to some unknown coast, increases at once' his knowledge and hardihood. Meantime, his bark adapts itself to nobler functions, enlarges its size, and improves in form : the rudder is added, the mast is better sustained, and the sail receives a more favorable application. And thus the art by which the ship is made, and that by which it is conducted, advance with equal steps. Deprived of the aid of surrounding objects, the land withdrawn from view, and nothing within the verge of the horizon but a waste of trackless water, the mariner casts his eyes in despair to the overhanging heavens. Aid is granted to his prayers: the constellations assist him in his course: among many revolving stars, he finds one steadfast, and makes it his perpetual guide Such do we find the actual state of navi gation among the savage tribes of our own day; and such was also the progress of the art among the earliest nations that improved it. Not the least of the improvements which we have made in this art, is that simplification in practice, by which it is rendered available with little study and capacity. Anomalous as it may seem, yet it is true, that more study, more experience, and laboriously acquired information, were necessary to form an Acestes, or a Palinurus, than are now required to furnish forth a La Perouse or a Parry. The master, or pilot, of ancient times, who had command of the sailors, and directed all the evolutions, was not merely required to know whatever related to the management of the sails, the oars, and the rudder: he was to be familiar with all the ports that lay in the track of his navigation, the landmarks by which they were designated, and all the rocks, quicksands, and dangers of the intervening deep: he was to know the course of the winds, and the indications which preceded them j also the movements of the celestial bodies, not merely for the purpose of directing his course by them, but to understand the winds and weather, which some of them, as Arcturus and the Dog star, were believed to portend. Moreover, he had to be skilled in reading the various omens, which were gathered from the sighing of the wind in the trees, the murmurs of the waters, and their dash upon the shore, the flight of birds^nd the gambols of fishes. A voyage was, in those days, a momentous and awful undertaking. When the time arrived for the sailing of a ship or fleet, the rnasts were raised, th"as was most usual, the ships were hauled up on the shore, the mariners placed their shoulders at the stern of the ships, and, at the word of command, pushed their bows forward into the sea, leaping aboard when they floated. Levers were used to move the heavier vessels, and, in later times, the helix (probably jackscrew), which Archimedes had invented for that purpose. Before putting to sea, the gods were ever solemnly invoked, and propitiated by numerous sacrifices; thus we find all Homer's heroes sacrificing to the gods before they undertake a voyage; and Virgil's Anchises ventures forth only after having devoted a bull to Neptune and a bull to Apollo. Nor did the voyagers alone supplicate protection: the crowds of friends and countrymen, who thronged the shore, joined fervently in prayers for their deliverance from danger, and, like the Venusian poet, commended their departing friends to the presiding deities of the winds and waves. All omens were carefully regarded; the entrails of the sacrifices examined, with every possible prognostic of good or evil; and a very small matter, the perching of swallows on the ships, or an accidental sneeze to the left, was enough to delay departure. As this, however, never took place without the most favorable auspices, it was always joyful. The ships were adorned with streamers and garlands of flowers; and, when the signal was given from the admiral ship, by sound of trumpet, a shout of rejoicing rang through the fleet, sent back by the responding blessings of the friends that remained. After advancing a short space, doves, which the mariners had brought from their homes, were released, and their safe arrivalnot unfrequently charged with the last adieu of a departing loverwas considered auspicious of the return of the fleet. The admiral led the van, conspicuous by his painted sails and streamers, and opened a path in which many followed. In moderate weather, the ships often sailed side by side; but, as the wind freshened, and the sea grew rough, the order became more open, to avoid contact. At all times, they kept close to the land, following the indentations of the coast. When night approached, it was customary to anchor, or else to bea&h the vessels, that the crews might repose, each rower sleeping on his bench, ready to renew his labors with the returning sun. If the amenity of the weather, the friendly aid night, the plummet or the soundingpole directed their course, or it was shaped, as by day, from headland to headland. If the land were not visible, the known direction of the wind continued, with the aid of the stars, to guide them. Cynosura was the favorite star of the Phoenicians: the Greeks abandoned themselves to the direction of Helice. Having escaped the multiplied dangers of such a navigation, and having accomplished their object, the ships returned home with songs and rejoicings. If they were to be stranded, the sterns were turned towards the shore, and the vessels forced backwards upon it with the oars, until the crew landing, drew them beyond the reach of the surf. Sometimes they were taken into the beautiful moles, or artificial harbors, which the ancients constructed, with great labor and ingenuity, within the natural ones. These were in the shape of crab's claws, or horns, the ends, which formed the entrance, so overlapping as to exclude the swell of the sea. Castles defended their approach, and a lighttower, placed at the entrance, guided those who sailed along the coast, or desired to enter by night. It was called Pharos, from the island at the mouth of the Nile, where the first tower had been erected. Here the vessels were not hauled up, but simply fastened to the rings, or pillars, provided for the purpose, while at the inner port were docks and stores for building and repairing. In this port, too, were temples devoted to the gods, and especially to the patron of the place, where propitiatory sacrifices were made, and vows fulfilled or recorded: here, too, were numerous taverns, and places of more licentious gratification. Whether, however, they stranded their vessels on the beach, or moored them in the harbor, the mariners, before repairing to these resorts, fulfilled the vows made before departures, or in seasons of peril, offered thanks to Neptune, and sacrifices to Jupiter, for having granted them release from the durance of their ships. Upon those who had escaped shipwreck, gratitude was more deeply incumbent. In addition to other sacrifices proportioned to their means, they usually offered the garment in which they were saved, together with a picture descriptive of the disaster. If nothing else remained to them, the hair was shorn from the head, and consecrated to the tutelar deity; hence offering the hair was the last vow of the distressed mariner. There is much thatis beautiful in these simple acts of piety; but, except in some Catholic countries of the Mediterranean, where pictures of rescue and garments are still hung before the shrine of an invoked intercessor, and where processions are still made, after escape from shipwreck, none of these touching customs now remain. What can be more beautiful than the grateful sense of divine interference with which Columbus and his followers hasten to fulfil their vows after their safe return to Palos? Such piety, if it availed not to avert present danger, at least served to inspire confidence to meet it; and, when past, the gratitude which it occasioned must have tended at once to refine the sentiments and ennoble the heart.Imperfect as were the means and knowledge of the ancients in this noble art, yet the Carthaginians, who superadded the greatest commercial enterprise to the greatest skill which had yet been attained, achieved results which may even now be esteemed brilliant. They made the whole of the old world tributary to their city: not contented with exploring every nook and corner of the Mediterranean, they left behind the ne plus ultra which had hitherto almost entirely bounded the excursions of their predecessors, visited the Atlantic coasts of Europe, the British isles, and, pursuing the grand idea which afterwards led the Portuguese to India, discovered a vast extent of the western coast of Africa. Pliny even states that Hanno completed its circumnavigation, and returned home by the Red sea. Had not Carthage prematurely fallen beneath the rivalry of a nation braver and more barbarous, Vasco da Gama might have had to seek laurels elsewhere than by sailing first to India, and even Columbus been spared the most brilliant and enviable of all the achievements of navigation,the discovery of the New World. The art of navigation gained nothing after the fall of Carthage ; and the invasion of the northern barbarians effectually extinguished the few gleams of science which had survived her catastrophies. Every thing remained stationary for centuries, until the returning day of civilization began once more to dawn upon the world. It was not until the close of the twelfth century, that man became sensible of the existence of the most singular property which an allwise and allbeneficent Creator has provided to be his guide upon the deep ; nor until a still later period, that the genius to improve itthe gift of the same good Beingat Iengtn rendered it available to so noble a purpose. We allude to the polarity of the magnet, and the invention of the mariner's compass. The property of that mysterious mineral to attract iron was early known to the Greeks and Chinese ; but the far more singular one of assuming a particular direction, was not even suspected. Pliny himself, wiio records every thing known or fancied in his time concerning the magnet, makes no allusion to its polarity. The first accounts of this speak of it as known in the twelfth century, and that mariners sometimes made use of it to ascertain their course. Of the mariner's compass, we can only say that it was either invented or revived in 1802, by one Flavio Gioia, an obscure individual in the kingdom of Naples; and even this is not known with certainty. While the heroes of the remotest times come down to us, not only with an accurate account of battles fought and thousands slain, but with a minute detail of their private lives, and most insignificant peculiarities, pos terity is at a loss to know whom to bless for a recent discovery, of all others the most useful in its results, the most important in its influence upon the destinies of man. The effects of this discovery upon navigation were not, however, immediate ; for the mariner, as much as any one the slave of habit, could not at once appreciate and confide in the excellence of his new guide. This is the only excuse for the uncertainty which hangs about the identity of the discoverer. The experience of half a century, however, showed the value of this new assistant. Navigation now assumed a bolder character. Prince Henry of Portugal, son of king John, hav ing gained a brilliant reputation in a wai with the Moors, turned from these fierce pursuits to the more congenial one of science. Retiring from court, he established himself in a retreat upon the promontory of St. Vincent, and, calling round him astronomers and mathematicians from every nation, he collected and systematized all the science of the day. Nor were his researches of a mere speculative character ; they were directed to enlighten the field of discovery in which he was engaged, in search of a nearer route to India, and which, though he attained not the grand object of his ambition, repaid him well by the inferior discoveries to which it led. It was to aid these enterprises that he caused charts to be drawn, which, though they involved the monstrous supposition of the earth's being an extended plane, were of no inconsiderable use to the navigator, as they brought together whatever was known of the relative position of the different points of the earth, and enabled him to see, at a single glance, as in a picture, not only the direction of the port which he desired to visit, but also the various coasts, rocks, and quicksands to be avoided in the way. He also invented the astrolabe, which was simply a quadrantal arch, graduated at the rim into degrees and half degrees, and by directing one edge of which towards the heavenly body whose altitude it was desired to measure, a plummet suspended from the centre was made to mark the angle of elevation. This was u sed at first to discover the latitud e from the elevation of the pole star; for, as that star is in the horizon when viewed from the equator, and rises gradually in approaching the pole, so that it would at length become vertical, it follows, that the elevation is always equal to the observer's distance from the equator, which is the latitude. The error resulting from the star's not being exactly polar, was of little note in those primitive days of the art. Soon after, by causing tables of the sun's declination to be computed, prince Henry enabled the manner to deduce his latitude more correctly from the meridian altitude of that star. Yet all these improvements^ though they added much to what was already known, left the art in its infancy. Columbus was the most accurate navigator of his day; still we find him often making an error of so many degrees in his latitude, that the mistake of an equal number of minutes would not be excused in a modern navigator. To mention one of many instances,he places San Salvador three degrees north of its true position. But if Columbus made his discovery with such imperfect means, the greater was his merit: to him belongs the credit, by pushing boldly forth amid the uncertainties of the ocean, of forcing navigation, as well as shipbuilding, to provide against new difficulties, and march rapidly onwards in the career of improvement. From the moment that the hitherto hidden mysteries of the ocean were thus solved, we find improvements and inventions multiplying in rapid succession: First, the log is introduced, to measure the ship's rate of sailing: Nunes, a Portuguese mathematician, next shows that the shortest distance from place to place upon the surface of the globe, must always be ilong a great circle of the sphere: he also proves the fallacy of the plane chart: Gerard Mercator, a Fleming, next suggests the idea of extending the meridian Tines ou the plane chart, in receding from the equator, in a ratio equal to the error occasioned by supposing the meridians parallel, instead of gradually converging as they do towards the poles. By this means, the advantage of a plane surface was retained without the error of the old chart, or the inconvenience and imperfection of the globular projection. Wright, an Englishman, improving the suggestion of Mercator, calculates a table of meridional parts, increasing the length of the arches of meridians in due proportion towards the pole, and furnishes, thereby, data to determine, in any latitude, the difference of longitude from the departure, or distance sailed east or west. At the same time, lord Napier's invention of logarithms wonderfully diminishes the labor of calculation, enabling the mathematician, by their help, to substitute for the tedious operations of multiplication and division the simpler ones of addition and subtraction. Now, too, Gunter presents the seaman with his admirable scale, containing the logarithmic lines, by aid of which and a pair of dividers, all the problems of geometry are easily and accurately performed. The circumference of the earth is ascertained by measuring a given portion of its arch; and, the length of a degree being known, the logline is mark ed accordingly. The quadrant, or rather octant, is invented, and measures the altitude of the heavenly bodies to the nearest minute, undisturbed, like the astrolabe, by the motion of the ship. The sextant and circle still improve upon the octant and each other. And now the tables of the moon's motions, invented by Mayer, with a view to ascertain the longitude, are im proved by Maskelyne, and published periodically at the expense of the British government. The idea of finding the longitude by the watch had been early suggested as an important use of that admirable machine; but it continued too imperfect until the last century, when the munificent rewards offered by Britain so stimulated mechanical ingenuity, that it has at last become admirably adapted to this important purpose. To those who are ignorant of the means by which men are enabled to trace their way over a trackless deep, and to whom the whole art is a mystery, it may be interesting to learn how seamen, and often very ignorant ones, are able successfully to practise it. We shall, therefore, in conclusion, briefly explain the actual practice of navigation: and, first, it may be necessary to premise that, ii. order to determine and designate positions on the surface of the glooe, lati tude and longitude have been invented. Nor is this system entirely arbitrary, since nature herself furnishes the data. We have the poles, determined points of that axis round which the earth performs its daily revolutions : equidistant from these poles, and midway between them, nature aids us to conceive a line called the equator, and about which, by the motion of the earth in its orbit, the sun seems to perform an equal movement, accomplishing the beautiful scheme of the seasons by an annual excursion on either side. What idea more obvious, and, at the same time, more beautiful and complete, than that of measuring latitude from the equator towards the poles, upon meridional lines perpendicular to it, and formed upon the surface of the earth by planes of its axis? But the latitude, though it indicates the distance from the equator, does not alone determine the position; for the same latitude may correspond to an infinity of places, except only a latitude of ninety degrees : hence, then, the necessity of longitude, measured round the wrorld upon the equator, and small circles parallel to it; for, crossing each other at right angles, the same latitude and longitude can only concur at one given point. Latitude and longitude are measured in degrees, minutes and seconds; the first, from the equator to the poles, a quadrant of ninety degrees ; the second, from the first meridian east and west, a semicircle, or 180 degrees, and meeting at the antipodes. In this there are two things deeply to be regretted: one is, that the circle had not been divided into 400 degrees, instead of 360, each quadrant being of 100 degrees, and each degree further subdivided centesimally into minutes and seconds. Every one having any knowledge of figures may appreciate the advantage. The second subject of regret is, that, nature having furnished no data for a line at which to begin the measurement of longitude, the first meridian is arbitrarily chosen, almost every nation selecting its own capital for the purpose: hence charts and nautical almanacs must undergo a tedious reduction before they can be used by mariners of different nations. The cause of science, as well as general convenience, would be greatly promoted by adopting some ocean island as a common first meridian, whence the longitude might be universally estimated, without shocking national vanity. Let us now show the means by which the mariner guides his bark across the ocean, and is able, at all limes, to determine his progress and posi tion. The most important instrument used by the navigator is the compass. It consists of a magnetized needle, freely suspended, by an agate or metal socket at its centre, upon an upright spindle, and possessing the singular property of pointing to the poles. The magnetic virtue is communicated to the needle, which is a flat, oblong piece of hard steel, by applying a natural or artificial magnet to its two extremities. The magnet has two distinct properties, corresponding to its two extremities or poles, the one called boreal^ the other austral magnetism, and which have a near analogy to the equally mysterious principles of positive and negative electricity. When, then, the poles of the magnet are applied respectively to the intended poles of the needle, magnetic influence is developed, not imparted; for the magnet, instead of losing virtue, has itself gained a new accession; and the needle assumes the wonderful power of pointing to the poles of the earth. Attached to the needle is a circle of paper, called the card, upon which the points of the compass are marked, the north and south points corresponding to the poles of the magnet, with which it revolves. The spindle rises from the bottom of a brass or wooden hemisphere, called the shell: this shell is connected to the compassbox by means of two concentric rings, or jimbals, which allow it to swing freely as the ship rolls and pitches, so as to maintain a perpetual level. The box, in turn, is placed within the binnacle, which stands in front of the helm. It were vain here, or any where, to speculate upon the cause of magnetism. We are acquainted, indeed, with the effects of the phenomenon, but all beyond continues to baffle the search of science. It only becomes us to avail ourselves of its guidance, with humble and devout thankfulness for a gift obviously bestowed to open to us the highways of the deep. Having in the compass an instrument for directing our course, we next seek the means of ascertaining the distance run. This we find in the log. The log is a long cord, having a piece of wood attached to one end, and called the chip. This is of a quadrantal form, and being slung at the corners with line, and loaded at the circumference, when thrown overboard, it remains erect and stationary, and drags the line off as fast as the ship passes through the water. The line is divided into knots and half knots, representing miles and half miles, or minutes of a degree, to which they bear the same proportion as the logglass doesto an hour. Thus the logglass being filled with sand, to run through in 30 seconds, the length of a knot must be 51 feet, the first being the same proportion of an hour, that the last is of a mile. As, however, the log is found to come home a little in the effort to draw the line out, it is customary to mark the knot a foot or two less than the true length. The mode of heaving the log to measure a ship's rate, is as follows: The logreel, upon which the line is wound, being held by one of the sailors, the officer places himself on the rail to leeward, and a third person holding the glass, he proceeds to prepare the chip, so that the peg of one of the lines holding the chip in a perpendicular direction, will draw out, by the force of the water, when the reel is stopped, and allow it to haul in easily. Then, having gathered a sufficient quantity of line into his hand, he throws it far to leeward, that it may not be affected by the eddies which follow in the wake. The stray line, which allows the chip to get astern, now runs off, and the instant that the white rag, which marks its termination, passes through the hand of the officer, he cries, " Turn 1" and continues to veer out line until the glass runs out, and the person holding it cries, "Stop!" Then the line is grasped, and the number of knots that have passed off mark the speed of the ship. When this exceeds five miles, it is usual to use a glass of 15 instead of 30 seconds, counting the knots double. The rate of sailing, per hour, multiplied by the hours sailed, thus gives the mariner the measure of his run. In addition to these essential instruments for directing the course and ascertaining the distance, the navigator must be provided with octants of double reflection, to measure the altitude of the heavenly bodies; and a circle, or sextant, more nicely graduated, to measure distances between the moon and stars. He should also have with him a book containing the logarithms of numbers, sines, tangents and secants, to facilitate trigonometrical calculations ; tables for correcting altitudes for dip, parallax and refraction ; also lists of latitudes and longitudes for every part of the world; and of time of high water at every port, at the period of full and change of the moon, from which, at all times, to be able to find the tide; and a variety of tables, to facilitate the various problems of navigation. He should also have with him the Nautical Almanac, containing the places and declinations of the fixed stars and planets, and especially the distances of the moon from the sun and other stars, and all that relates to that body, With a view to calculate the longitude by observation. Finally, he must be provided with the general and local charts applicable to his contemplated voyage. Thus furnished, the mariner may set sail with confidence; many do so with no other aids than their compass, log, quadrant, a single chart and book of navigation, and arrive in safety. But it is less our business to show with how little care a ship may be navigated, than to show how she may be carried from port to port with the greatest possible certainty. Having taken leave of the port, and, when the last land is about to disappear from view, either from the growing distance or the intervention of night, the mariner selects some conspicuous headland, of which the latitude and longitude are noted in his tables, and, placing a compass in some elevated position, remote from any iron object to disturb its polarity, proceeds to determine its bearing, and estimate his distance from it, either by the progress made from it, or by the ready estimate of a practised eye. Or, taking the simultaneous bearings of two distinct points of coast, he has still surer data for deducting his position. This is called taking the departure, and is carefully noted on the logslate, with the time of making the observation. Thenceforth the log is thrown every hour, and the course and distance are entered upon the slate, to be copied into the logbook at the end of the day. The first thing which the navigator attends to, after making the offing which prudence dictates to clear the dangers of the land, is to shape his course for the port of his destination. And first he searches in the chart if there be any point of land, island, or rock, intervening in his way. If there be, the course is primarily shaped with reference to the danger; if not, the differences of latitude and longitude between the two places being taken, the course and distance are obtained by the aid of trigonometry. The shortest distance between any two places on the surface of our sphere, is the arc of a great circle passing through those two places. Thus, between cape Henry, in latitude 37°, and the island of St. Mary, in the same latitude, but 50° Ion. farther E., the distance is 30 miles less in sailing on a great circle, than if you were to sail due E. on a parallel of latitude, and consequently on a lesser circle of the sphere. In a higher latitude, the difference between sailing on a great or small circle becomes more considerable, as the small circles grow smaller; thus,m the latitude of 60°, a distance equal to that between cape Henry and St. Mary would offer a disparity of near 200 miles. But, as it is only in sailing on the equator, or on a meridian, that the compass points us uniformly along a great circle of the sphere, in most cases it would be necessary to change the course at short intervals, in order to attain even an approximation towards this desideratum. For :nstance, in sailing from cape Henry to St. Mary, on a great circle, it would first be necessary to sail more than a point northward of E., gradually approaching that direction towards the middle of the distance, when the course should be due E.; thence declining southward, until the land would be made upon a course as much south of E. as, on starting, it was north of it. In high latitudes, when the reduction of distance would offer a sufficient inducement, it may be advantageous to attempt following a great circle; but m the seas ordinarily traversed by mariners, the trifling increase of distance which results from following a uniform course, as obtained by Mercator's sailing, is far more than compensated by its convenience and freedom from all perplexity. For the rest, the wind not unfrequently deprives the fastidious navigator of all choice between a great circle and a loxodromic. At the first noon succeeding the time of taking his departure, the mariner works up his reckoning. This is an epoch fixed by nature, being determined by the passage of the sun over the meridian, and is therefore well chosen as the beginning of the day. The logslate being marked, he copies the courses and distances, if from head winds or other cause they have been various; the departure from the land is also converted into a course ; as is also the current, if there be any known one. He next proceeds to find the difference of latitude and departure from the meridian corresponding to each course, either by geometrical calculation, or, more expeditiously, by reference to tables ; then he adds the several differences of latitude and departure, and, if they be of different names, as some north and some south, some east and others west, deducts the less from the greater. With the remaining difference of latitude and departure, he not only finds the course and distance made good, but also the latitude and longitude in; the difference of ^titude being applied to the latitude left, by adding or subtracting, in sailing from or towards the equator, at once gives the latitude of the ship. But before the depart ure can be thus applied to find the iwigi tude, it is necessary to reduce it for the converging of the meridians towards the poles; for, though all degrees of longitude are divided, like those of latitude, into 60 minutes or miles, yet they decrease in length, from being equal to a degree of latitude at the equator, until they become nothing at the poles. There are many ways, more or less accurate, of deducing the difference of longitude from the departure, the latitude being known; they are founded upon this principle; the circumference of the earth at the equator is to its circumference at any given parallel of latitude, as the departure is to the difference of longitude. The most easy and correct way of obtaining the difference of longitude, on an oblique course, is by the aid of a table of meridional parts; for, having taken out the meridional difference of latitude, the mariner has this simple proportion : the proper difference of latitude is to the meridional difference of latitude, as the departure to the difference of longitude. The difference of longitude thus obtained, is applied to the longitude left, adding or subtracting, in sailing to or from the first meridian, and the result will be the ship's longitude; which, with the latitude previously ascertained, determines her position on the chart. The method of navigating thus described is called dead reckoning. It is far from infallible, and leaves much to desire. It will, indeed, do pretty well in short runs; but as errors daily creep in from many causes escaping calculation, such as bad steerage, leeway, heave of the sea, unknown currents, and as these accumulate, and become considerable at the end of a long voyage, it becomes necessary for the mariner, removed from all reference to terrestrial objects, to resort to the immovable guides in the heavens, whose motions the God that placed them there has given him capacity to comprehend. Let us now see how the ship's position on the ocean, represented by latitude and longitude, may at any time, without reference to course sailed, or distance, to capricious winds and stealthy currents, be ascertained with ease and accuracy. And, in the first place, to find the latitude, we have abundant data. All the heavenly bodies are, by the revolution of the earth, daily brought to the meridian, at which time, if their altitude be measured, their declination or distance from the equinox being known, the latitude is readily deduced; it may also be deduced from single or jBut the meridian altitude of the sun is what furnishes at once the easiest and most correct method of finding the latitude. The meridian altitudes of the stars, and frequently of the moon, must be taken at night, when the horizon is vaguely marked ; moreover, their minuteness and want of brilliancy make observation troublesome and uncertain; but when the sun comes to the meridian, the observer brings a brilliant and palpable object down to a welldefined horizon; then, too, he has the advantage of observing, at a selffixed epoch, the beginning of a new day. So great, indeed, are the advantages offered by the meridian altitude of the sun, that no other means of finding the latitude are used, except when these have failed from a clouded atmosphere, or when the momentary expectation of making the land quickens the mariner's anxiety. We shall, therefore, now explain the method of deducing the latitude from the sun's meridian altitude. Furnished with a sextant, circle, or octant of reflection, the observer goes upon deck, and, having examined the adjustment of his instrument, proceeds to bring down the image of the sun reflected by its mirror, until the lower limb just sweeps the horizon. He continues to follow and measure its ascent, until it ceases to rise ; the moment that it begins to fall, and the lower limb dips in the horizon, the sun has passed the meridian. The altitude marked by the index being read off, it is next corrected. And first, the observer adds the semidiameter, in order to make the altitude apply to the centre of the object; next, he subtracts the dip, to meet the error caused by the extension of the horizon, in consequence of the rotundity of the earth, and the elevation of his eye above its surface ; also the refraction of the atmosphere, by which the object, when not vertical, is made to appear higher than its true place; lastly, he adds the parallax (a small correction, inconsiderable from the sun's distance), in order to reduce the calculation for the centre of the earth ; for which point all calculations are made, and which is ever supposed to be the station of an observer. Having made all these corrections, which many mariners despatch summarily, by an addition of 12 minutes, he has the true meridian altitude of the sun. Taking this from a quadrant, or 90 degrees, gives its zenith distance, or distance from that point in the heavens which is immediately over the observer, and would be metif the sun were for ever on the equinoctial, the zenith distance would always be the latitude ; for, whilst the zenith is the observer's position, referred to the heavens, the equator is there, in like manner, represented by the equinoctial; and we have already seen that latitude is the distance from the equator. But, as the sun is only twice a year upon the equinoctial, and as his distance from it, at times, increases to more than 20°, it becomes necessary to take this distance (called his declination) into the estimate. The sun's declination is given, in the Almanac, for the noon of each day; by correcting it for the time anticipated or elapsed, according as the sun comes first to him or to the first meridian, by his position east or west of it, the observer obtains the declination for noon at his own position. This declination applied to the zenith distance, by adding when the sun is on the same side of the equator, by subtracting when on the opposite side, gives the true latitude. A daily and accurate knowledge of his latitude is, then, to the mariner of our day, a desideratum of easy attainment. By its aid, nothing is easier than to sail clear of any rock or shoal that crosses his track, either by a watchful lookout at the moment of passing its latitude, or else by avoiding its parallel entirely, until it be surely passed. Moreover, this is his best and surest guide in aiming at his destined port; for he has but to attain the exact latitude it lies in, and then sail directly upon it, east or west, to be sure of success. And here nature is again his friend: by a singular coincidence, discoverable in glancing at the map of the world, most coasts and continents lie in a northern and southern direction. Hence the value attached, by seamen, to an accurate knowledge of the latitude; and hence the saw of "Latitude, lead and lookout." But if it be possible to obtain the longitude with any thing like an equal ease and certainty, no one will dispute its advantage. Although, as we have stated, most coasts follow a northern and southern direction, there are yet not a few, such as both coasts of Cuba and San Domingo, which lie east and west, so that points along them are only determined by the longitude. And even to have the satisfaction, not merely to run his finger along the chart, and say, " I am somewhere along that parallel," but to be able to point to the spot, and say, " I am there," is, to the nice navigator, no insufficient motive. Various ways have been devised to find the longitude, in all of which the great element is time. Inasmuch as the earth performs her diurnal revolution in 24 hours, from the time any given meridian is brought under the sun until it reaches it again, it follows that 24 hours and 360 degrees are both equal to a circle, and that the equator and other circles of longitude may be indifferently estimated by either of these divisions. Hence the difference of time between two places, is no other than the difference between the sun's coming to their respective meridians, or, in a word, their difference of longitude; and hence it follows that if we, by any means, simultaneously ascertain the time at the first meridian, and the time at ship, we shall have ascertained the longitude. The easiest method of solving this problem is by means of the chronometer. This is a watch so nicely constructed as to go with perfect uniformity, either having no error whatever, or else losing or gaining a known quantity every day. This watch is set to the time of the first meridian, and its rate is carefully ascertained, before leaving the land. To find the longitude by means of it, the mariner has merely to take an observation of the sun or other star, when rising or falling rapidly, and deduce the time of ship ; this, compared with the time at the first meridian, simultaneously given by the chronometer, determines the longitude. Several chronometers concurring with each other, may make the mariner sure of his position; but a single one, unchecked by other data, and liable, from its nicety of construction, to easy derangement, is a dangerous guide. The many noble ships so inexcusably lost, in late years, between the entrances of the Delaware and Hudson, owe their destruction to a blind dependence on a single chronometer. The most expeditious and certain way of observing the longitude, is by the eclipses of Jupiter's satellites. Their times of immersion and emersion at the first meridian are noted in the Almanac, and these, compared with the times at which the telescope shows the observer the occurrence of the same phenomena, determine the longitude. But the unsteadiness of a ship at sea deprives the mariner of this expeditious method. Fortunately, there yet remains open to him one of sufficient accuracy: this is, by observing the distance of the moon from the sun and other fixed stars, and comparing the time of observation with that time at which the Almanac shows a similar distance for the first meridian. The only difficulty attending this beautiful method, which the rapid movement of the moon in her orbit, and her consequent change of distance from the stars, renders proportionably correct, consists, in the first place, in nicely observing the distance, and then in correcting it trigonometrically for the errors occasioned by parallax and refraction. A single lunar observation, like a single chronometer, has been confided in to the loss of many a gallant ship ; but a series of them, taken from day to day, with stars on different sides of the moon, and concurring to show the same longitude, are worthy of all confidence. Thus, aided by these heavenly guides, is the mariner at all times able to determine his position. He should not, however, be inattentive to any means of information ; he should, by observing the difference between the magnetic bearing of some heavenly body, and what calculation shows to be its true bearing, daily inform himself of that wonderful phenomenon, the magnetic variation; he should, in calm weather, ascertain the direction and force of the current, by lowering a boat and anchoring it to an iron vessel let down below the superficial strata of the ocean; in approaching the land, he should be attentive to the changing color and temperature of the sea, which last is, especially on our coast, an admirable monitor; also to the floating of weeds, and the flight of birds, such as do not stray far from it. All these little cares, the watching of the barometer, and profiting by its friendly predictions, and the frequent inspection of the chart, whilst they take from the dangers of navigation, amuse the mariner, and beguile the tedium of the sea. Thus, then, is a ship conducted from port to port; thus are dangers avoided, difficulties overcome. Though they who traverse the vast ocean leave neither track nor way mark for the guidance of those who follow, it is thus converted into a plain and convenient highway, extending to the extremities of the earth. (See the article Ship.)