the centre of the earth, for which the rational horizon is calculated. This operates to sink the sun's apparent place lower than its real, though not enough to counterbalance the contrary effect of refraction. When the proper corrections are made of these errors, and others of less consequence arising from the aberration of light, &c. the true altitude is found, and the true latitude follows.

It still remains to find the ship's longitude. Having found thus much, the navigator may point out the parallel of latitude upon which he is, but for all he can tell from celestial observations, he may be any where between Nova Scotia and the mouth of Oregon. In former days, not far removed, the entire dependence for longitude was upon "dead reckoning," the whole mystery of which consisted in a careful record of the ship's bearings, and rate of sailing, from which her course was traced and her present position ascertained. By these means latitude was found at the same time as longitude, but as the means of ascertaining the former, from the bearing of the heavenly bodies, was introduced into general use before the more complicated methods of finding the latter, "dead reckoning" continued to be relied on for that purpose, long after it had ceased to be used for the former, except in cloudy weather, when it still is the navigator's sole reliance. Upon leaving the land, the ship's course is carefully noted, both from the shore and the compass, and thus the "departure" taken. After this, every thing depends for "dead reckoning" upon the compass, and the log; the former to ascertain the course, the latter the distance sailed. The log is a long line fastened to a piece of wood, something like a buoy, which being loaded at one end stands upright in the water. The cord is divided into a number of equal parts, by knots, which are termed knots, and half-knots; each knot being the 120th part of a mile. When every thing is ready to throw the log, an officer stands ready with the logglass, which is emptied of its sand in 30 seconds, or the 120th part of an hour. After the log is in the water the glass is turned, and the number of knots noted, which run out as the vessel passes on her course in the 30 seconds. Each knot

bearing the same proportion to a mile, which the time does to an hour, of course the number of knots sailed in that time indicate the number of miles the ship is going by the hour. This process is repeated hourly, and, for a few days together, affords the data for a pretty good guess at the true position of the vessel, though on long voyages it is far from satisfactory. The navigator having started from the point A in the annexed figure, finds by his compass that his bearing is in the direction AB, and by his log that the distance AB is twenty miles. In order to find his longitude at B, he must ascertain the difference

between that of A, (which having been his starting point he knows,) and B. The line AC is the parallel of latitude which passes through A; BC, the meridian, or parallel of longitude of B, and of course perpendicular to AC; AC, is then the difference of longitude, and BC of latitude, between A and B. ACB being a right angled triangle, if he has found his latitude at B, by astronomical observation, and consequently the length of the side BC, he may find the side AC by the 47th proposition of the first book of Euclid: which proves the square of AB to be equal to the sum of the squares of AC and BC, and consequently the square of AC to be equal to the difference between the square of AB and that of BC. But if, as is generally the case, the navigator, has recourse to his dead reckoning, for want of an opportunity for observation, and knows no more of his latitude than of his longitude, he has no data, but the length of the side AB and the angle at C, which being the point at which the meridian BC intersects the parallel of latitude AC is a right angle, and therefore 90 degrees, and the angle at A, which he has found by calculating the difference between the ship's course AB and a due east course AC. He has therefore two angles and the side of a triangle, and easily finds the other sides, one of which is the side AC the difference of longitude, and BC the difference of latitude, between A and B. That difference, added to the latitude and longitude of A, makes those of B. The veteran Commodore Rogers is said to have been singularly skilful in this kind of navigation, and to have made his famous cruise during the last war, in the President, without other help, and, what is the more remarkable, without miscalculation.

At this day, longitude is found whenever observations can be had from the lunar distances, and from the chronometer, the one being used to test the accuracy of the other. The moon, as every one knows, has a rapid motion among the fixed stars, and other heavenly bodies. Upon this, the whole system of lunar observations depends. The almanac contains the data from which the precise time the moon at Greenwich

appears at a certain distance from the sun, can be deduced. By noting, then, the time at which they assume the same relative position to the observer, he has the difference between his time and that of Greenwich. That difference will give the difference of longitude between the two places. The time of any place depends upon the sun's passage over its meridian. When that takes place, it is twelve o'clock, noon, and the other hours are counted from that period. The sun, rising in the east, and progressing westwardly, reaches, the meridian of the more eastward of two places, before that of the other; and the time of the former will be faster than that of the latter, by precisely the time it takes the sun to travel from the former meridian to that of the latter. If, then, it be ascertained that the sun and moon have assumed to the observer, on shipboard, a relative position, two hours after the former passed his meridian, or at two o'clock, which they held at Greenwich at noon, his time is two hours behind that of his prime meridian, and he is evidently west of it. He knows that the sun passes over 15° of space in an hour of time, and that he must consequently be twice that many degrees west from Greenwich.

The method of finding longitude by chronometer, is only another mode of finding the difference of time between the ship's position, and the prime meridian. After that is accomplished, the process is the same as in the last described mode. The time, at the ship's position, having been carefully ascertained, as described in our account of the mode of finding latitude, from the sun's passage over the meridian, it only remains to fix the time at the prime meridian. For this purpose, modern ingenuity has constructed the chronometer, a kind of watch, of peculiarly accurate workmanship. Before leaving port, it is carefully regulated, and set to the true time of the prime meridian, found by astronomical observations. During the whole course of the voyage, it therefore preserves that time, and enables the navigator, whenever he can find his own true time, to tell with great accuracy his longitude. For the sake of greater certainty, it is usual to take at least three chronometers. Two are worse than one, since, if they differ, neither will be trusted. With three, there must always be a majority, unless, like the vanes on our court-houses, they make a point of no two ever agreeing. Even if this should be the case, the moon may be brought in as umpire, to decide upon their veracity, though it be something new under the sun, to make her the standard of truth. The use of these instruments has become universal in the merchant service. Our government is the only shipowner who refuses them to her vessels. The smaller of our national ships are not considered worth a chronometer. What particular amount of tonnage, or VOL. XVIII.-No. 37.

13

how many immortal souls are estimated at head-quarters equal to that value, we have not learned.

These more accurate modes of computing longitude, simple as they are, have been of more recent introduction into our naval service than is generally supposed. As late as the year 1817 we remember to have heard it mentioned, as an affair of congratulation, that a certain frigate about to sail for Europe contained one officer acquainted with the method of taking a lunar observation, and he used to shut himself up in his room to work out the calculation. If we are not mistaken, there was a general order issued that none, on pain of the captain's displeasure, should, upon any account, interrupt him while engaged in that arduous duty. One of the advantages of the new arrangement by which midshipmen are compelled to undergo an examination before their promotion, is the attention they now find it necessary to devote to this subject. The officers who commanded our vessels during the late war, were inferior to none then or now in existence, in the discipline or sailing of their ships, or their management in battle. But the ignorance of some of them, touching the science of navigation, (an ignorance attributable to their defective education in that respect,) on some occasions has produced results that, but for good fortune, would have been more serious than ridiculous. A frigate, some years after the peace, sailed for the other hemisphere (we do not mean to be particular), intending to touch first at an island which shall be nameless. She sailed entirely by dead reckoning. The Gulf Stream, which in some parts runs two miles in the hour, was not considered worth notice in the calculation, and with a few other trifles was wholly neglected in the log. The commander on a certain day, when according to the best of his knowledge and belief he was fully three days' sail at sea, was startled by an announcement from the mast head of land. At first he refused to believe the fact, and threatened to punish the lookout for his carelessness, but the confirmation came in time to save him. Fortunately for the island, the ship came in sight by daylight. Had it occurred of a dark night, the unfortunate inhabitants would have been fairly run down before the unconscious ship's crew had found their mistake. In a subsequent part of her cruise, the same frigate was bound for a port, which, according to the calculation of her officers, formed upon the same accurate premises, was sixty miles to windward. The wind, as head winds are apt to be, was very strong, and after twenty-four hours' hard work the ship had made but eighteen miles. By this time a midshipman, who, to the amazement of the crew, had made a lunar observation, found that the official calculation, though right as to the distance from the port, contained a slight error in regard

to its bearing from the ship, inasmuch as, instead of sixty miles to windward, it was that far to leeward. Feeling more confidence in the moon than the dead reckoning, he ventured to advise, since sailing before the wind was much more expeditious, and in this instance more direct withal, for the haven where they would be, that the ship should be put right about. It was not, however, until another day's unprofitable beating had wearied the general patience, and a second observation had confirmed the first, that the experiment was tried, and in six hours the ship was in port.

Still more ludicrous miscalculations have arisen from mistakes in the allowance for the variation of the compass. It is known that the needle rarely points due north, but in some places has a permanent variation of one or two degrees to the east or west of that point. If the variation at a given place should be one degree east, a navigator wishing to sail due north, would steer one degree west of the magnetic pole. One would think this plain enough; and yet it has actually occurred, under those circumstances, that commanders of vessels have made the allowance on the wrong side, until suddenly finding themselves off a lee shore, have become convinced of an error, which even then they were not perhaps able to detect. Our navy, it is notorious, contains officers whose path has been but sparingly on the "mountain wave." We have heard of an incident connected with one of these personages, who had risen to the command of a vessel, and was just making seaward upon, to him, a voyage of discovery. Sirius was in the southwest, just lifting himself in all his glory from above the deep, when a mischievous midshipman, who had seen more service than his commander, affecting a laudable desire for information, pointed to the star, and enquired what light it was. captain looked all round the horizon, as if in hope of finding some mark by which to frame a reply, and then, lifting up his arms with a ludicrous air of abandonment, exclaimed, "Heaven knows, I don't." We remember ourselves having wasted a precious half hour in a vain effort to convince another gentleman who had attained some rank in the service, that the great bear was not the Pleiades, or as he was pleased to call them the "Peleads." Of course, having sailed by them upon the trackless deep, and found his haven under their auspices, he knew better than we did, and retained his opinion, in despite of our benevolent efforts for his instruction.

The

Let it not be supposed for a moment that, by these incidents, we desire to do injustice to our gallant and victorious navy. They all occurred some years since, and even then formed exceptions to the general rule. We have only adduced them to show, that officers may attain the command of ships, and