NAVIGATION, ART os', We shall give a few indications of the manner of ducting the course of a ship at sea, referring to the various headings, such as SENTAxr, LATITCDE and LONGITUDE, GREAT-CIRCLE SAILING, etc., for the more scientific expla nation of the operations in use for determining position.
A vessel having completed her lading, she is steered out of port by a pilot, who lays his course by the ranges with which long familiarity has made him acquainted. Arrived off soundings, or at a point where his local knowledge is no longer of value, he leaves the vessel to the captain, who then assumes all responsibility. While off the coast the captain steers by hiS chart and by the lead, assisted by landmarks and buoys by day, and by lights at night. It is his duty. without waiting for foggy weather, nor for any doubt of his situation, to keep the lead going, and a careful watch, while on soundings. When, finally, he is about to lose sight of the coast, he determines a last position, called the point of departure, which serves as the base of his reckoning, The problems involved in a long voyage are many, some intricate, but position is always ascertainable, either by observations or dead reckoning. Two things must be known, speed end direction. The first is found by the log, whose unit, the knot, predicates the number of nautical mks, 1851.85 meters, traversed per hour. The second is indicated by the compass, from which is read the angle, known as the course, between the magnetic meridian and the axis of the keel. But to reduce this to the true course with reference to the terrestrial meridian, the magnetic variation must he known and applied. The log is not en accu rate instrument, nor is it possible, in a sailing vessel, to throw it as often as slight changes in the rate of motion occur; besides, it seldom happens that the course of a vessel is exactly that read from the compass, for decomposing into two forces the normal line of action of the wind on the sails, there results a certain side-push, forming an angle wills the keel, and resulting in a falling-off from the true course known as drift. Allowance must also be made for the influence of local, tidal, or ocean currents, the force of which, even where not known by experience nor laid down on the chart, must be carefully judged. and anxiously watched for. The amount of drift is ascertained from the wake,
either by a back-sight of the compass, or by means of a quadrant and eye-pieces, and can be always combined with the magnetic variation to obtain the true course. Then make allowance for the set of the current, the effect on a course of given length and direction of the known speed and trend of the stream.
When, as is necessary every day, it is desired to follow a rhumb-line, the course is de duced by making allowance inversely for variation, drift, and set. This course the captain lays down, and the officer of the deck continually oversees the steersman, so that, granting this course continually kept, the path of the v.essel successively intersects each meridian at the same angle, called the angle of rhumb, and this line, a curve of double flexure, is the lo.rodrome, or lo.rodromic line. But for a vessel to sail directly from point of departure to destination is almost impossible, whether from baffling winds, interven ing coasts, or adverse currents., the best that can be done then is to substitute a series of loxodromic curves, as little removed from the true course as possible, to make as few and as advantageous stretches as possible, and to take advantage of known currents and favorable winds to substitute for a short but questionable passage a more circuitous but quicker route. The log must always be thrown whenever wind, sails, or course may change; the course and the speed are noted, say every half-hour, on a tally, and at the end of each watch the course is transferred to the log-took. Finally, every day at noon, or oftener as advisaLlo, the reckoning is cast up, and the position of the vessel marked on the chart, taking as point of departure the last calculated position. The future course is deduced froni this. All navigation by reckoning should be checked at least once a day, and as often and in as many different ways as can be accomplished by observations, repeated if possible, and the mean taken.