RECKONING AT SEA is the process of computing the several elements which relate to the determination of the ship's place at any time. The term may include the operations which are performed in finding the latitude and longitude of the ship, the variation of the the needle, &c., from celestial observations ; and the part which is in dependent of these is called the It is this last only which we purpose here to explain.
When a ship crosses the seas towards the place of its destination, its path, on account of the various winds, currents, fie., by which it is impelled, is always indirect, and generally consists of numerous zigzags, whose portions are Biwa of a few miles in length. The length of each of these lined, and the angle which it makes with the terrestrial meridian passing through one of its extremities (all necessary corrections having been made) are the data obtained by the teg.fine and compass ; and the earth being supposed to be a sphere, those lines might he considered AR area of great circles. Hence the mks of spherical trigonometry might be employed to find the length of an arc joining the two ex tremities of the series of indirect lines, and the angle which it makes with the meridian passing through either of those extremities; and, from these, the geographical position of the ship. But, because this process is considered laborious, others possessing greater facilities are, according to circumstances, employed, and these will be described after it has been shown what are the corrections which the observed elements require before they can lie used in the computations.
The reckoning may be said to commence when the ship is on the point of quitting a harbour or road ; and the first circumstances to be recorded are the observed bearing and the estimated distance of some remarkable object on the coast whose geographical position is known, together with the bearing of the ship's line of motion at the time, and her velocity on that line.
Let it be here observed that the said object on the coast is called the point of departure, and that the angle which the line of a ship's motion at any time makes with the meridian passing through the actual position of the ship is called,her course. Now, while the angle indicated
by the compass remains the same, the ship's path, except when it coincides with the meridian, or with a line tending due east and west, is a portion of that which is called the loxodromic curre [Itnuma LINE] ; vet, to the extent of a few miles, it is the custom to consider it as a right line, and, therefore, as making a constant angle with the meridian passing through one of its extremities. The deviation of the magnetic from the true meridian (the declination or variation of the needle) differing in different places, the amount of that variation (ascertained by celestial observations as often as possible) must be added to or subtracted from the angles observed with the compass, in order to have the bearing, or course, from the true meridian. But while a ship is sailing with the wind in a direction oblique to the line of her keel, she is compelled, by the force of the wind and the resist ance of the water against her side, to move in the direction of a line which makes some angle with her keel on the side opposite to that from which the wind is blowing ; this angle is called the lee-may, and as it differs for different ships, it must always be determined by trial in some one of the ways proposed in treatises on navigation. The esti mated amount of the lee-way is a second correction, which must be applied to the course observed with the compass, in order to obtain the correct angle with the meridian.
The velocity of the ship is ascertained by means of the log-line [Loc-urrs], which at once indicates the number of geographical miles (equatorial minutes) she has passed over in an hour ; and consequently, supposing her motion to be uniform, the space through which the ship has sailed on a particular course in a given number of hours is known. This is technically called the distance.