We have been the more particular ,in describing the formation of the compass card, because a perfect knowledge of that important aid is indispensably necessary for all who attempt the study, or follow the practice, of navigation. Under the head of MAaarrisat we have explained the properties of the load-stone, and spoken of the dip of the needle, as well as of the variation which exists between the true and apparent polar directions ; as indicated by the compass. Therefore we shall briefly observe, that variation is eitl ier Easterly or Westerly ; and, when ever it prevails, must be computed in the reckoning ; always making allowance for the difference, and laying down the ship's course accordingly. Thus if a ship sails due North, and that six degrees of West ern variation are known to prevail in that part where the vessel is sailing, instead of sailing due North, as indicated by the compass, the vessel must sail six degrees more Easterly, or, in other words, North half East, i. e. half a point Easterly of North, in which direction the true North point would be found by observation of the heavenly bodies. Currents must also have allowance made for them according to their bearing', or the points to which they run : it is self-evident, that if such were not duly ascertained, and set of from the dead reckoning, the ship's place would never be accurately laid down, and destruction would inevitably follow the neglect.
The way, or distance, the ship sails within the day, is ascertained by means of a small piece of board called the Loa, (which see) ; that being fastened to a thin, but stout line, and lowered over the stern, occasions the line to run off from a large reel. The line being marked at certain distances with small pieces of string, whereon one, two, three, &c. knots are made, at distances corresponding with the rate per and the person who super intends the process, having a minute or a half minute glass, the to is allowed to run the line from the reel, during such interval ; the number of knots, on the proximate string, indicating the number of miles the ship sails within the hour. Hence the technical term of so many "knots per hour;" or "an eight knot breeze," &c.
The whole world is supposed to be comprised under a circle, which, in every direction, contains 360 degrees of equal measurement. Such as pass through the meridian of any place, and from North to South, cutting the equator at right angles, are called meridional lines, and are each divided into nine degrees, counting from the line towards the poles respectively : those proceeding to the north are called degrees of North latitude ; those towards the South being called degrees of South latitude. In this respect all civilized na tions are agreed: but in their estimation of longitude, they generally differ, each taking some particular point within their own dominions as zero, and counting 180 degrees East, and as many West ; calling the former East longitude, the latter West longitude. Consequently, the union of those adverse designations takes place at the antipodes of the zero from which they proceeded ; and the moment a vessel passes 180 degrees either way, she en ters upon 179 of the opposite semicircle, and reckoning the degrees of latitude are equal from the equator to the pole; and each in general measures about 69h of British statute miles. But the degrees of longitude vary greatly ; decreasing regu larly from the equator to the poles, where they all meet, and are, as it were, annihi lated. The regular declension of the cir
cles of longitude, which are the same as parallels of latitude, may be seen under the head of DIALLING ; where, in the struction of lines of latitude, their gradual decrease is fully exhibited : see also Lou Grimm, for a table of longitudes in vari ous latitudes.
A rhumb-line is a right line drawn from the centre of the compass to the horizon, and is named from that point of the hori zon it falls upon. The course is the an gle which any rhumb-line makes with the meridian, and is sometimes reckoned in degrees, and sometimes in points of the compass ; so that if a ship sail upon the second rhumb, or N. N. E. the course is 22° 30', and so for any other. When a ship makes a direct course from one point or port to another, and that there is no current nor any variation of the compass, she sails "on a rhumb;" that is, she is guided invariably from one to the other throughout her course by one point of the compass, being governed through out her passage by that line only. This is different from what is called traverse sailing, which arises from adverse winds, or sometimes from currents, and obliges a vessel to change her course occasional ly; especially where the vicinity of land renders it necessary to steer at times dif ferently ; lest the current, which gene rally changes from one side or direction to another, should set her against the shore. When the wind is diametrically, or ob liquely, against a ship's direct course, she must make traverses, 1. e. ziz-zags, which is effected by laying her head as close to the wind as may allow her sails to be fill ed when close hauled ; (see Plate XI. Miscel. fig. 12.) in which A is the place of departure, B the point of destination, from which the wind blows direct, and A II, b c, c 13, &c. the course the ship must steer to arrive at B. Square rigged ships generally can lay within six points of the wind ; but sloops, &c. commonly lay up within four • points and a half. When working in this manner, it is called "beat ing," or " playing to windward ;" when the wind blows straight upon the side of the vessel, it is said to he "on the beam" when between her side and stern, it is called "a quartering wind," or " on her quarter ;" when direct astern, or near it, she is said to he " before the wind," or to " sail large." When the wind from being fair becomes suddenly foul, it is said to " take her aback." In traverse sailing, the vessel's head is usually turned up "into the wind" when she is " put about." This is called " tack ing ;" but if, instead of " throwing her up" in that manner, she is allowed to go round from the wind until it comes or is met by her on the other side, it is called "wearing." When she has the wind on her starboard, or right bow, she is said to "have her starboard tacks aboard," and vice versa, when the larboard or left bow is to the wind. To know how close a ship will lay to the wind, observe the course she goes on each tack, say north on one, and south-west on the other ; di vide in the middle, and her course will appear to be west-north-west. But allow ance must be made for lee-way, which is the loss made by the impression made on the vessel as she is working to windward; when the wind presses her from the di rect line of her course, and occasions her to " drift to leeward." See LzawAy.