NAVIGATION. 



153 



quadrant, or rather octant, is invented, and measures 

 the altitude of the heavenly bodies to the nearest 

 ninute, 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 improved 

 by Maskelyne, and published periodically at the ex- 

 pense of the British government. The idea of find- 

 ing the longitude by the watch had Jt>een early sug- 

 gested 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 ex- 

 plain the actual practice of navigation : and, first, it 

 may be necessary to premise that, in order to deter- 

 mine and designate positions on the surface of the 

 globe, latitude 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 excur- 

 sion 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 world 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 longi- 

 tude 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 

 ndergo 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 

 l>e 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 

 times, to determine his progress and position. 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 apply- 

 ing a natural or artificial magnet to its two extremities. 

 The magnet has two distinct properties, correspond- 

 ing 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 compass-box 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 phenome- 

 non, 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 arid half miles, or minutes of a degree, to 

 which they bear the same proportion as the log-glass 

 does to an hour. Thus the log-glass being filled with 

 sand, to run through in thirty seconds, the length of a 

 knot must be fifty-one 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 log-reel, \ipon 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!" 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 



