NAVIGATION. 



155 



in from many causes escaping calculation, such as bad 

 steerage, leeway, heave of the sea, unknown currents, 

 and as these accumulate, and become considerable at 

 the end of a long voyage, it becomes necessary for 

 tlie mariner, removed from all reference to terrestrial 

 objects, to resort to the immovable guides in the 

 heavens, whose motions the God that placed them 

 thfre has given him capacity to comprehend. 



Let us now see how the ship's position on the 

 ocean, represented by latitude and longitude, may at 

 any time, without reference to course sailed, or dis- 

 tance, to capricious winds and stealthy currents, be 

 ascertained with ease and accuracy. And, in the 

 first place, to find the latitude, we have abundant 

 data. All the heavenly bodies are, by the revolution 

 of the earth, daily brought to the meridian, at which 

 time, if their altitude be measured, their declination 

 or distance from the equinox being known, the 

 latitude is readily deduced ; it may also be deduced 

 from single or double altitudes of bodies not in the 

 meridian, the times being accurately known. But 

 the meridian altitude of the sun is what furnishes 

 at once the easiest and most correct method of 

 finding the latitude. The meridian altitudes of 

 the stars, and frequently of the moon, must be 

 taken at night, when the horizon is vaguely marked ; 

 moreover, their minuteness and want of brilliancy 

 make observation troublesome and uncertain ; but 

 when the sun comes to the meridian, the observer 

 brings a brilliant and palpable object down to a 

 well-defined horizon ; then, too, he has the advan- 

 tage of observing, at a self-fixed epoch, the be- 

 ginning of a new day. So great, indeed, are the 

 advantages offered by the meridian altitude of the 

 sun, that no other means of finding the latitude are 

 used, except when these have failed from a clouded 

 atmosphere, or when the momentary expectation of 

 making the land quickens the mariner's anxiety. 

 We shall, therefore, now explain the method of 

 deducing the latitude from the sun's meridian alti- 

 tude. Furnished with a sextant, circle, or octant of 

 reflection, the observer goes upon deck, and, having 

 examined the adjustment of his instrument, proceeds 

 to bring down the image of the sun reflected by its 

 mirror, until the lower limb just sweeps the horizon. 

 He continues to follow and measure its ascent, until 

 it ceases to rise; the moment that it begins to fall, 

 and the lower limb dips in the horizon, the sun has 

 passed the meridian. The altitude marked by the 

 index being read off, it is next corrected. And first, 

 the observer adds the semkliameter, in order to 

 make the altitude apply to the centre of the object ; 

 next, he subtracts the dip, to meet the error caused 

 by the extension of the horizon, in consequence of 

 the rotundity of the earth, and the elevation of his 

 eye above its surface ; also the refraction of the 

 atmosphere, by which the object, when not vertical, 

 is made to appear higher than its true place ; lastly, 

 he adds the parallax (a small correction, inconsider- 

 able from the sun's distance), in order to reduce the 

 calculation for the centre of the earth ; for which 

 point all calculations are made, and which is ever 

 supposed to be the station of an observer. Having 

 made all these corrections, which many mariners 

 despatch summarily, by an addition of twelve min- 

 utes, he has the true meridian altitude of the sun. 

 Taking this from a quadrant, or ninety degrees, 

 gives its zenith distance, or distance from that point 

 in the heavens which is immediately over the ob- 

 server, and would be met by a straight line passing 

 from the centre of the earth through his position. 

 Now, if the sun were for ever on the equinoctial, the 

 zenith distance would always be the latitude ; for, 

 whilst the zenith is the observer's position, referred 

 to the heavens, the equator is there, in like manner, 



represented by the equinoctial ; and we have already 

 seen that latitude is the distance from the equator. 

 But, as the sun is only twice a year upon the equi- 

 noctial, and as his distance from it, at times, in- 

 creases to more than 20, it becomes necessary to 

 take this distance (called his declination) into the 

 estimate. The sun's declination is given, in the Al- 

 manac, for the noon of each day ; Toy correcting it 

 for the time anticipated or elapsed, according as the 

 sun comes first to him or to the first meridian, by his 

 position east or west of it, the observer obtains the 

 declination for noon at his own position. This decli- 

 nation applied to the zenith distance, by adding when 

 the sun is on the same side of the equator, by sub- 

 tracting when on the opposite side, gives the true 

 latitude. A cUiily and accurate knowledge of his 

 latitude is, then, to the mariner of our day, a deside- 

 ratum of easy attainment. By its aid, nothing is 

 easier than to sail clear of any rock or shoal that 

 crosses his track, either by a watchful look-out at 

 the moment of passing its latitude, or else by avoid- 

 ing its parallel entirely, until it be surely passed. 

 Moreover, this is his best and surest guide in aiming 

 at his destined port ; for he has but to attain the exact 

 latitude it lies in, and then sail directly upon it, east 

 or west, to be sure of success. And here nature is 

 again his friend : by a singular coincidence, discover- 

 able in glancing at the map of the world, most coasts 

 and continents lie in a northern and southern direc- 

 tion. Hence the value attached, by seamen, to an 

 accurate knowledge of the latitude ; and hence the 

 saw of " Latitude, lead, and look-out." But if it be 

 possible to obtain the longitude with any thing like 

 an equal ease and certainty, no one will dispute 

 its advantage. Although, as we have stated, most 

 coasts follow a northern and southern direction, 

 there are yet not a few, such as both coasts of Cuba 

 and San Domingo, which lie east and west, so that 

 points along them are only determined by the longi- 

 tude. And even to have the satisfaction, not merely 

 to run his finger along the chart, and say, " I am 

 somewhere along that parallel," but to be able to 

 point to the spot, and say, " I am there," is, to the 

 nice navigator, no insufficient motive. 



Various ways have been devised to find the longi- 

 tude, in all of which the great element is time. Inas- 

 much as the earth performs her diurnal revolution in 

 twenty-four hours, from the time any given meridian 

 is brought under the sun until it reaches it again, it 

 follows that twenty- four hours and 360 degrees are 

 both equal to a circle, and that the equator and other 

 circles of longitude may be indifferently estimated 

 by either of these divisions. Hence the difference of 

 time between two places, is no other than the diffe- 

 rence between the sun's coming to their respective 

 meridians, or, in a word, their difference of longi- 

 tude ; and hence it follows that if we, by any means, 

 simultaneously ascertain the time at the first meri- 

 dian, and the time at ship, we shall have ascertained 

 the longitude. The easiest method of solving this 

 problem is by means of the chronometer. This is a 

 watch so nicely constructed as to go with perfect 

 uniformity, either having no error whatever, or else 

 losing or gaining a known quantity every day. This 

 watch is set to the time of the first meridian, and its 

 rate is carefully ascertained, before leaving the land. 

 To find the longitude by means of it, the mariner has 

 merely to take an observation of the sun or other 

 star, when rising or falling rapidly, and deduce the 

 time of ship ; this, compared with the time at the 

 first meridian, simultaneously given by the chrono- 

 meter, determines the longitude. Several chrono- 

 meters concurring with each other, may make the 

 mnrinei -ure of his position ; but a single one, 

 unchecked by other tl?.tH. and liable, from its nicety 



