LUXAR METHOD. 



never go down, nor fall into disrepair ; a chronometer which is 

 exempt from the accidents of the deep ; which is undisturbed by 

 the agitation of the vessel ; which will at all times be present 

 and available to him wherever he may wander over the trackless 

 and unexplored regions of the ocean. Such a chronometer has 

 been found ; made by an Artisan who cannot err, and into 

 whose works imperfection can never enter. Such a chronometer 

 is supplied by the firmament itself. The unwearied labours of 

 modern astronomers have converted the face of the heavens into 

 a clock, and have taught the mariner to read its complicated but 

 infallible indications. We may regard for this purpose the 

 firmament as the dial-plate of a chronometer on an immense scale. 

 The constellations and the fixed stars upon it, which, for count- 

 less ages, are subject to no change in position, serve as the 

 hoar and minute-marks. The sun, the moon, and the planets, 

 which move continually over the surface of this splendid 

 piece of mechanism, play the parts of the hands of the clock. 

 The positions of these bodies from day to day and from hour 

 to hour, and every change of their positions, are accurately 

 foreknown and exactly registered in a book published some two 

 or three years in advance, called the " Nautical Almanac," and 

 circulated for the benefit of mariners. In this work the navigator 

 is told what the hour is or will be at Greenwich for every 

 variety of position which the sun, moon, and planets shall have 

 from time to time upon the heavens. But of all objects in the 

 heavens, that which is best suited for this species of observation 

 is the moon, and hence this method of determining the longitude 

 at sea has been distinguished by the appellation of the lunar 

 method. By the use of Hadley's sextant, which we have already 

 alluded to, it is easy, whenever the heavens are clear, to observe 

 the angular distance of the moon either from the sun or from the 

 most conspicuous stars or planets. The motion of the moon in 

 the firmament is so rapid that its change of position is perceptible, 

 even by such observations as can be made on board a ship from 

 hour to hour. 



How, then, it may be asked, can such observations be made 

 subservient to the discovery of the longitude of a ship ? Nothing 

 can be more simple. The navigator requires only to know what 

 is the hour at Greenwich at the time he makes his observation. 

 This he discovers in the following manner : He observes with 

 the sextant the distance of the moon from the sun, or from some 

 of the most conspicuous stars ; he then, after certain pre- 

 liminary calculations not necessary to detail here, examines the 

 ' Nautical Almanac," where he learns what the hour is at Green- 

 wich, when it has these particular distances from the sun or the 



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