540 THE LATITUDES AND LONGITUDES. 



sively relied upon. If we only required their indications for short spaces of 

 time, such as a few days, or even weeks, we might perhaps place a secure re- 

 liance upon them ; especially if the voyager were provided with more than one 

 instrument of this kind. But in voyages or journeys which occupy mon.ns, 

 we cannot rely on the indications of these instruments, even when most liberally 

 provided and most perfectly constructed. 



In the absence, then, of a chronometer, how, it will be asked, can the lon- 

 gitude of a place be ascertained at all. The first method that will occur to the 

 min 1, will be that of some conspicuous signal which can be seen at the same 

 time at. the two places, whose difference of longitude is to be determined. 

 For this we require two observers ; but it is perhaps the method of all others, sus- 

 ceptible of the greatest accuracy. Let us suppose that on some elevated posi- 

 tion between two distant places, such as New York and Boston, a sudden 

 and conspicuous light is produced, such as the celebrated Drummond liijht. 

 which might be exhibited on the top of a high mountain so as to be visible a 

 great distance. Let this signal be exhibited at any required moment, so as to 

 render it suddenly visible at the two places. Let the observers at these places 

 note precisely the hour of the day or night at which the light is seen. By 

 comparing afterward, these times, their difference will at once give us the 

 difference of the longitude at the two places. 



But this method is evidently applicable only on a limited scale, and under pe- 

 culiar circumstances ; it is altogether unavailable to the mariner. Now the 

 . astronomer supplies him with a chronometer of unerring precision ; a chro- 

 nometer which can never go down, nor fall into disrepair; a chro : 

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

 s iion 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 labors of modern as 

 omers have converted the face of the heavens into a clock, and have : 

 the 'iiariner to read its complicated but infallible indications. We may ; 

 for this purpose the firmament as the dial-plate. of a chronometer on an im- 

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

 less ages are subject to no change in position, serve as the hour and minute- 

 marks. The sun, the moon, the planets, and the satellites, which move 

 continually over the surface of this splendid piece of mechanism, play.hr 

 of the hands of the clock. The positions of these bodies from day to d,. 

 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. 



the " Nautical Almanac,'' and circulated for the benefit of ti.arii!ers. 

 this work, the navigator is told what the hour is or will be at Greenwich for 

 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 deter- 

 mining the longitude at sea has been distinguished by the appellation ol 

 lunar method. By the use of Hadley's sextant, which we 1, 



to, it is easy, whenever the Leavens are. clear, to obser.v the angular distance \ 

 of the moon either from the sun or from the most conspicuous stnrs cr pi; 

 The motion of the moon in the firmament is so rapid that it:' c ;;;:(. oi 

 tion is perceptible, even by such observations as can be mat <- IT, c^r.l a 

 from hour to hour. 



How, then, it may be asked, can such observations be i>: 

 the discovery of the longitude of a ohip I Nothing can be more, oiiii! 



