LATITUDES AND LONGITUDES. 



continue to inform him what the time is from hour to hour at 

 Greenwich. When he arrives at New York, he will find that 

 when the chronometer points to 12 o'clock, or noon, it will be 

 early in the morning ; and if he ascertains the hour exactly, he 

 wall find that it will be 4 minutes after 7 o'clock. He will there- 

 fore know that the time at New York is 4 hours 56 minutes 

 earlier than at Greenwich, and, consequently, that New York must 

 be 74 west of Greenwich. It is for these reasons that the perfec- 

 tion of chronometers has always been considered so essential to the 

 progress of navigation. Every ship that makes a long voyage 

 ought to be supplied with one, at least, of these instruments ; but 

 as they are liable to accident, and as even the best of them 

 cannot be rendered perfect, it is usual with ships that are well 

 provided for long voyages to carry more than one chronometer. 



Although the art of constructing time-keepers has been 

 brought to a high degree of perfection by the skill of modern 

 artisans, these instruments are even yet, and probably will ever 

 continue to be, too imperfect to be implicitly and exclusively 

 relied upon. If we only required their indications for short 

 spaces of time, such as a few days, or even weeks, we might per- 

 haps place a secure reliance upon them ; especially if the voyager 

 were provided with more than one instrument of this kind. But 

 in voyages or journeys which occupy months, 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 longitude of a place be ascertained at all ? The first 

 method that will occur to the mind, will be that of some con- 

 spicuous 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 susceptible of the greatest accuracy. Let us suppose 

 that on some elevated position, between two distant places, such 

 as London and Birmingham, a conspicuous light is produced, such 

 as the celebrated electric light, which might be exhibited on 

 the top of a high mountain so as to be visible at once from both 

 places. Let this signal light be suddenly extinguished, and let 

 the observers stationed at London and Birmingham note the 

 exact moment at which this extinction takes place. By com- 

 paring afterwards these times the exact difference of longitude of 

 the two places will be found. 



12. But this method is evidently applicable only on a limited 

 scale, and under peculiar circumstances ; it is altogether unavail- 

 able to the mariner. Now the astronomer supplies him with a 

 chronometer of unerring precision ; a chronometer which can 

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