THE LATITUDES AND LONGITUDES. 539 



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> means by which we can discover, at the same time, what the hour of the day 



> is at Greenwich, and that by these means we know that it is 56 minutes past 

 j 6 o'clock. We know, then, that the time is 4 hours 5G minutes earlier at New 

 J York than at Greenwich, and consequently we infer that New York must be 



west of Greenwich by a longitude which corresponds to 4 hours 56 minutes. 

 ] Now 4 hours correspond to 60, and 56 minutes correspond to 14 ; therefore 



> it follows, that the longitude of New York must be 74 west of Greenwich, 

 j We can, then, always discover the longitude of any place, provided we can 



ascertain, at any moment, the hour of the day at the place in question, and 

 I know, at the same time, what the hour of the day is in that place from which 

 ; the longitude is measured.* 



There are simple methods of observation and calculation by which the 



> hour of the day in the place where we are can be determined, with more or 

 less accuracy, according to the circumstances of our position. If we are on 

 land, and supplied with a proper transit instrument, we can, by its means, ob- 

 serve the moment at which the centre of the sun's disk passes the meridian. 

 Thus, as the moment of noon arrives, by observing it, we can set a good clock, 

 which will inform us of every other hour of the day. But even in the absence 

 of a clock we can determine the hour of the day at any moment at which the 

 sun is visible, by observing its altitude, having previously ascertained the lati- 

 tude of the place at which we are. 



If we are at sea, where we cannot command a transit instrument, nor use it if 

 \ve could, the latitude of the place of the ship is first determined, and then the- 

 hour is found by observing the altitude of the sun at any convenient time in the 

 afternoon or forenoon. The hour being once found, the time can be kept by a 

 chronometer for any number of hours afterward. Thus it appears, under all cir- 



) cumstances, whether by sea or by land, there is no practical difficulty in de- 

 termining what o'clock it is where we are. This at once reduces the problem ( 

 of the longitude to the simple discovery of the hour of the day, at any given * 

 time, at the place from which the longitudes are reckoned. 



The first and most obvious method of accomplishing this which would occur j 

 to the mind, would be to carry a good chronometer from the place from < 

 \\hich the longitude is reckoned. Supposing this chronometer subject to no { . 

 error, it will continue to inform you of the hour of the day at that place. Thus, i 

 suppose that on leaving London the mariner takes with him a chronometer set | 

 according to the time at Greenwich, and with it makes his voyage to New < 

 York ; the chronometer will 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 will find that it will be 4 minutes after 

 7 o'clock. He will therefore 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 perfection of chro- 

 nometers 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 de- 5 

 gree of perfection by the skill of modern artisans, these instruments are even yet, 



'l and probably will ever continue to be, too imperfect to be implicitly and exclu- 



* There are several corrections to be attended to in the practical working of the methods of deter- 

 S mining latitude and longitude which I have purposely omitted, as they do not affect the ipint of 

 ) the method, which is all I would here convey. 



C-V.-X 



