LONGITUDE MEASURED BY TIME. 



New 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. 

 We can, then, always discover the longitude of any place, pro- 

 vided we can ascertain, at any moment, the hour of the day at 

 the place in question, and 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 circum- 

 stances of our position. If we are on land, and supplied with a 

 proper transit instrument, we can, by its means, observe the 

 moment at which the centre of the sun's disc 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 latitude 

 of the place at which we are. 



If we are at sea, where we cannot command a transit instru- 

 ment, nor use it if we 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 circumstances, whether at sea or on land, there is no 

 practical difficulty in determining 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 to the mind, would be to carry a good 

 chronometer from the place from which 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, 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 



* There are several corrections to be attended to in the practical working 

 of the methods of determining latitude and longitude which we have pur- 

 posely omitted, as they do not affect the spirit of the method, which is all 

 we would here convey. 



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