LATITUDES AND LONGITUDES. 



stars. Knowing this, and knowing the hour where he is, the 

 difference of the longitude of a ship and the observatory at 

 Greenwich is known to him. 



13. To supply ships leaving the Thames on long voyages with 

 the exact Greenwich time, the following expedient is adopted : 



The Royal Observatory, built on an elevated ridge, forms a 

 conspicuous object from the- river. It was, therefore, decided 

 that a signal should be given at the instant of one o'clock 

 in the afternoon of each day ; by observing which, navigators 

 within view of the observatory could correct their chronometers. 

 The signal adopted for this purpose was the sudden fall of a 

 large black ball, placed upon a pole raised from the top of one of 

 the towers of the observatory. 



Before elevating the ball, at five minutes before one o'clock, a 

 signal is made of the intention to do so by raising it half-mast 

 high. Observers are then instructed to prepare their chronome- 

 ters ; and as the descent of the ball occupies several seconds, 

 they should confine their attention to observing the moment when 

 the ball leaves the top, 'as it is that alone which indicates the 

 hour. 



The use of this signal is not merely confined to the indication 

 of the mean time at Greenwich for navigators going down the 

 river. By observing the drop of the ball, repeated day after 

 day, mariners who are in the river will be enabled to ascertain 

 the daily rate of their chronometers. 



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