CHAPTER XIII 



Shad- 

 well's 

 "Notes on 

 Chrono- 

 meters." 



Absolute 

 Longi- 

 tudes. 



Remarks 

 confined 

 to Differ- 

 ential 

 Longitude 

 and Sex- 

 tant. 



OBSERVATIONS FOR ERROR OF CHRONOMETER 



General Remarks ou Obtaiumg Longitude — Error by Equal Altitudes 

 Error by Two Star? at Equal Altitudes. 



The whole question of obtaining longitude by means of chrono- 

 meters is so ably and exhaustively treated by Captain Shad- 

 well in his " Notes on the Management of Chronometers," 

 both as regards the treatment of the watches, the method of 

 observation, and the various systems of obtaining meridian 

 distances, that we refer the reader to that work for full in- 

 formation on the subject. Here we do not pretend to give 

 more than the broader principles of the general question, but 

 a work of this kind, intended for the perusal of young sur- 

 veyors, would be incomplete without some reference to it. 



The methods of obtaining longitude, called " absolute 

 methods," which give the longitude of the place as measured 

 from the first meridian, directly and independently, such as 

 observations of occultations of the stars by the moon, moon 

 culminating stars, eclipses of Jupiter satellites, etc., are now 

 rarely employed in nautical surveying, and may be said to 

 be decidedly inferior in value to the results of good chrono- 

 metric runs. 



Similarly, altazimuths, portable transits, and other like 

 astronomical instruments, are now seldom or never supplied 

 to a surveying vessel. The sextant in a practised hand will 

 give results equal to those obtainable by fixed instruments 

 of small size, and has the great advantage of being more 

 portable, and always ready. 



To the sextant, telegraph, and chronometers, therefore, our 

 remarks ^\^ll be confined. 



By the use of the two latter we obtain only the " difference 



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