CHAPTER XVI 



As re- 

 gards Sur- 

 veying 

 Opera- 

 tions. 



Refrac- 

 tion. 



Position 

 Early in 

 the Day. 



SEA OBSERVATIONS 



Do'^.ble Altitude — Sumner's Method — • New Navigation — Short Equal 

 Altitude — Circum-meridian Altitudes of Sun. 



For surveying purposes, observations at sea are mainly re- 

 quired for fixing the ship's position when sounding banks, or 

 looldng for vigias. 



We cannot hope to attain to any very great accuracy, and 

 are much dependent on weather and the state of the sea and 

 clearness of the horizon. As longitude must depend entirely 

 on the chronometers, we must in cases where we require all the 

 accuracy we can get, as in fixing the position of banks far away 

 in mid- ocean, wait until we can again obtain Error and rates to 

 give the final positions ; but with ordinarily good chronometers 

 our daily positions obtained whilst sounding will be correct, 

 comparatively one with the other, so that we can at once plot 

 and delineate the shape of the banks, which is what we want at 

 the time. 



In all observations at sea it must be remembered that the 

 horizon may be affected by obnormal refraction, and no depen- 

 dence can be placed on a latitude or longitude deduced from 

 altitudes observed on one side alone. In certain cases the error 

 may amount to 2' or even 3'. 



With a high smi at noon, when accuracy is aimed at, it is 

 well to observe the opposite side of the horizon ; an awkward 

 observation at first, which practice will render easy. 



One great object when sounding or looking for banks is to 

 obtain a position as early as possible in the day, after lying-to 

 probably all night, as in the vicinity of banks currents are nearly 

 always set up, and in variable directions, so that we cannot at 

 all depend upon dead reckoning, or upon finding ourselves 



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