70 



HYDROGRAPHY. 



[Sect. III. 



dries, state what water there is over it at high-water 

 springs, and at what time of the tide it becomes dry. 



13. When nearing a coast, and at all times when at a 

 greater distance from the shore in miles than the amount of 

 dip in rmrmes due to the heighi of your eye, the height of 



mountains, or of other objects, may be determined with 

 considerable accuracy if the weather be clear, and proper 

 precautions be taken. To do this, if the distance of the 

 object be not known, it must be found by measuring 

 a, base mth the patent log. There are various methods 

 -" m navigation-books for determining this p, oblem • 

 shall therefore here merely describe the observations 

 required to be made. At each end of the ba.P Tr,.n..„. 



given 

 I 



# 



on ; and it one of Gary's double sextants be on board 

 measure the terrestrial refraction by bringing the opposite 

 horizons m contact with the arc both above and below the 

 index, and then reading off each time, divide the differ- 

 ence by 4 : this will give the dip and terrestrial refraction 

 combined, which is the proper quantity to be allowed in 

 correcting the observed angle.* 



In Raper's ' Navigation,' p. 90, 2nd edition, theme- 

 thod of determining a ship's distance from an object by 

 two bearings is briefly explained; and in Belcher's SSur. 

 veymg ' it is set forth in a manner so clear and ample as 

 to leave nothing to be desired : I shall therefore merely 

 observe here, that accord 



observatioas and the 



value 



mg to the accuracy of the 



of the means adopted, will 



* If the terrestrial refraction alone he required, 

 tny the true dip due to the heiglit of the eve, and 

 terrestrial refraction required. ' 



take from this quan- 

 the remainder i^ :he 



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f 



Sect- 



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be 



the 



col 





shou 



Id, ii"? 

 in the best 

 the ol^^ct 



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that vi 

 iU same tn 

 distance ru 



r^." 



arc 0')Sv, v.- 

 Hauled iu ^ 

 ^ ship sh( 



^v. 



operation, i 

 and fresh b 



Two obst 

 nations n 

 With the 



may often 1 



^liencr/.l] 

 ^e^it and p 



^y it as o. 

 •^^ --nt for 



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s. 



