98 



HYDROGRAPHY. 



[Sect III 



m 



To find the Distance. 



Rule.— From the observed altitude subtract the dip less the terrestrial 

 refraction,* and call the remainder corrected altitude. To the constant 

 above mentioned 6*5424481 add the cosine of the corrected altitude -dnA 

 irom the cosine of the snm subtract the corrected altitude. The re- 

 mainder is the log of the approximate distance in arc. Divide the 

 approximate distance so found by the proportion of terrestrial refraction 

 allowed, aid subtract the quotient from the before found corrected alti- 

 tude for the true altitude. 



Lastly, add the cosine of the true altitude to the constant due to the 

 height of the object (a) ; find the cosine of the sum, and subtract from it 

 the true altitude ; the remainder is the distance in arc required. 



ExAMPT^. — Observed the altitude of Snowdon to be, On 45 

 its height being 3565 feet = 1188 yards, 



00 

 Off 45 10 



required its Constant and its Distance, height of eye 

 being 14 feet. 



Log of height 3=0792499 

 Constant. . 6'5424481 



45 5 mean. 



2)16'52980I8 



■T- 



Sine 



S'S;649009 



Cosine ™ 9^9999265 Constant required. 



To find the Distance. 



■i?i^>?-* 



m.'^ 



Constant for Snowdon 9 f* /Ssr? 

 Cosine corrected Alt, 9*999968*5 



/« 



1 ib 33 ~ Cosine 9-9998951 

 -41 20 Ait. 



y|5)34 13 Approx. dist 

 3 25 Correction. 



Observed Alt. * • , 45 

 Dip for 1 4 feet , * - 3 



W P 



05 



45 



4! 



Terrestrial ref. i^j of Dip + 



20 

 2S 



Corrected Alt. , . .41 

 Correction ♦ . , — 3 



42 



25 





n 



Constant . 

 Cosine true Alt, 



, 9 -9999205 

 , 9'999973! 



o 

 1 



IVue Alt. 



IS 

 38 



55 

 17 



Cosine : 

 miles. 



9-9998996 



35 38 = .35'6 = distance of object. 



* Tht? terrestrial refraction varies from | to -%% of the arc. 



- _ X Hh 



