APPENDIX. 



Whenever the diftance of the object, vvhofe angle is 

 taken by the megameter, does not exceed that of the 

 vifible horizon, the very fmali portion of the earth's 

 furface intercepted between the objed and obferver, may 

 be confidered as a plane, to which the objeft is perpendi- 

 cular, and the diftance may be concluded by refolving the 

 right-angled triangle, formed by the upright objedl, and 

 lines drawn from the obferver's ftation to the top and 

 bottom of it. . 



But in greater dillances, the bottom of the objedl being 

 concealed from the fight of the obferver, it becomes ne- 

 ceffary to have recourfe to a different calculation. 



The only cafes which can occur in pradice are two ; 

 the one when the height is given to find the diftance ; 

 the other when, the diftance being known, the height of 

 the objeft is to be deduced from the obfervation: both 

 which are eafily folved by the following praftical rules. 



To find the Diftance. 



To the apparent altitude of the object above the fen- 

 fible horizon, add the complement of the dip'anfwering 

 to the height of the obferver's eye above the fea; the 

 fum is the angle B AE (fig. i:); and fay : As the femi- 

 diameter of the earth increafed by the height of the objed:, 

 is to the femidiameter increafed by the height of the 



eye ; . 



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