Relating to the FIGURE of the EARtH. ir 



(^*cof^^ + A'fin^y ^^ ^"^^^ ^' ^as been already fliewn that 



'^' r « 



« — U i. <» cof«* 1 ^ 



3* cofc' > ^"^ *' = B' '» tlierefore otD' = 





T— and <i — 



Now, I +^, — — - — 15^ = J. , therefore 



— ctD' cof » 



17. Tars value of a is very convenient for logarithmical cal- 

 culation ; for if fin <p 1/57 be computed, it will always be lefs 

 than I, becaufe D' is greater than D, and therefore may be ta- 

 ken for the fine of an arch i//, of which arch ^r^ — jj? fi" ?' will 

 of courfe be the cofine, fo that a = " ^^^'^° . 



The fame method may be ufed for finding j from the for- 

 mula in § 15. 



In the fame manner that a has been found, we will, obtain 



7 toD' cof ip* 



If we examine thefe formulas in the extreme cafes, viz. 

 when <p ■=: go", and when (pi — o, we fhall have in the for- 

 mer cafe « = 5-, becaufe cof (p = o, and alfo D' = D, fo that 



