COD wDCO 



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The Calculation. 



D N : D M 1 , ■ 



f. $BFD : f. fCFD J :: rad# : f * A % 



rad. : t. i_ x v> 45*° »t. |BFC:t. 



If % >45°> -iCOD>DGO 



And 

 if%z.45°, *- c OD < DCO. 



If the intervals between the obfervations are fo 

 fmall, that the fines differ not much from the arches, 

 the arches BC, CD may be counted in time, and 

 the calculation may be abbreviated thus : 



DM :DN:: arc. B D : Z (for DO) 

 DC + Z : aDC:: |BC: SR. 



Or, 

 DMxDC+DNxBD : DMxDC :: f BC : SR. 



XXXI. On 



