[ 446 ] 
The Calculation. 
. DN : DM 1 , 
f. |BFD : f. 1 C FD J :: rad * 
rad. : t. /. x co 45 0 :: t. | B F C : t. 
: ^ X 
COD c/s D C O 
I f%> 45 °> COD > D C O 
And 
if x A 45°> z. COD c DCO. 
If the intervals between the obfervations are fo 
fmall, that the lines differ not much from the arches, 
the arches BC, CD may be counted in time, and 
the calculation may be abbreviated thus: 
D M : D N :: arc. B D : Z (for D O) 
DC-l-Z : * DC:: jBC: SR. 
DMx DC-j-DN x BD : DM x DC 
\ BC 
SR. 
• XXXI. On 
