on the Atmosphere of Venus. 329 
Log. T S = 4997758 
Log . c S = 4857022 
Log. tang. 10,140736 
belongs to 54 0 7' 30" 
Subtract 45 
9° 7 ' 3°" 
Log. tang. 9 0 7' 30" = 9205803 
Log. tang. 87° 29' = 11357018 
Log. tang, semid. 10,562821 
Half the difference of the angles = 74 0 41' if F 
Half the sum = 87 29 o 
Angle S c T =162° io' 15" 
Angle cTS = 12 47 45 
Now the angle at Venus being = 162° 10' 15". Its comple- 
ment will be = 17 0 49' 45". But this is = the angle C c G = 
fcg, fig. 7, Tab. VI. and hence we farther derive the sine of 
the arc/ g= sin./ c x sm.fcg ; whence 
Log. sin. 15 0 19' o" = 9,4218566 
+ Log. sin. 17 49 45 = 9>4 8 597 68 
Log. sin .fg = 8,9078334 
which gives fg = 4 0 38' 30". 
Such, therefore, is the length of the arc of a great circle 
of Venus over which its twilight extends, as deduced from its 
appearance to the inhabitants of our globe under the most fa- 
vourable circumstances : and if the true semi-diameter of the 
