MODERN GUNS AND GUNNeRY.
297
Rectilinoar Figures. llThe ar~a of any plane rectilinear figure may be obtained graphica y ~y d.lViding it into triangles and rectangles, as ABC, or by re?ucmg It to one triangle, working on the principle that any two tnangles on the same base and between the same parallels are equal.
B
Circle.
Let r be the radius; then Circumference 27rr = 7rr~ Area
=
22
Where 7r
= 3.14159 or-.
7 I
Log.
7r
= .4971496 : log. - = 1.5021508. 7r
Sector of Circle. Let A be the number of degrees which the arc subtends at the centre: 7T'rA Then length of arc 180
= --
Area of sector
= I arc
X radius
7rr~X A