You will need
  • line;
  • the compass.
Find the radius of the arc of a circle. To do this, take a compass and draw a new circle at three points. Point it is advisable to choose are located far enough away from each other, therefore advisable to take the extreme point of the arc and a point approximately in the center. Every two circles must intersect at two points. Swipe through these two points line. Where these two lines intersect is the center of the arc of a circle. The radius is the distance from the center to any point on the circle.
Guide segments from the center to the extreme points of the arc. They form an angle, called Central. If possible, measure it. Length of an arc in m of degree equal to the product of PI, radius, arc and m degrees divided by 180 degrees. pm=π*r*m/180.
It may be that to measure the angle nothing. In this case, the output angle of the triangle, if possible, or use the formula of Huygens.
Connect the extreme points of the arc A and B. C Find the midpoint of the segment AB. Note the arc at the middle of m. It lies on the perpendicular drawn to AB through C.
Calculate the length of the arc according to the formula of Huygens, having measured the required values: p≈2k+1/3*(2k-d). Here k=AM d=AB. The formula of Huygens estimated and has an error.