The field of view of mangnifying glass 1, located at the position of the tracing stylus of the pantograph, has two lines d and b perpendicular to each other. Curve a is traced with the intersection of the lines so that line d is always tangent to the curve and line b is along a normal to the curve. Cords 6 and 5, running tightly over disks 2, 3 and 4, transmit rotation of magnifying glass 1 to recording wheel 7, mounted on link 8 of the pantograph. As a result, recording wheel 7 rolls along the plane of the drawing so that the plane of the wheel is always parallel to line d. Therefore, the point of contact of wheel 7 with the plane of the drawing describes a curve similar to the given curve and recording wheel 7 measures the length of an arc of this curve.
$1971$FL,M$