The mechanism is used to draw circular arcs of small curvature. When link 2 is adjusted a long link 1, which is permitted by the difference in diameter between the hole for, and the stem of, screw 6, all the links change their position so that the distance between hinges, B͞B', B͞'B" , B͞''B''', . . . , remains constant, distances C͞C', C͞'C'', . . . , increase and distances A͞A', A͞'A'', . . . , are reduced. Hence, the ruler is bent to the required arc and screw 6 holds it in this position.
$745$LW,MO$