Straightedge supporting device for rail type drawing instrument

A rail type drawing apparatus having a straightedge supporting device, said apparatus comprising a cross rail disposed at a side portion of a drawing board, a cross cursor slidably mounted on the cross rail, a longitudinal rail connected to the cross cursor at right angles to the cross rail, a weight means mounted for shifting movement along the longitudinal rail, a longitudinal cursor mounted on the longitudinal rail for shifting movement therealong, a head connected to the longitudinal cursor, a ruler mounting plate rotatably mounted on the head, a straightedge means adjustably mounted on the ruler mounting plate, means for fixedly engaging the ruler mounting plate to the head and disengaging it therefrom, a rope means having one end connected to said weight means and having exerted thereon a force in one direction due to the effect on gravity on said weight, the other end of said rope means being coupled to said longitudinal cursor and having a force exerted thereon by the weight of said cursor, said head, said plate and said straightedge means in the opposite direction, and turning force converting means mechanically coupled between said ruler mounting plate and said rope means for converting the rotary motion of the straightedge means around the head in a plane parallel with the surface of the drawing board under the effect of gravity into a force opposite to the direction of the force exerted on the rope by the weight means.