Instruments for drawing dashed or similar lines e.g. dot-dashed or, alternatively, continuous lines are known. The patent to Simmons U.S. Pat. No. 2,081,439 teaches an instrument for drawing dashed lines wherein a horizontal carriage has fixed along one edge a downwardly extending fin which is slideably received in a longitudinally extending, open-topped groove in a ruler. The carriage extends beyond one longitudinal edge of the ruler and at its outer end it carries a vertically adjustable scriber such as a drawing pen. Intermediate the one longitudinal edge of the ruler and the slot is a follower which engages a longitudinally extending changeable plate having on its upper edge undulations defining peaks and valleys. As the carriage is moved longitudinally the follower follows the undulations and periodically lifts the scriber point from the paper being marked whereby a dashed line is drawn. The lengths of the dashes, that is, the parts of the line drawn by the scriber as the carriage is moved relative to the ruler, can be controlled by adjusting the scriber relative to the carriage so that the scriber pointer is closer to or farther away from the carriage. If it is lowered a substantial distance, the carriage and the follower are lifted relative to the ruler and the follower may just hit only the tops of the peaks whereby only short blank portions appear in the line between long dashes. If the point is lowered still more the follower is lifted entirely clear of the peaks and a continuous straight line is drawn.
In theory, the Simmons unit performs as intended. However, because its guide fin rides in an open top groove and is rigidly fastened to the carriage, as the follower traverses the peaks and valleys on the plate edges, the carriage must perforce rock which causes the fin to periodically cock in the groove thus impeding movement of the carriage, or the groove must be wide enough to cater to this problem thus producing a zig-zag line, or the entire carriage and its fin must move vertically. It is difficult for an operator who is moving the carriage by a knob intermediate the groove and guide to control his manual vertical pressure on the knob so that the entire carriage can rise an fall and if he accidently lifts the carriage the pen can skip more than intended resulting in an uneven dashed line.