[Fig. 164]. Parallel ruler, consisting of a simple straight ruler B, with an attached axle C, and a pair of wheels A A. The wheels, which protrude but slightly through the under side of the ruler, have their edges nicked to take hold of the paper and keep the ruler always parallel with any lines drawn upon it.
Figs. 163, 164. Balance—Ruler
[Fig. 165]. Compound parallel ruler, composed of two simple rulers A, A, connected by two crossed arms pivoted together at the middle of their length, each pivoted at one end to one of the rulers, and connected with the other one by a slot and sliding pin, as shown at B. In this the ends as well as the edges are kept parallel. The principle of construction of the several rulers represented is taken advantage of in the formation of some parts of machinery.
[Fig. 166]. A simple means of guiding or obtaining a parallel motion of the piston rod of an engine. The slide a moves in and is guided by the vertical slot in the frame, which has been planed to a true surface.
Figs. 165, 166. Ruler—Parallel motion
[Fig. 167]. Parallel motion for direct-action engines. In this, the end of the bar B C is connected with the piston-rod, and the end B slides in a fixed slot D. The radius bar F A is connected at F with a fixed pivot, and at A midway between the ends of B C.