Fig. 943. Parallel ruler, consisting of a simple straight ruler B, with an attached axle C, and 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.

Fig. 944. Compound parallel ruler, composed of 2 simple rulers A, A, connected by 2 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. 945. Parallel ruler composed of 2 simple rulers A, B, connected by 2 pivoted swinging arms C, C.

Fig. 946. 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 is planed to a true surface.

Fig. 947 differs from Fig. 946 in having rollers substituted for the slides on the cross-head, said rollers working against straight guide-bars a, a, attached to the frame. This is used for small engines in France.

Fig. 948. A parallel motion invented by Dr. Cartwright in the year 1787. The toothed wheels C, C, have equal diameters and numbers of teeth, and the cranks a, a, have equal radii, and are set in opposite directions, and consequently give an equal obliquity to the connecting rods during the revolution of the wheels. The cross-head on the piston-rod being attached to the 2 connecting rods, the piston-rod is caused to move in a right line.

Fig. 949. A piston-rod guide. The piston-rod A is connected with a wrist attached to a cog-wheel B, which turns on a crank-pin, carried by a plate C, which is fast on the shaft. The wheel B revolves around a stationary internally-toothed gear D, of double the diameter of B, and so motion is given to the crank-pin, and the piston-rod is kept upright.

Fig. 950. The piston-rod is prolonged and works in a guide A, which is in line with the centre of the cylinder. The lower part of the connecting rod is forked to permit the upper part of the piston-rod to pass between.

Fig. 951. Table engine. The cylinder is fixed on a table-like base. The piston-rod has a cross-head working in straight slotted guides fixed on top of cylinder, and is connected by 2 side connecting rods with 2 parallel cranks on shaft under the table.

Fig. 952. An engine with crank motion like that represented in Fig. 753 and Fig. 926, the crank-wrist journal working in a slotted cross-head A. This cross-head works between the pillar-guides D, D, of the engine framing.

Fig. 953. A parallel motion used for the piston-rod of side-lever marine engines. F, C is the radius bar, and E the cross-head to which the parallel bar E, D, is attached.

Fig. 954. A parallel motion used only in particular cases.

Fig. 955 shows a parallel motion used in some of the old single-acting beam-engines. The piston-rod is formed with a straight rack gearing with a toothed segment on the beam. The back of the rack works against a roller A.

Fig. 956. An arrangement of parallel motion for side-lever marine engines. The parallel rods connected with the side rods from the beams or side levers are also connected with short radius arms on a rack-shaft working in fixed bearings.

Fig. 957. A parallel motion commonly used for stationary beam-engines.

Fig. 958. 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.

Fig. 959. Mode of obtaining 2 reciprocating movements of a rod by one revolution of a shaft, patented in 183G by B. F. Snyder, has been used for operating the needle of a sewing machine, by J. S. McCurday, also for driving a gang of saws. The disc A on the central rotating shaft has 2 slots a, a, crossing each other at a right angle in the centre, and the connecting rod B has attached to it 2 pivoted slides c, c, one working in each slot.

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Fig. 960. Another parallel motion. Beam D, C, with joggling pillar support B, F, which vibrates from the centre F. The piston-rod is connected at C. The radius bar E, A, produces the parallel motion.

Fig. 961. Grasshopper beam-engine. The beam is attached at one end to a rocking pillar A, and the shaft arranged as near to the cylinder as the crank will work. A is the radius bar of the parallel motion.

Fig. 962. A modification, in which the radius bar is placed above the beam.

Fig. 963. Old-fashioned single-acting beam pumping engine on the atmospheric principle, with chain connection between piston-rod and a segment at end of beam. The cylinder is open at, top. Very low pressure steam is admitted below piston, and the weight of pump-rod and connections at the other end of beam helps to raise piston. Steam is then condensed by injection, and a vacuum thus produced below piston, which is then forced down by atmospheric pressure, thereby drawing up pump-rod.

Fig. 964. Parallel motion for upright engine. A, A are radius rods connected at one end with the framing, and at the other with a vibrating piece on top of piston-rod.