Fig. 797. A continuous rotary motion of the shaft carrying the 3 wipers produces a reciprocating rectilinear motion of the rectangular frame. The shaft must revolve in the direction of the arrow for the parts to be in the position represented.

Fig. 798. Chinese windlass. This embraces the same principles as the micrometer screw, Fig. 779. The movement of the pulley in every revolution of the windlass is equal to half the difference between the larger and smaller circumferences of the windlass barrel.

Fig. 799. Shears for cutting metal plates. The jaws are opened by the weight of the long arm of the upper one, and closed by the rotation of the cam.

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Fig. 800. A system of crossed levers, termed lazy tongs. A short alternating rectilinear motion of rod at the right will give a similar but much greater motion to the rod at the left. It is frequently used in children's toys. It has been applied in France to a machine for raising sunken vessels; also applied to ships' pumps three-quarters of a century ago.

Fig. 801. This is a motion which has been used in presses, to produce the necessary pressure upon the platen. Horizontal motion is given to the arm of the lever which turns the upper disc. Between the top and bottom discs are 2 bars which enter holes in the discs. These bars are in oblique positions, as shown in the drawing, when the press is not in operation; but when the top disc is made to rotate, the bars move toward perpendicular positions and force the lower disc down. The top disc must be firmly secured in a stationary position, except as to its revolution.

Fig. 802. On rotating the disc carrying the crank-pin working in the slotted arm, reciprocating rectilinear motion is imparted to the rack at the bottom by the vibration of the toothed sector.

Fig. 803. A simple press-motion is given through the hand-crank on the pinion-shaft, the pinion communicating motion to the toothed sector, which acts upon the platen, by means of the rod which connects it therewith.

Fig. 804. Uniform circular motion into rectilinear, by means of a rope or band, which is wound several times around the drum.

Fig. 805. Modification of the triangular eccentric, Fig. 761, used on the steam engine in the Paris Mint. The circular disc behind carries the triangular tappet, which communicates an alternate rectilinear motion to the valve-rod. The valve is at rest at the completion of each stroke for an instant, and is pushed quickly across the steam-ports to the end of the next.

Fig. 806. On turning the cam at the bottom a variable alternating rectilinear motion is imparted to the rod resting on it.

Fig. 807. A cam-wheel, of which a side view is shown, has its rim formed into teeth, or made of any profile form desired. The rod to the right is made to press constantly against the teeth or edge of the rim. On turning the wheel, alternate rectilinear motion is communicated to the rod. The character of this motion may be varied by altering the shape of the teeth, or profile of the edge, of the rim of the wheel.

Fig. 808. Expansion eccentric, used in France to work the slide-valve of a steam-engine. The eccentric is fixed on the crank-shaft, and communicates motion to the forked vibrating arm, to the bottom of which the valve-rod is attached.

Fig. 809. The internal rack, carried by the rectangular frame, is free to slide up and down within it for a certain distance, so that the pinion can gear with either side of the rack. Continuous circular motion of the pinion is made to produce reciprocating rectilinear motion of rectangular frame.

Fig. 810. Endless band-saw. Continuous rotary motion of the pulleys is made to produce continuous rectilinear motion of the straight parts of the saw.

Fig. 811. The toggle-joint arranged for a punching machine. Lever at the right is made to operate upon the joint of the toggle by means of the horizontal connecting link.

Fig. 812. Movement used for varying the length of the traversing guide-bar, which in silk machinery guides the silk on to spools or bobbins. The spur-gear turning freely on its centre, is carried round by the larger circular disc, which turns on a fixed central stud, which has a pinion fast on its end. Upon the spur-gear is bolted a small crank, to which is jointed a connecting-rod attached to traversing guide-bar. On turning the disc, the spur-gear is made to rotate partly upon its centre by means of the fixed pinion, and consequently brings crank nearer to centre of disc. If the rotation of disc was continued. the spur-gear would make an entire revolution. During half a revolution the traverse would have been shortened a certain amount at every revolution of disc, according to the size of spur-gear; and during the other half it would have gradually lengthened in the same ratio.

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