Fig. 857. Another modification of Fig. 852.

Fig. 858. A screw-clamp. On turning the handle the screw thrusts upward against the holder, which operating as a lever, holds down the piece of wood or other material placed under it on the other side of its fulcrum.

Fig. 859. A variety of what is known as the mangle-wheel. One variety of this was illustrated by Fig. 748. In this one the speed varies in every part of a revolution, the groove b, d, in which the pinion-shaft is guided, as well as the series of teeth, being eccentric to the axis of the wheel.

Fig. 860. Another kind of mangle-wheel, with its pinion. With this as well as with that in the preceding figure, although the pinion continues to revolve in direction, the mangle-wheel will make almost an entire revolution in one direction and the same in an opposite direction; but the revolution of the wheel in one direction will be slower than that in the other, owing to the greater radius of the outer circle of teeth.

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Fig. 861. Another mangle-wheel. In this the speed is equal in both directions of motion, only one circle of teeth being provided on the wheel. With all of these mangle-wheels the pinion-shaft is guided, and the pinion kept in gear, by a groove in the wheel. The said shaft is made with a universal joint, which allows a portion of it to have the vibratory motion necessary to keep the pinion in gear.

Fig. 862. The pinion B rotates about a fixed axis, and gives an irregular vibratory motion to the arm carrying the wheel A.

Fig. 863. A modification of what is called a mangle-rack. In this the pinion revolves, but does not rise and fall as in the former figure. The portion of the frame carrying the rack is jointed to the main portion of the frame by rods, so that when the pinion arrives at the end it lifts the rack by its own movement, and follows on the other side.

Fig. 864. An illustration of the transmission of rotary motion from one shaft to another, arranged obliquely to it, by means of rolling contact.

Fig. 865. Another form of mangle-rack. The lantern pinion revolves continuously in one direction, and gives reciprocating motion to the square frame, which is guided by rollers or grooves. The pinion has only teeth in less than half of its circumference, so that while it engages one side of the rack, the toothless half is directed against the other. The large tooth at the commencement of each rack is made to ensure the teeth of the pinion being properly in gear.

Fig. 866. A regular vibrating movement of the curved slotted arm gives a variable vibration to the straight arm.

Fig. 867 represents a wheel driven by a pinion of 2 teeth. The pinion consists in reality of 2 cams, which gear with 2 distinct series of teeth on opposite sides of the wheel, the teeth of one series alternating in position with those of the other.

Fig. 868. A continuous circular movement of the ratchet-wheel, produced by the vibration of the lever carrying 2 pawls, one of which engages the ratchet-teeth in rising and the other in falling.

Fig. 869. By turning the shaft carrying the curved slotted arm, a rectilinear motion of variable velocity is given to the variable bar.

Fig. 870. A modification of Fig. 853, by means of 2 worms and worm-wheels.

Fig. 871. A pin-wheel and slotted pinion, by which 3 changes of speed can be obtained. There are 3 circles of pins of equal distance on the face of the pin-wheel, and by shifting the slotted pinion along its shaft, to bring it in contact with one or the other of the circles of pins, a continuous rotary motion of the wheel is made, to produce 3 changes of speed of the pinion.

Fig. 872 represents a mode of obtaining motion from rolling contact. The teeth are for making the motion continuous, or it would cease at the point of contact shown in the figure. The fork catch is to guide the teeth into proper contact.

Fig. 873. What is called the Geneva-stop, used in Swiss watches to limit the number of revolutions in winding-up; the convex curved part a, b, of the wheel B serving as the stop.

Fig. 874. Another kind of stop for the same purpose.

Fig. 875. A continuous rotary motion of the large wheel gives an intermittent rotary motion to the pinion-shaft. The part of the pinion shown next the wheel is cut of the same curve as the plain portion of the circumference of the wheel, and therefore serves as a lock while the wheel makes a part of a revolution, and until the pin upon the wheel strikes the guide-piece upon the pinion, when the pinion-shaft commences another revolution.

Fig. 876, 877. Other modifications of the stop, the operations of which will be easily understood by comparison with Fig. 873.

Fig. 878. The two crank-shafts are parallel in direction, but not in line with each other. The revolution of either will communicate motion to the other with a varying velocity, for the wrist of one crank working in the slot of the other is continually changing its distance from the shaft of the latter.

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