Fig. 933 is the movable head of a turning lathe. By turning the wheel to the right. motion is communicated to the screw, producing rectilinear motion of the spindle, in the end of which the centre is fixed.

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Fig. 934. Too and lifter for working poppet-valves in steam engines. The curved toe on the rock-shaft operates on the lifter attached to the Lifting rod to raise the valve.

Fig. 935. Conical pendulum, hung by a thin piece of round wire. Lower end connected with and driven in a circle by an arm attached to a vertical rotating spindle. The pendulum-rod describes a cone in its revolution.

Fig. 936. Mercurial compensation pendulum. A glass jar of mercury is used for the bob or weight. As the pendulum-rod is expanded lengthwise by increased temperature, the expansion of mercury in the jar carries it to a greater height therein, and so raises its centre of gravity relatively to the rod sufficiently to compensate for downward expansion of the rod. As rod is contracted by a reduction of temperature, contraction of mercury lowers it relatively to rod. In this way the centre of oscillation is always kept in the same place, and the effective length of pendulum always the same.

Fig. 937. Compound bar compensation pendulum. C is a compound bar of brass and iron, or steel brazed together with brass downward. As brass expands more than iron, the bar will bend upward as it gets warmer, and carry the weights W, W, up with it, raising the centre of the aggregate weight M, to raise the centre of oscillation as much as elongation of the pendulum-rod would let it down.

Fig. 938. Watch regulator. The balance-spring is attached at its outer end to a fixed stud R, and at its inner end to staff of balance. A neutral point is formed in the spring at P, by inserting it between 2 curb-pins in the lever, which is fitted to turn on a fixed ring concentric with staff of balance, and the spring only vibrates between this neutral point and staff of balance. By moving lever to the right, the curb-pins are made to reduce the length of acting part of spring, and the vibrations of balance are made faster, and by moving it to left an opposite effect is produced.

Fig. 939. Compensation balance, t, a, t' is the main bar of balance, with timing screws for regulation at the ends. t and t' are 2 compound bars, of which the outside is brass and the inside steel, carrying weights b, b'. As heat increases, these bars are bent inward by the greater expansion of the brass, and the weights are thus drawn inward, diminishing the inertia of the balance. As the heat diminishes, an opposite effect is produced. This balance compensates both for its own expansion and contraction, and that of the balance-spring.

Fig. 940. Endless chain, maintaining power on going barrel, to keep a clock going while winding, during which operation the action of the weight or main-spring is taken off the barrel. The wheel to the right is the going wheel, and that to the left the striking wheel. P is a pulley fixed to the great wheel of the going part, and roughened, to prevent a rope or chain hung over it from slipping. A similar pulley rides on another arbor p, which may be the arbor of the great wheel of the striking part, and attached by a ratchet and click to that wheel, or to clock frame, if there is no striking part. The weights are hung, as may be seen, the small one being only large enough to keep the rope or chain on the pulleys. If the part b of the rope or chain is pulled down, the ratchet-pulley runs under the click, and the great weight is pulled up by c, without taking its pressure off the going wheel at all.

Fig. 941. Harrison's going barrel. Larger ratchet-wheel, to which the click R is attached, is connected with the great wheel G by a spring S, S'. While the clock is going the weight acts upon the great wheel G, through the spring; but as soon as the weight is taken off by winding, the click T, whose pivot is set in the frame, prevents the larger ratchet from falling back, and so the spring S, S', still drives the great wheel during the time the clock takes to wind, as it need only just keep the escapement going, the pendulum taking care of itself for that short time. Good watches have a substantially similar apparatus.

Fig. 942. A very convenient construction of parallel ruler for drawing, made by cutting a quadrangle through the diagonal, forming two right-angle triangles A and B. It is used by sliding the hypothenuse of one triangle upon that of the other.