Fig. 900 represents a pantograph for copying, enlarging, and reducing plans. One arm is attached to and turns on the fixed point C. B is an ivory tracing point, and A the pencil. Arranged as shown, if we trace the lines of a plan with the point B, the pencil will reproduce it double the size. By shifting the slide attached to the fixed point C, and the slide carrying the pencil along their respective arms, the proportion to which the plan is traced will be varied.

Fig. 901. Union coupling. A is a pipe, with a small flange abutting against the pipe C, with a screwed end; B, a nut which holds them together.

Fig. 902. Anti-friction bearing. Instead of a shaft revolving in an ordinary bearing, it is sometimes supported on the circumference of wheels. The friction is thus reduced to the least amount.

Fig. 903. A mode of releasing a sounding weight. When the piece projecting from the bottom of the rod strikes the bottom of the sea, it is forced upwards relatively to the rod, and withdraws the catch from under the weight, which drops off and allows the rod to be lifted without it.

Fig. 904. Releasing hook used in pile-driving machines. When the weight W is sufficiently raised, the upper ends of the hooks A, by which it is suspended, are pressed inward by the sides of the slot B, in the top of the frame; the weight is thus suddenly released, and falls with accumulating force on to the pile-head.

Fig. 905. A and B are two rollers, which require to be equally moved to and fro in the slot C. This is accomplished by moving the piece D, with oblique slotted arms, up and down.

Fig. 906. Centrifugal check-hooks, for preventing accidents in case of the breakage of machinery which raises and lowers workmen, or ores, in mines. A is a framework fixed to the side of the shaft of the mine, and having fixed studs D, attached. The drum on which the rope is wound is provided with a flange B, to which the check-hooks are attached. If the drum acquires a dangerously rapid motion, the hooks fly out by centrifugal force, and one or other, or all of them, catch hold of the studs D, and arrest the drum, and stop the descent of whatever is attached to the rope. The drum ought, besides this, to have a spring applied to it, otherwise the jerk arising from the sudden stoppage of the rope might produce worse effects than its rapid motion.

Fig. 907. A sprocket-wheel to drive or to be driven by a chain.

Fig. 908. A differential movement. The screw C works in a nut secured to the hub of the wheel E, the nut being free to turn in a bearing in the shorter standard, but prevented by the bearing from any lateral motion. The screw-shaft is secured to the wheel D. The driving shaft A carries 2 pinions F and B. If these pinions were of such size as to turn the 2 wheels D and E with an equal velocity, the screw would remain at rest; but the said wheels being driven at unequal velocities, the screw travels according to the difference of velocity.

Fig. 909. A combination movement, in which the weight W moves vertically with a reciprocating movement, the down-stroke being shorter than the up-stroke. B is a revolving disc, carrying a drum, which winds round itself the cord D. An arm C is jointed to the disc and to the upper arm A, so that when the disc revolves, the arm A moves up and down, vibrating on the point G. This arm carries with it the pulley E. Suppose we detach the cord from the drum and tie it to a fixed point, and then move the arm A up and down, the weight W will move the same distance, and in addition the movement given to it by the cord, that is to say, the movement will be doubled. Now, let us attach the cord to the drum, and revolve the disc B, and the weight will move verti-cally with the reciprocating motion, in which the down-stroke will be shorter than the up-stroke, because the drum is continually taking up the cord.

Figs. 910, 911. The first of these figures is an end view, and the second a ride view, of an arrangement of mechanism for obtaining a series of changes of velocity and direction. D is a screw on which is placed eccentrically the cone B, and C is a friction-roller, which is pressed against the cone by a spring or weight. Continuous rotary motion, at a uniform velocity of the screw D carrying the eccentric cone, gives a series of changes of velocity and direction to the roller C. It will be understood that during every revolution of the cone the roller would press against a different part of the cone, and' that it would describe thereon a spiral of the same pitch as the screw D. The roller C would receive a reciprocating motion, the movement in one direction being shorter than that in the other.

Fig. 912. The shaft has two screws of different pitches cut on it, one screwing into a fixed bearing, and the other into a bearing free to move to and fro. Rotary motion of the shaft gives rectilinear motion to the movable bearing, a distance equal to the differences of pitches at each revolution.

Fig. 913. Two worm-wheels of equal diameter, but one having one tooth more than the other, both in gear with the same worm. Suppose the first wheel has 100 teeth and the second 101, one wheel will gain one revolution over the other during the passage of 100 X 101 teeth of either wheel across the plane of centres, or during 10,000 revolutions of the worm.

Fig. 914. Variable motion. If the conical drum has a regular circular motion, and the friction-roller is made to traverse lengthwise, a variable rotary motion of the friction-roller will be obtained.

Fig. 915. Circular into reciprocating motion by means of a crank and oscillating rod.

Fig. 916. Continued rectilinear movement of the frame with mutilated racks gives an alternate rotary motion to the spur-gear.

Fig. 917. Rotary motion of the worm gives a rectilinear motion to the rack.

Fig. 918. Anti-friction bearing for a pulley.

Fig. 919. On vibrating the lever to which the 2 pawls are attached, a nearly continuous rectilinear motion is given to the ratchet-bar.