Figure 6 shows a combination of a lever and links, such as are used in what is called a " toggle-press." Hand-power is applied to the long arm of the bent lever, which has a wide range of motion; the short arm is con nected by a link to the "toggle," which is composed of two links end-on. The upper end of the upper link works on a fixed pin, and the lower end of the lower link works on a pin adapted to the sliding part, which moves toward the object to be pressed as the lever is drawn down and the toggle is straightened. Great advantage is gained by this simple contrivance, the principle involved being a rapid increase of ratio between the distance through which the sliding-piece is moved by the power applied and the object acted upon.
Figure 7 (/5/. 12o) shows a bell-crank or bent-lever devised for changing the direction of motion. The rod to the right in the Figure, moving verti cally, transmits its motion to a connecting-rod which moves horizontally, and through the medium of the crank rotary motion is produced by the vertical reciprocating action of the rod.
Figure 4 exhibits au equal-ended beam, such as is employed for weigh ing-balances, and, in a larger way, as the walking-beam of a steam-engine (see pl. 82), the opposite ends having equal motion simultaneously, but in a reverse direction. The forms of levers as well as the purposes to which they are applied are multitudinous.
Figure 5 b shows the relation an inclined plane bears to the screw. If a triangular piece of paper be wound around a cylinder, the inclined edge of the paper will mark a true spiral, as is shown in Figure 5 a. The screw shown in Figure 5 b may be regarded as a vise-screw, which is urged by the lever passing through its head. The great mechanical force exerted by the screw is due to the small distance between the threads, which deter mine the advance of the screw at each revolution, and to the extreme length of lever, which is applied in turning the screw.
Pulleys.—Figure 8 shows a single fixed pulley (A) and a single mov able pulley in combination with a fixed pulley (II). In the first the power and weight are equal, both moving equal distances, but in opposite direc tions. In the second the power required is but half the weight lifted, but must pass through twice the distance in an opposite direction. The weight hangs upon the running pulley which rests in the fold of the cord, one end of which is fast while other parts lie in the groove of the fixed pulley. As the weight is equally divided between the two parts of the cord, it is plain that a force equal to only half the weight will be necessary to lift it, and it is also plain that the power must move through a space twice that passed through by the weight, since it must take up both parts of the supporting cord. Figure 9 shows the common balance-pulley and buckets for raising
water, the empty bucket being pulled down to assist in raising the full one.
Au extension of the principle of the pulley-block is shown in Figure 13, known as White's system of concentric pulleys. It is a very ingenious combination, but is attended with more friction on the rope than the above forms, owing to the difficulty of preserving the working-pitch line of the ropes in the grooves in a proper proportion.
On Plate 123 (frp,-. 4) are shown the spring-box and conical wheel called a " fusee" which is designed to equalize the varying power of the main-spring of a watch or clock by winding the chain in a conical groove adapted to the tension of the spring. (See p. 362.) Figure To (pl ioS) exhibits the differential or Chinese windlass, in which both ends of the rope are fastened to the barrel, there being one runner pulley connected to the weight to be lifted, while the strips of the rope wind on different-sized parts of the barrel, the effect being that as at the same time that one strip is wound on one part of the barrel the other strip is unwound from the other part, and as the circumferences of the ends of the barrels are different, the rope must either be paid out or drawn up (according to the direction in which the drum is turned) a distance equal to the difference of the circumferences of the barrels multiplied by the number of turns made by the drum. The movement of the pulley to which the weight is attached, in every revolution of the windlass, is equal to half the difference between the larger and smaller circumferences, and its power must be rated accordingly.
In all the mechanical elements above described the motion is recip rocating, is limited to the length of the lever and of the ropes and chains, and may be successively repeated. In Figures to to 12 (pl. 120), which show an endless cord passing over two pulleys, the motion is continuous (running either way), first in the same direction (fig. to), secondly in reversed directions (fig. It), the axes of the pulleys being in both cases parallel to each other. In Figure 12 the axes may lie at any angle with respect to each other, but not in the same plane, and the motion is con tinuous in both. In these examples the number of revolutions may be increased or diminished according to the circumferences of the pulleys enfolded by the cord.