PULLEY

In Mechanics, one of the six simple machines, or mechanical powers. It consists of a wheel, moveable about an axis, and having a groove cut in its circumference, over which a cord pa es. The axle is supported by a box or sheave, called the block, which may either be moveable, or fixed to a firm support. A single pulley serves merely to change the direction of motion; but several of them may be combined in various ways, by which a mechanical advantage or purchase is gained, greater or le , according to their number and the mode of combination. The purchase gained by any.combination is readily computed by comparing the celerity of the weight raised with that of the moving power, according to the principle of virtual velocities, which is alike applicable to all machines of whatever kind. In fig. 1., which represents a system where the several portions of the cord are parallel to each other, suppose the weight W to rise 1 inch, the two blocks would approach each other by that quantity, and, consequently, the length of cord connecting a single pair of pulleys would be shortened by 2 inches, so that the power P would descend 2 inches. Let the number of pulleys in each block be n; then, while the weight ascends 1 inch, the power descends 2 n inches, and, consequently, when there is equilibrium, the power is to the weight as 1 to 2 n. In the combination represented in fig. 2., the purchase is much greater. Here the pulleys are all moveable, and each is supported by a separate cord, having one end fastened to a fixed obstacle and the other attached to the succeeding pulley, excepting the upper block which is fixed. It is evident that, for every inch the weight on the 2 ascends, the second, b, ascends two; the third, c, ascends four, and so on; the velocity being doubled by each additional pulley. The purchase finally obtained is, therefore, = 2"; or the power is to the weight as 1: 2". The third combination, fig. 3., has still greater efficacy. In this system, each cord is fastened to tiie weight, and, pa ing over a pulley, is attached to another pulley, excepting the last, which supports the power. While the weight W ris'es 1 inch, the first moveable pulley, /, will sink 1 inch, which allows the cord applied to it to slacken 2 inches, and this joined to the inch which the weight ascends allows the second moveable pulley, g, to descend 3 inches. This allows the next pulley in succe ion to descend 6 inches, which, joined to the 1 inch which the weight ascends, gives 7 inches for the descent of the third pulley. In like manner, it is found that the descent of the fourth pulley is 15 inches. Hence, one moveable pulley allows the weight to descend 2x1-1-1 = 3 inches; two such pulleys, 2x3+ 1=7 inches; 3 pulleys, 2x7 + 1 = 15 inches; four pulleys, 2 x 15 + 1 = 31 inches; and so on; so that the purchase obtained by n moveable pulleys is 2" + * — 1, or the power is to the weight as 1 to 2" + ^ — 1. The theoretical advantage thus computed is, however, in all the cases, greatly diminished by friction, and the rigidity of the rope. The two last combinations are of little, if any, use in practice, but various modifications of the first are common. Smeaton's pullf/, or Smeaton's tack, as it is usually 097 first pulley, PUMl'. called, contains two rows of wheels, one under the other, in each block, and a single cord is made to pa over them in such a manner that the power and the weight both act in the same line with the centres of the two blocks, so that there is no tendency to twist. But this ingenious arrangement is open to several objections, and particularly the great amount of lateral friction of so many independent wheels. In White's pulley (seey?^. 1.), the wheels in each block turn on the same axis, and consequently revolve in the same time; and they are of different sizes, their dimensions being so proportioned that a point on the circumference of any wheel moves with the velocity of the rope on that wheel. To effect this the diameter of the wheels in the upper block must be as the numbers 1, 3, 5, ., and in the lower as 2, 4, 6, . Instead of separate wheels, the upper arid lower blocks are cut in grooves in the above proportions, whereby the friction is reduced to that of one wheel in each block. PULMO GRADES, Pulmo grad a. (Lat. pulmo, a lung, gradior, / advance.) The name of a tribe of Acalephans, including those gelatinous species which swim by the contraction of the vascular margin of the disk shaped body, where respiration, also, probably takes place. PULMONARIES, Pubnonaria. The name of the order of Arachnidans, including those which breathe by means of pulmonary sacs or lungs. PULMONATES, Pulmonata. The name of an order of Gastropodous Mollusks, including those which breathe air, to which the blood is exposed while circulating through a vascular network which lines the internal surface of the bronchial cavity. The order is's ubdivided into the Pulmonata terrestria, comprehending the Linnsean genera Limax and Helix, with Clausilia, Drap., and A chat in a, Lam.; and the Ptdvionata aquatic a, comprehending the genera Onchidium, Buchanan; Plan orb is, Cuv.; Lytnncea, Lam.; P/ii/sa, Drap.; Auricula, Lam.; Conovulus, Lam. [s. 1010]