The Screw. microscope, is seen to be but a finer saw. 135 The sharpest razor may be pressed directly against the hand with considerable force, and will not enter, but, if drawn along ever so little, it starts into the lesh. 248. The screw is another of the simple machines. It may be called a combination of the lever and a winding inclined plane. If a sheet of paper shaped like an inclined plane be wound round a cylinder of wood, its edge will trace a perfect screw. Fig. 47. A screw may be described as a spindle, a d (fig. 47), having a thread or worm cut spirally round it, which turns or works in a nut c, within which there is a corresponding spiral furrow fitted to receive the thread. The nut is sometimes called the external screw. Every turn of the screw carries it forward in a fixed nut, or draws a movable nut along with it, by exactly the distance between two turns of its thread this distance, therefore, is the space passed through by the resistance, while the force moves in the circumference of the circle described by the handle of the screw, as at f in the figure. The disparity between these lengths or spaces is often as a hundred or more to one; hence the prodigious intensity of effect which a screw enables a moderate force to produce. 249. Screws are much used in presses: as in those for squeezing oil and juices from such vegetable bodies as linseed, rapeseed, almonds, apples, grapes, sugar-cane, &c. : they are used also in the cotton-press, for reducing a great spongy bale, of which a small number would fill a ship, to a compact or dense package, heavy enough to sink in water; also, in the common printing-press, which has to force the paper strongly against the broad expanse of type; in machinery for coining; and in letter-copying machines. It is a screw which draws together the iron jaws of a smith's vice, &c. The screw, although producing so much friction as to consume a notable part of the force used in working it, owes its value to this very friction; but for friction the screw would not retain its place against the pressure overcome. As a screw can be made with a great many turns of its thread in the space of an inch, at perfectly equal distances from each other, it enables the instrument maker and mechanic to mark divisions on his work with a minuteness and accuracy very marvellous. If we suppose such a screw to be pulling forward a plate of metal, or pulling round the rim of a circle, over which a sharp pointed steel marker can be let down perpendicularly from always the same place, clear lines may be drawn so fine and so close as to be readable only with the aid of a microscope. The instruments called micrometers, by which the sizes of the heavenly bodies and of microscopic objects are ascertained, are worked by fine screws. An endless screw is one which acts on a toothed wheel, producing a rotation of the wheel always in the same direction, one tooth passing for every turn of the screw. A common corkscrew may be regarded as the worm of a screw detached from the central spindle; it is used, not to produce motion or to balance opposing forces, but merely to pierce and fix itself in the cork. 250. The pulley is another simple machine, by which masses moving with different velocities may be connected, and thus forces of different intensities balanced. A simple pulley consists of a wheel, as ab (fig. 48), with a grooved circumference, by which a rope, c a b, may be passed round; the weight or resistance, e, being attached to the axle of the wheel. e Fig. 48. In such a construction, it is evident. that the weight (say, one hundred pounds) is equally supported by each ply or length of the rope, and that a man holding up one end, while the other is attached to a fixed support, only bears half the weight, or fifty pounds. But to raise the weight one foot, he must draw up two feet of rope; therefore the pulley enables him, by lifting fifty pounds two feet, to raise a hundred pounds one foot. There is here no saving in the expenditure of Energy, but an important modification, which adapts the limited intensity of power belonging to the animal frame to the overcoming of vastly increased intensity of resistance. The Principle of Reduplication. 251. Pulleys may be combined in several ways. is but one rope used, as shown in fig. 49, the relation of velocities, and therefore of power and resistance, is known by the number of plies or lengths of the rope supporting the weight, each bearing its due proportion. Here there are four supporting folds, so that a weight of twenty-five pounds moving four feet would balance a weight of one hundred pounds moving one foot. The upper fixed pulley evidently serves no other purpose than that of enabling a downward pull to give the necessary support to the weight. Fig. 50. In practice the pullies are usually arranged, as in fig. 50, in one block, and having a single axle, so that the sheaves are side by side, in place of being one below another. But the mode of estimating the velocityconversion is exactly the same. In modern 137 Where there Fig. 49. mechanical language, the pulley is denominated the machine of reduplication. ს a 252. In fixed pulleys, like those shown at a and c (fig. 51), there can be no difference of velocity at the two extremities of the rope, for the weight just moves as fast as the power; and such pulleys are of use only in changing the direction of forces. Yet this is often of very great importance. A sailor, without moving from the deck of his ship, may, by means of such a pulley, hoist the sail or the signalflag to the top of the loftiest mast. And in building, where heavy loads of material are to be elevated every few minutes, a horse, trotting away with the end of the rope from d, in a level court-yard, raises the weight or charged basket, b, as effectually Fig. 51. d as if he had the power of climbing, at the same rate, the perpendicular wall. There is a case, however, in which a fixed pulley may seem a balancer of different intensities of force; viz., where one end of a rope is attached to a man's body, and the other is carried over a pulley above, and brought down again to his hands, or to a weight 138 The Pulley.-Other Mechanical Powers. taken to assist him. By pulling then, with a force equal to half his weight or less, he supports himself, and may easily raise himself to the pulley. A man, by a pulley thus arranged, may let himself down into a deep well, or from the brow of a cliff, with assurance of being able easily to return, although no one be near to help him; and cases have often occurred where, by such means, a fellow-creature's life might have been saved. How easily, for instance, might persons either reach or escape from the elevated windows of a house on fire by means of such a pulley, when ladders could not be obtained ! This kind of pulley furnishes a convenient means of taking a plunge bath from the stern windows of a ship. The chief use of the pulley is on shipboard. It is there called a block, although strictly speaking, the block is only the wooden frame which surrounds the wheel or wheels of the pulley. It aids so powerfully in overcoming the heavy strains of placing anchors, hoisting the masts and sails, &c., that by means of it a smaller number of sailors are rendered equal to the duties of the ship. Pulleys are also used on shore, instead of cranes or capstans, fot lifting weights and overcoming other resistances. Surgeons in former days, when they trusted to mere force, used pulleys, with unnecessary violence, in the reduction of luxations. The cranks by which bell wires are carried round corners into the different rooms of a house, are nearly equivalent to fixed pullies. Railway signals are, by means of such pullies, now conveyed long distances, so that a man at a station can, without moving a step, lower or raise the warning hand to the approaching enginedriver. 253. Excepting old usage, there is no reason why the term mechanical power should have been confined to the six Any connection of solid or rigid parts moving with different velocities will equally transform Energy from one degree of intensity to another, and therefore equally merits the title of mechanical power. But the needs for the raising of great weights and the overcoming of great resistances were for a long time satisfied by the simple machines we have enumerated. In the light of the modern doctrine of Energy, we know that a machine cannot create the smallest amount of force or power. If a man, with a couple of five-sheave pulley blocks and a rope, can raise a weight which it would take ten men to move directly, still Classification of Machines. 139 he has to continue his exertion just ten times as long as the ten men would have to do ; and a work which would last the ten men a whole day would last the one man ten days; there would be just ten days' wages to pay in both cases, and therefore no saving of human effort. 254. Machines, then, are but modifiers of motive energy; and this modification may take one or other of the follow- (i.) The connected parts may be so arranged as to move with different velocities; thus, a mass of one pound moving at one velocity will correspond to a different mass (say two or three pounds) moved at another velocity. The simple machines fall under this head; the lever, the pulley, the inclined plane, the wedge, the wheel and axle, and the screw, all exemplify this interchange of a smaller mass moving with greater speed into a larger mass moving with less speed. There are, however, many other arrangements serving the very same purpose and equally meriting the name of mechanic powers. One of the most notable of these is the Hydrostatic press, which will be described in the section on "Hydrostatics." 255. Oblique action, the explanation of which belongs to the theory of resolved forces already given in p. 57, is another mode of connecting different velocities. This is exemplified by the knee-joint, knuckle-joint, or toggle-joint, represented in skeleton form in fig. 52, and often employed in machinery where a very great pressure has to be exerted through a small space, as in punching, or shearing iron, or in the printing press, where the types have to be powerfully pressed against the paper. In the figure, ca and cb represent two links or rods, hinged together like a carpenter's folding rule. By force applied to the joint, the two links will be straightened or carried towards d. a ს Fig. 52. d But the motion of c is much more rapid than that of the ends, a and b; consequently, the outward pressure on the confining guides or obstacles will be correspondingly increased. This is the principle of the Stanhope lever, introduced in the end of last century by Lord Stanhope in his improvement of the printing press. |