The screw. proportionate to the increased length of the arms of the lever formed by the sides of the cleft. The wedge is a mechanical power of singular efficacy, and the percussion by which its action is obtained, is precisely that force which we can with the greatest convenience, almost indefinitely increase. By means of the wedge, the walls of houses may be propped, rocks split, and the heaviest ships raised up-operations to which the lever, the wheel and axle, and the pully, are incompetent. To the wedge are referred the axe, spade, chisels, needles, knives, punches, and, in short, all instruments which, beginning with edges or points, grow gradually thicker. A saw is a number of chisels fixed in a line, and a knife, if its edge be examined with a microscope, will be found to be only a fine saw. OF THE SCREW. The sixth and last mechanical power which we have to notice, is the screw. The screw, strictly speaking, consists of two parts, which work within each other. One of these parts, and which is always meant when the word screw is used alone, is a solid cylinder on the circumference of which is cut a spiral groove; it is, as we have formerly observed, when specifically named, called an outside or convex screw. The other part, is a hollow cylinder, or, at least, whatever its external form may be, it contains a cylindrical hole, within which is cut a spiral groove corresponding to that of the convex screw, which can be turned with it, and the spiral projections of the one lock into the spiral hollows of the other. For the sake of necessary contradistinction, this latter part is called an inside, a concave, or socket screw, when spoken of generally, without reference to any other use than its principal one, of an indispensable companion to the convex screw; but when it consists of a small piece of metal, as for drawing tight bolts of any description, it is most commonly called a nut; and when it is of considerable size, as for a large press or vice, it is usually called the box. The thread of a screw is its spiral projection; the pace or step of a screw is the distance between the threads; and the groove or gorge is the hollow between the threads. To obtain an idea of the nature of the screw, and of its -affinity to the inclined plane, cut a piece of paper in the form of an inclined plane, or half wedge, as LMN, fig. 9, pl. III, and then wrap it round a cylinder, fig. 10, the edge of this plane or paper, LMN, will form a spiral round the cylinder, The screw. which will give the thread of the screw. The height of the plane is the pace of the screw, or distance of one thread from another; its base is the circumference of the screw, and its length is estimated by this circumstance and the height of the pace. A screw is seldom used without the application of a lever to assist in turning it; it then becomes a compound machine of great force, either in compressing the parts of bodies together, or in raising great weights. As the lever or winch must turn the cylinder once round, before the weight or resistance can be moved from one spiral winding to another, or before the screw working in its box can rise or sink the distance between the threads, as from a to b, therefore as much as the circumference of the circle described by the lever is greater than the pace of the screw, or distance between the threads, so much does the force of the screw exceed the motive force. For example, suppose the pace or distance of the threads to be half an inch, and the length of the lever 12 inches, the circle described by the extremity of the lever where the power acts, will be about 76 inches, or 152 half inches, consequently 152 times as great as the distance between two contiguous threads; therefore, if the intensity of the power at the end of the lever, be equal to one pound, that single pound will balance 152 pounds acting against the screw. If as much additional force be exerted as is sufficient to overcome the friction, the 152 pounds may be raised; and the velocity of the power will be to the velocity of the weight as 152 to 1. Hence we may clearly perceive, that the longer the lever, and the nearer the threads to one another, so much the greater is the force of the screw. The friction of the screw is very great, but we are indebted to this circumstance for a peculiar advantage in the use of the screw, which will sustain a weight, or press upon a body against which it is driven, after the power is removed or ceases to act. To enumerate all the uses of the screw would be impossible. Among other purposes, it is applied to great advantage for measuring or subdividing small spaces; when thus applied it is called a micrometer, which may be made to indicate on an index plate, a portion of a turn, advancing the screw less than the fifty thousandth part of an inch. The threads of screws are differently formed, according to the materials of which they are made, or the use for which they are intended. The threads of wooden screws are generally angular, that they may rest upon a broad base, and thereby have their strength increased to the utmost. Small screws, whatever material they are made of, are generally angular also, not only The endless screw. for the same reason as the wooden ones, but because the angular thread is the most easily made. The metal screws which are used for large presses, vices, &c. generally have a square thread, a form which augments the surface of each thread, and occasions an increase of friction, but is attended with the advantage of greater steadiness in its motion. In the common screw, to which the preceding observations are exclusively applicable, the threads are one continued spiral from one end to the other; but where there are two or more separate spirals running up together, as in the worm of a jack, or the principal screw of a common printing press, the descent of the screw in a revolution will be proportionately increased; and therefore whatever be the number of the spirals, they must, in calculating the power, be measured and reckoned as one thread. Of the Endless Screw. A screw is sometimes cut on an axle, to serve as a pinion, and either turns or is turned by a wheel; it is then called an endless screw, because it may be turned perpetually without advancing or receding, that is, without any other motion than a rotatory one. The threads of this screw are of the square form, and fit exactly into the spaces between the teeth of a wheel, which teeth are cut obliquely to answer to the threads. When the endless screw has been turned once round, the wheel has only made a portion of a turn equal to the distance between one of its threads, that is, the wheel has moved one tooth, and therefore the number of its teeth is always the same as the number of the revolutions made by the screw before it is once turned round. The construction and mechanical advantage gained by this screw may be best illustrated by a figure: let the wheel C, fig. 11, pl. III. have an endless screw B, on its axis, working in a wheel D, of 48 teeth. The screw B, and the wheel C, being on the same axis, every time they are turned round by the winch, the wheel D will be moved one tooth forward by the screw, and therefore 48 revolutions of the winch will be required to turn the wheel D once round. Then if the circumference of the circle described by the handle of the winch A, be equal to the circumference of a groove round the wheel D, the velocity of the handle will be 48 times as great as the velocity of any given point in the groove; consequently, if a line G, goes round the groove, and has a weight of 48 pounds hung to it, a power equal to one pound at the handle will balance and support the weight. If an apparatus were constructed for the The endless screw. purpose, this might be proved by making the circumferences of the wheel C and D equal to one another; and then it a weight H, of one pound were suspended by a line going round the groove of the wheel C, it would balance a weight of 48 pounds hanging by the line G, and a small addition to one of the Weights will cause it to move the other, as in every other case aner the equilibrium takes place. If a line G, instead of going round the groove of the wheel D, goes round its axle I, the power of the machine will be as much increased as the circumference of the groove exceeds the circumference of the axle. If then the circumference of the groove, be six times greater than the circumference of the axis, one pound at H will balance six times 48, or 288 pounds, hung to the line on the axle; the power gained will therefore be as 28 to 1, and a man whose natural strength would enable him to lift one hundred weight, will be able to raise 288 hundred weight by this engine. The use of the endless screw affords a very ready means of greatly diminishing or increasing a rotatory motion, and accomplishes at once what would otherwise require the intervention of two or three wheels. It possesses the advantage, too, of moving or being moved by a wheel with much more steadiness than a pinion, when the workmanship of both is of equal quality. This circumstance is not so much regarded by mechanics as perhaps it ought, and therefore the endless screw is often not used when it would be advantageous. Cast-iron wheels are becoming increasingly general for all kinds of large machinery requiring wheel-work. Their durability, and the little room they take up, when compared with the wooden ones to answer the same purpose, entitle them to a decided preference; but still there are many great impediments to their being made true. They must necessarily partake of the defects of the pattern made for casting them; very few millwrights make good patterns, and even their best efforts are sometimes rendered in some measure abortive by the warping or shrinking of the wood they have used. But supposing all difficulties with respect to the pattern to be conquered, the liability to incorrect work from the imperfections of the mould, the presence of stagnated air, and the unequal contraction of different portions of the metal, are very considerable, and never absolutely overcome by the most careful workmen. It is true that cast-iron wheels are better than any other, and that they may be rectified by hand; but this operation, if carried beyond a certain point, would prove very expensive; it is besides indesirable, and seldom attempted in any great degree, because as the surface of The nature of the advantages of machines. cast-iron is far harder than the interior, it would remove that portion of the metal which always wears the best. Whoever, therefore, attends to the motions of machinery, will frequently observe some parts to move by jolts or starts; the cause of which is, the inaccuracy of the teeth or cogs. In consequence of this, engineers, when they want a steady motion, as for a lathe, resort to the expedient of employing a large train of wheels. They thus commonly attain their prime obiect, for the probability is greatly in their favour, that opposite imperfections will balance each other; but they have introduced a great increase of friction, and a consequent necessity for a greater motive force, not to mention the expense of the additional parts of the machinery. To avoid in part these disadvantages, they might more frequently employ the endless screw, for the reasons already alledged. The principal restriction to the use of this screw is, its being so liable to wear when its motion is very rapid; a rapid motion therefore, should not be assigned to it, unless it be made of hardened steel, when the objection will not apply. OF COMPOUND MACHINES. When two or more of the simple mechanical powers are made to act in conjunction to produce a given effect, the contrivance resulting from the union is called a compound machine or engine. Though any one of the mechanical powers is capable of overcoming the greatest possible resistance in theory; yet, in practice, if used singly, they would frequently be so unmanageable as to render their properties nugatory. It is therefore generally found best to combine them together; by which means the power is more easily applied, and many other advantages obtained. As the mechanical powers, in whatever manner they are combined, still preserve their properties; so, in compound as in simple engines, whatever is gained in power is lost in time; consequently, if a given power will raise one pound with a given velocity, it will be impossible, for that power by the help of any machine, to raise two pounds with the same velocity; yet by the assistance of a machine, two pounds may be raised with half that velocity, or one thousand pounds with a thousandth part of it; but still there is no greater quantity of motion produced when a thousand pounds are moved, than when only one pound is moved; because the greater weight moves proportionately slower than the lighter. The power, then, of machines, consists only in this, that by their means the velocity of the weight may be diminished at pleasure, so |