The present generation, standing on the vantage ground of scientific knowledge, are too apt to contemn the laborious researches of the past seekers after truth, forgetting that the heights ol science, whenee they extend their gaze, were raised by the accumulated toil of ages now gone by, and that, moreover the science of the present will, perhaps be only the ordinary knowledge of the luture. Actuated by such a feeling as that alluded to above, the first impulse is to smile at a mode adopted only fifty years since, for screw cutting, when the thread was traced first on paper, then the lines tranferred to an iron cylinder, and a guiding thread having been cut by a chisel and file, the operation was completed by a fixed cutter- Yet this rough plan was at that time regarded as the climax ol ingenuity, and was carefully described for imitation; in truth it was a great advance over the previous methods, and its importance is increased ten-fold by the supposition that it was intended for the '' boring-bar," then just invented, and which has so effectually superseded ?the boring machine previously used for engine cylinders, which boring machine was simply a boy who scrubbed the rough casting with a piece of pumice stone. To a person unversed in mechanics, the stress laid upon the necessity of obtaining excellence in screws may appear absurd and pedantic, but those most conversant with the subject, are also the most precise in their requirements on this point. A few words will explain the reason for this discrepancy of opinion ; when a screw is employed merely to bind or attach one body to another, such exactness of workmanship is not required, and whether the pitch of the thread has any exact relation to the inch, is a matter of indifference as regards its individual usefulness. But in screws of a superior kind, or those which are termed " regulating " and " micrometrical," it is not alone sufficient that the screw shall be good as respects its general character, and as nearly as possible a true " helix " (a word denoting the peculiar shape* of trieJcrSw), but it mist also bear some denned proportion to the standard foot or inch, or other measure. This will be understood by explaining that micrometrical screws are employed in engines for the graduation of right lines and circles, and likewise lor astronomical and mathematical instruments. In these latter the requirements of science appear ever to outstrip the most refined methods of execution, and a single instance will give an idea of the indefatigable perseverance with which some individuals have pursued this object. About the year 1800 two eminent mechanicians undertook to reform the old, imperiect, and accidental practice of screw-cutting, and succeeding in the attempt, have introduced the exact and systematic mode now generally adopted. As a preliminary proceeding, it was necessary to cut a very exact screw; each of them, therefore, cut a similar screw, 15 inches long, by a distinct process, and the two having been compared, they were found to agree exactly, but on being examined by a powerful microscope, these screws were discovered to be exceedingly defective. This rigid scrutiny led both parties to fresh and ultimately successful efforts. Without, however, entering into an account of the complex arrangements, by which such results were obtained, it will be more advisable to recur to the method adopted by hand turners to cut a screw, as their mode of procedure, perhaps, imparts most simply to the inexperienced, the theory of the subject. All elementary works on mechanics tell us that the screw consists of an inclined plane wrapped around a cylinder; or, in other words, that it is a continuous circular wedge. These definitions afford an index to the manner in which a screw is made,for the turner, having formed a piece of metal to a cylindrical shape, next proceeds to cut the thread. This he does with a " screw tool," which is 4 straight flat piece of steel having projections and recesses exactly corresponding to the thread, but previously he " starts the thread," that isj lightly traces on the cylinder, as it revolves, a few threads similar, as near as possible, to the pitch ot the screw he is to form, then stopping the motion of the lathe, he carefully examines his work to note the correspondence of the pitch, and if not sufficiently correct, he effaces the spiral mark he had made, with a turning tool, and commences denova. If correct, however, he grasps the " screw tool," and presses it against the cylinder, which is rotating before him, in such a manner that the teeth or projections of the lormer touch the spiral line he had lightly cut, the incision being sufficiently deep to give a tendency to the " screw tool " to follow the course of the spiral. This inclination the workman encourages by a light pressure in the longitudinal direction, whilst, at the same time, he maintains the tool in a position favorable lor deepening the incision in the cylinder. When the tool has traversed as far as it is intended to cut the screw, it is withdrawn from the work and again placed in its first position, which process is continued until the thread is cut as deep as desired. Had the screw tool been held at rest it would have made a series of rings, but no spiral, as many an amateur has found to his discomfiture. Should the tool fail to drop exactly into the groove at the commencement ot the process a tolerably good screw may nevertheless be formed, as the error can be rectified. But if the difference should happen to be great, the tool finds its way into the groove with an abrupt break in the curve, and this error is olten beyond correction. Should the tool be moved too rapidly, a double thread is sometimes the result, if too slowly the screw has only half the inclination intended, and the grooves are as fine again as the tool. The assemblage of points in the "screw tools," proper for metals and hard woods, renders the striking of screws in these materials comparatively certain and excellent, but the soft woods require tools with very keen edges, and therefore the "screwtool "' is made with only a single point. With a tool thus constructed, no skill could cut a correct screw, unless a lathe with a traversing mandrel were used when guide screws are fitted as rings to the extreme end of the mandrel, and they work in a plate,of brass, whieh has six scol-'lopsor semi-circular serews-KpoHsjts--"ige,' It will be understood that the tool remains stationary, whilst the work, which is chucked traverses with the mandrel, whose motion is determined by the above-mentioned guide screw. The alternating motion is effected by giving a swinging movement or partial revolution to the foot wheel, but the use of this arrangement is limited to a lew trades. The self-acting screw-cutting lathe is the best machine lor cutting accurate screws of considerable length or ot great diameter. In this lathe, the traverse or longitudinal motion of the tool is effected by a long guide screw, which revolves in bearings and gives motion to the slide rest. The screw receives motion from the mandrel by the intervention of geared wheels, so that the traverse ot the tool is regulated by the number of revolutions of the mandrel. This affords a simple means of obtaining a screw of any desired pitch, for it is only requisite to regulate the ratio between the revolutions of the screw, and those of the mandrel, to obtain any desired result. This latter purpose is effected by changing the geared wheels, and therefore every lathe of this description is furnished with a number of " change wheels." The accuracy of the result now depends almost entirely upon the perfection ot the guide-screw, which should possess, very exactly, some whole number oi threads per inch, for, in fact, every screw cut by its aid is either a reduced or enlarged copy of its merits and defects. It will here occur to the reader, that if only a pair of wheels were used to move the guide-screw, its direction would be the reverse of that ot the mandrel and work, for adjoining wheels always travel in opposite directions, but by introducing one or two intermediate wheels, either right or left-handed screws may be cut. The screw or chasing tools employed for this lathe resemble, generally, the fixed tools, except as regards their cutting edges. Angular screws are sometimes cut with a single point tool, the general angle ot the point being from 35 to 60s, and when it is allowed to cut on only one side or bevel it may be used fearlessly, but if './oth sides are allowed to cut, more caution is required ; for angular threads it is more usual and expeditious to employ a chaser. For square threads, a rectangular-shaped tool is general and the end tlone is used to cut, a side tool being sometimes employed to regulate the width of the groove ; there are, however, a number of peculiar tools and modes of working adopted according to the exigency ol the case or the fancy ot the workman.