The improvement of the screw has given rise to many valuable schemes and modes of practice, which have not been noticed in the foregoing sections, notwithstanding their collective length. These practices, indeed, could not consistently have been placed in the former pages of this chapter, because some of them must be viewed as refinements upon the general methods, the earlier notice of which would have been premature; and others exhibit various combinations of methods pursued by different eminent individuals with one common object, and are therefore too important to be passed in silence, notwithstanding their miscellaneous nature.

To render this section sufficiently complete, it appears needful to take a slight retrospective glance of the early and the modern modes of originating screws and screw apparatus; some account of the former may be found in the writings of Pappus, who lived in the fourth century.*

The progressive stages which may be supposed to have been formerly in pretty general use for originating screws, may be thus enumerated:

1. The first screw-tap may be supposed to have been made by the inclined templet, the file, and screw-tool; it was imperfect in all respects, and not truly helical, but full of small irregularities.

2. The dies formed by the above were considerably nearer to perfection, as the multitude of pointed edges of 1, being passed

• The author has been told by a classical friend, that in the works of Pappus Alexandrinus, a Greek mathematician of the fourth century, are to be found practical directions for making screws.

The process is simply to make a templet of thin brass of the form of a right-angled triangle, the angles of which are made in accordance with the inclination of the proposed screw. This triangle is then to bo wrapped round the cylinder which is to be the desired screw, and a spiral line traced along its edge. The screw is subsequently to be excavated along this line. Minute practical directs are given not only for every step of this process, but also for the division, setting out, and shaping the teeth of a worm-wheel of any required number of teeth to suit the screw. (Vide Pappi Math. Col. lib. viii.. prob. xriii.) through every groove of the die, the threads of the latter became more nearly equal in their rake or angle, and also in their distances and form.

3. The screw cut with such dies would much more resemble a true helix than 1; but from the irregularities in the first tap, the grooves in the die 2 would necessarily be wide, and their sides, instead of meeting as a simple angle, would be more or less filled with ridges, and 3 would become the exact counterpart of 2.

4. A pointed tool applied in the lathe, would correct the form of the thread or groove in 3, without detracting from its improved cylindrical and helical character; especially if the turning tool were gradually altered, from the slightly rounded to the acute form, in accordance with the progressive change of the screw. The latter is occasionally changed end for end, either in the die-stocks or in the lathe, to reverse the direction in which the tools meet the work, and which reversal tends to equalise the general form of the thread.

5. The corrected screw 4, when converted into a master-tap, would make dies greatly superior to 2; it would also serve for cutting up screw tools; and lastly,

6. The dies 5 would be employed for making the ordinary screws and working taps; and this completes the one series of screwing apparatus.

One original tap having been obtained, it is often made subservient to the production of others; for example, a screw tool, with several points cut over the corrected original 4, would serve for striking, in the lathe, other master-taps of the same thread but different diameters. The process is so much facilitated by the perfection of the screw-tool, that a clever workman would thus, without additional correction, strike master-taps sufficiently accurate for cutting up other dies larger or smaller than 4. Sometimes also the dies 5 are used for marking out original taps a little larger or smaller than 4.

As a temporary expedient, the screw tool may be somewhat spread at the forge fire to make a tool a little coarser, or it may be upset for one a little finer, and afterwards corrected with a file; or screw tools may be made entirely with the file, and then employed for producing, in the lathe, master-taps of corresponding degrees of coarseness and of all diameters.

These are in truth some of the progressive modes by which under very careful management, great numbers of good useful screwing apparatus have been produced, and which answer perfectly well for all the ordinary requirements of "binding" or "attachment" screws; or as the cement by which the parts of mechanism, and structures generally, arc firmly united together, but with the power of separation and reunion at pleasure.

In this comparatively inferior class of screws, considerable latitude of proportion may be allowed, and whether or not their pitches or rates have any exact relationship to the inch, is a matter of indifference as regards their individual usefulness; but in superior screws, or those which may be denominated "regulating" and "micrometrical" screws, is does not alone suffice that the screw shall be good in general character, and as nearly as possible a true helix; but it must also bear some defined proportion to the standard foot or inch, or other measure. The attainment of this condition has been attempted in various ways, to some of which a brief allusion was made in the second section, and a few descriptive particulars will now be offered.

Fig. 610.

The apparatus for cutting original screws by means of a wedge or inclined plane, appears to be derived from the old fusee engine, a drawing of which is given in fig. 610; in principle it is perfect and it is also universal within the narrow limitation of its structure *.