ffer little from that shown in _Das
mittelalterliche Hausbuch_ of 1483. The double work-support is, of
course, a great improvement, while the tool-support is regressive since
it lacks a feed screw.

The development of engineering theory, coupled with the rising needs of
industry, particularly with the advent of the Industrial Revolution,
brought about accelerated development of screw-cutting lathes through
the combination of screw-cutting machines with simple lathes as seen in
figure 9 and in detail in figure 11. One important advance shown here
is driving the machine by means of a cord or band so that any means of
rotary power could be applied, not just hand or foot power. Of greater
interest and technical importance to this study is the provision, seen
to better advantage in figure 11, for readily changing from one master
lead screw to another. This had already been achieved in the Manuel
Wetschgi machine, as far as versatility is concerned, although not in
quite such a convenient way.

[Illustration: Figure 7.--THREAD OF MODERN FORM recently cut, using the
old screw and nut but with a new tool. The material threaded is
carbon-steel drill rod. (Smithsonian photo 49276A.)]

Figure 12, the headstock of another and more advanced lathe than shown
in figures 9 and 11 but of the same type, shows "keys" (D), each of
which is a partial nut of different pitch to engage with a thread of
mating pitch. The dotted lines in figure 13 show the engaged and
disengaged positions of one of these keys, and figure 14 shows the
spindle with the various leads, C. At D is a grooved collar to be
engaged by the narrow key shown in operating position at the left in
figure 12 for the purpose of controlling the endwise movement of the
spindle when used for ordinary turning instead of thread-cutting. In
return for greater convenience and freedom from the expense of the many
separate spindles, as typified by the Wetschgi machine, a sacrifice has
been made in the length of the thread which can be cut without
interruption.

[Illustration: Figure 8.--BINDING SCREW seen at A in figure 3, showing
the long smooth fadeout of the thread below the shoulder. (Smithsonian
photo 49276.)]

[Illustration: Figure 9.--MAKING SCREWS IN FRANCE in the third quarter
of the 18th century. From _L'Encyclopedie, ou dictionnaire raisonne des
sciences, des arts et des metiers ... receuil de planches sur les
sciences, les arts liberaux, et les arts mechaniques, avec le