end and withdraw the screw from its split nut[2] and from the
driving clutch near the headstock. This split nut also would have to be
changed to one of a pitch corresponding to that of the screw. While more
expensive than a solid nut, it neatly circumvents the need (and saves
the time involved) to reverse the screw in order to get the tool back to
the point of beginning preliminary to taking another cut. David
Wilkinson's lathe of 1798 (fig. 17) which was developed in Rhode Island
at the same time shows the same method of mounting and driving the
master screw. At least in the United States, this method of changing the
lead screw instead of using change gears remained popular for many
years. Examples of this changeable screw feature are to be found in the
lathes constructed for the pump factory of W. & B. Douglas Company,
Middletown, Connecticut,[3] in the 1830's. Middletown, at that time one
of the leading metal-working centers in one of the chief industrial
States, had been for many years the site of the Simeon North arms
factory which rivaled Whitney's. In this atmosphere, it is reasonable to
expect that machinery constructed by local mechanics, as was the custom
in those days, would reflect the most accepted refinements in machine
design.

[Illustration: Figure 12.--WELL-DEVELOPED EXAMPLE of lathe headstock
having several leads on the spindle and provision for mounting the work
or a work-holding chuck on the spindle. Adapted from _L'Encyclopedie_,
vol. 10, plate 13.]

[Illustration: Figure 13.--END VIEW OF THE HEADSTOCK seen in figure 12,
showing the keys or half nuts which engage the threaded spindle, in
engaged and disengaged positions. From _L'Encyclopedie_, vol. 10, plate
13.]

[Illustration: Figure 14.--SPINDLE OF FIGURES 12 AND 13, showing the
several leads and the many-sided seat for the driving pulley. Note the
scale of feet. From _L'Encyclopedie_, vol. 10, plate 16.]

Roughly twenty years later, Joseph Nason of New York patented[4] the
commercially very important "Fox" brassworker's lathe (fig. 18). While
this does have a ratio in the pair of gears connecting the work spindle
and master screw, it is clear from the patent that various pitches are
to be obtained by changing screws, not by changing gears. The patent
sums it up as follows:

A nut upon the end of the stud ... is unscrewed when the guide
screw is to be removed or changed. The two wheels ... should have
in their number of teet