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 teeth a common multiple. They are seldom or never removed and their diameters are made dissimilar only for the purpose of giving to the guide screw a slower rate of motion than that of the mandrel whereby it may be made of coarser pitch than that of the screw to be cut and its wear materially lessened. The introduction of gearing between the spindle and the lead screw, for whatever purpose, could not help but introduce variable factors caused by inaccuracies in the gears themselves and in their mounting. These were of little consequence for common work, particularly when coupled to a screw which, itself, was of questionable accuracy. The increasing refinements demanded in scientific instruments and in machine tools themselves after they had reached a relatively stable form dictated that attention be dedicated to improved accuracy of the threaded components. [Illustration: Figure 15.--MAUDSLAY'S WELL-KNOWN screw-cutting lathe of 1797-1800, showing the method of mounting and driving changeable master screws. (_Photo courtesy of The Science Museum, London._)]