solution of the lines
of the solar spectrum, such machines being required to rule many
thousands of lines on an inch of space by a marking device which is
reciprocated over the glass plate and is fed by the action of a
leading screw after the formation of each line. Great difficulty
has been experienced in constructing a leading screw for this and
other purposes, in which the thread is so nearly correct as to
produce no perceptible variation in the microscopic spaces between
the ruled lines or gratings.... Various causes prevent the
formation of a thread on the rod or blank, which is absolutely
uniform and accurate from end to end of the rod. Among other causes
are the variations of temperature from time to time, the
imperfections of the operating leading screw, the springing of the
leading screw and of the rod that is being threaded, and other
unavoidable causes, all of which, although apparently trivial and
producing only slight variations in the thread at different parts
of the rod or blank, are of sufficient moment to be seriously
considered when a screw of absolute accuracy is desired.

[Illustration: Figure 18.--NASON'S LATHE, patented in 1854, showing a
master lead screw driven at less than work speed so that the master
could be of a coarser and more durable pitch than the work. U.S. patent
10383.]

It is interesting to note in figure 19 that Vander Woerd's machine, to
avoid the problems outlined in his patent, has returned to a starkly
simple design. We are not told, however, how he originated this master
screw which is used to produce the accurately threaded work pieces.
Later generations, in the search for ever-greater accuracy, also
returned to the fundamental simplicity of a master screw as we shall see
when we consider the refinements in mechanism necessary to the extended
development of the automobile and the airplane.

[Illustration: Figure 19.--VANDER WOERD'S PATENT, seen here, covered the
combination of a master screw, toolslide and work in a rigid frame to be
supported and driven by outside means of no required precision. U.S.
patent 293930 dated February 1884.]

As the power and speed of automobiles and aircraft increased, critical
parts became more highly stressed. Gears and threaded parts were
particularly troublesome details of the mechanism because of the
stresses concentrated in them, and, in the case of gear