The inventor then thought of securing the dial to the pointer die so
that they would revolve together, the zero of the dial being in line
with the pointer, as illustrated in Fig. 7. This would obviate the
necessity of setting the zero of the dial up to the pointer at the
initial imprint.
[Illustration: FIG. 5.
Initial imprint of zero mark alone and final imprint of hand (and
zero). Elapsed time, 8 minutes. No subtraction and no uncertainty as to
which imprint first, but counting still required.]
But again the improvement involved a difficulty. As the dial rotated,
its final impressions would never register with its initial impressions
and would therefore always destroy them. As the first imprint of the
dial was the only useful one, and as the second imprint only made
trouble, the inventor conceived the idea of not making any imprint of
the dial at the close of the period, and this he accomplished by making
the annular portion of the platen covering the dial so that it could
be advanced to print or not as desired. As the zero of the dial always
marked the beginning of the measuring arc, it served the same purpose
as the mark in line with the pointer, and the latter could now be
omitted.
The final machine then consists simply of a revolving die which, as
shown in Fig. 8, consists of a graduated and progressively numbered
dial, having a pointer revolving in line with the zero, and the machine
has a platen consisting of an inner circular portion over the pointer
and an annular portion over the dial, each portion being operated by a
separate handle so that the dial can be printed at the beginning of the
period and the pointer alone, at its close.
The final record has an initial imprint of the dial, Fig. 9a, the
zero of the dial showing the position of the pointer at the beginning
of the period, and a final imprint of the pointer alone, as shown
in Fig. 9b, the complete final record, Fig. 9c, consisting of the
superimposition of these two records, and showing the pointer in line
with that graduation whose number is the value of the period. Here is
a record not only involving no subtraction and no uncertainty but not
even, counting in its record, and, as it was made without disturbing
the motions either of the pointer or dial, any number of records of
other periods could have been begun or finished while the machine was
measuring the period in question.
[Illustration: FIG. 6.
Dial moved up to initial position of zero mark. Elapsed time, 11
minutes. No subtraction, no counting, no uncertainty; but only one
record possible at a time.]
Hiding all the intermediate steps in the evolution of this invention,
it seems the result of spontaneous creation, but considering the
steps in their successive order, it will be seen that the invention
is an evolution from the time-stamp; that logic rendered the effort
of the imagination at any one step small by comparison, and that the
individual steps might be well within the capacity of a person to
whom the spontaneous creation of the final invention might be utterly
impossible.
A most interesting example of the evolution of an invention is that of
the cord-knotter of the self-binding harvester. The problem here was to
devise a mechanism which would take place of the human hands in tying a
knot in a cord whose ends had mechanically been brought together around
a bundle of grain.
[Illustration: FIG. 7.