de use only of the instant centers involving the fixed link of a
linkage. Like others before him, he considered the fixed link of a
mechanism as something quite different from the movable links, and he
did not perceive the possibilities opened up by determining the instant
center of two movable links.

Many other books dealing with mechanisms were published during the
middle third of the century, but none of them had a discernible
influence upon the advance of kinematical ideas.[74] The center of
inquiry had by the 1860's shifted from France to Germany. Only by
scattered individuals in England, Italy, and France was there any
impatience with the well-established, general understanding of the
machine-building art.

[Footnote 74: Several such books are referred to by Reuleaux, _op. cit._
(footnote 68), pp. 12-16.]

In Germany, on the other hand, there was a surge of industrial activity
that attracted some very able men to the problems of how machines ought
to be built. Among the first of these was Ferdinand Redtenbacher
(1809-1863), professor of mechanical engineering in the polytechnic
school in Karlsruhe, not far from Heidelberg. Redtenbacher, although he
despaired of the possibility of finding a "true system on which to base
the study of mechanisms," was nevertheless a factor in the development
of such a system. He had young Franz Reuleaux in his classes for two
years, from 1850. During that time the older man's commanding presence,
his ability as a lecturer, and his infectious impatience with the
existing order influenced Reuleaux to follow the scholar's trail that
led him to eminence as an authority of the first rank.[75]

[Footnote 75: See Carl Weihe, "Franz Reuleaux und die Grundlagen seiner
Kinematik," Deutsches Museum, Munich, _Abhandlung und Berichte_, 1942,
p. 2; Friedrich Klemm, _Technik: Eine Geschichte ihrer Probleme_,
Freiburg and Munich, Verlag Karl Alber, 1954, translated by Dorothea W.
Singer as _A History of Western Technology_, New York, Charles
Scribner's Sons, 1959, p. 317.]

Before he was 25 years old Franz Reuleaux published, in collaboration
with a classmate, a textbook whose translated title would be
_Constructive Lessons for the Machine Shop_.[76] His several years in
the workshop, before and after coming under Redtenbacher's influence,
gave his works a practical flavor, simple and direct. According to one
observer, Reuleaux's book exhibited "a recognition of the claims of
practice such as