ulton and Watt erector,
may have suggested this arrangement. _Ibid._, p. 56.]

[Footnote 14: Muirhead, _op. cit._ (footnote 3), vol. 3, note on p. 39.]

[Illustration: Figure 7.--James Watt's five alternative devices for the
conversion of reciprocating motion to rotary motion in a steam engine.
(British Patent 1306, October 25, 1781). From James P. Muirhead, _The
Origin and Progress of the Mechanical Inventions of James Watt_ (London,
1854, vol. 3, pls. 3-5, 7).]

[Illustration: (a) "Inclined wheel." The vertical shaft at _D_ is
rotated by action of wheels _H_ and _J_ on cam, or swash plate, _ABC_.
Boulton and Watt tried this device but discarded it.]

[Illustration: (b) Counterweighted crank wheel.]

[Illustration: (c) "Eccentric wheel" with external yoke hung from
working beam. The wheel pivots at _C_.]

[Illustration: (d) "Eccentric wheel" with internal driving wheel hung
from working beam. Wheel _B_ is pivoted at center of shaft _A_.]

[Illustration: (e) Sun-and-planet gearing. This is the idea actually
employed in Boulton and Watt engines. As the optional link _JK_ held the
gearwheel centers always equidistant, the annular guide _G_ was not
used.]

The sun-and-planet arrangement, with gears of equal size, was adopted by
Watt for nearly all the rotative engines that he built during the term
of the "crank patents." This arrangement had the advantage of turning
the flywheel through two revolutions during a single cycle of operation
of the piston, thus requiring a flywheel only one-fourth the size of the
flywheel needed if a simple crank were used. The optional link (JK of
fig. 7e) was used in the engines as built.

From the first, the rotative engines were made double-acting--that is,
work was done by steam alternately in each end of the cylinder. The
double-acting engine, unlike the single-acting pumping engine, required
a piston rod that would push as well as pull. It was in the solution of
this problem that Watt's originality and sure judgment were most clearly
demonstrated.

A rack and sector arrangement (fig. 8) was used on some engines. The
first one, according to Watt, "has broke out several teeth of the rack,
but works steady."[15] A little later he told a correspondent that his
double-acting engine "acts so powerfully that it has broken all its
tackling repeatedly. We have now tamed it, however."[16]

[Footnote 15: James Watt, March 31, 1783, quoted in Dickinson and
Jenkins, _op. cit._ (footnote