Environmental Engineering Reference
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by radial cross-arms. The power take-off is through a right-angle drive composed of gears
with pin teeth. Figure l-4(b) illustrates the action of the sails as the rotor turns. Each sail
is mounted asymmetrically on its mast and held against the wind by a rope (positions
G,
H, A,
and
B
)
until it reaches a point in its rotation where it
jibes
,
reversing its orientation
and swinging outward (C and
D
),
coming thence into the eye of the wind or
lufing
,
which
gives little resistance to shaft rotation (
E
and
F
)
.
Thus, it does not require shielding walls
like the S¯st¯n models and can utilize wind from any quarter. This type of mill is still used
in eastern China.
Figure 1-4. The vertical-axis Chinese windmill, with eight junk slat-sails
[Need-
ham 1965].
(a) General arrangement of sails, masts, main shaft, and power take-off. (b)
Automatic positioning of sails during rotation. At
E
and
F
,
sails are lufing, presenting
little resistance to the wind. (
Reprinted by permission of Cambridge University Press; from
Chhen Li, Khao Hsung Thung Pao, Vol. 2, No. 3, 1951, p. 266
)
The real mystery, however, lies in the fact that the vertical-axis Persian windmill never
came into use in Northwest Europe. At the end of the twelfth century, there was an eflor-
escence of a completely different type, the
horizontal-axis windmill.
This development
presents a second enigma in the technical development of the wind turbine that occurred
some thousand years after the enigma left by Heron's device. But this is a story in itself,
and it requires a separate presentation.
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