Environmental Engineering Reference
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Now Wulff gives some igures relating to the performance of these mills. When he
visited Neh in 1963, 50 mills were still operating. Each milled an average of a ton of grain
in 24 hours, so in a 120-day windy season, they were said to produce a total output of
6,000 tons. Based on a wind speed of about 30 m/s, an effective exposure at any time of
1.5 blades, and a mill eficiency of 50 percent, Wulff estimates a power output of about 75 hp
per mill. It is dificult to accept this high igure. A power calculation based on
aerodynamic drag in accordance with Equation (5-2) would yield a maximum 22 hp in a
30 m/s wind for the same assumptions.
The basic design of these primitive vertical-axis mills has lasted at least 1,000 years,
although a major change has come about in that the machine has been inverted, placing the
sails above the millstones, as we see in comparing Figures 1-2 and 1-3. Wulff suggests that
the earlier concept may simply have been taken from the ancient Norse or Greek water mill,
in which the mill itself had to be placed over the water wheel [1966]. No comment on
when the change might have been made seems to be available, but it certainly made
operation much more convenient. Elevating the driver to a more open exposure improved
the output by exposing the rotor to higher wind speeds. Other noticeable changes are the
use of reeds instead of cloth to provide the working surface and the use of a single entry
port in place of the four described by al-Dimashq¯, although this may have been limited to
the region around Neh, where the summer wind is almost constantly from the north.
It would seem, then, that we can take the tenth century as the earliest known date for
acceptable documentation of the vertical-axis windmill, and the location as most probably
West or Central Asia. Forbes apparently goes as far as to take the birthplace to be S¯st¯n
and to place the invention in early Muslim or even pre-Muslim times. If we take the
founding of Islam to be in the irst part of the seventh century, this is a good deal earlier
than most science historians would consider to be proven. Forbes also asserts that, after
having been irst conined to Persia and Afghanistan, the invention subsequently spread in
the twelfth century throughout Islam and beyond to the Far East. On the other hand, Lynn
White states that there is no evidence that mills of this type ever spread to other parts of
the Islamic world [1962].
Chinese Vertical-Axis Windmills
The belief seems to be quite widespread that the Chinese invented the windmill and
have been using it for 2,000 years. This might well be so, considering that they developed
so many engineering artifacts, but there is little or no evidence that the windmill was one
of them. The eminent scholar Joseph Needham, whose monumental work in many volumes,
Science and Civilization in China
[1965],
is the recognized classic text in the ield, states
that the earliest really important reference dates back to 1219. There is a report of a visit
in that year to Samarkand by a celebrated Chinese statesman and patron of astronomy and
engineering, Yehlü Chhu-Tshai, who in a poem wrote that stored wheat was milled by the
rushing wind and that the inhabitants used windmills just as the people of the south used
water mills. Later Chinese references to the windmill again all point to its transmission
from lands adjacent to western China as being the most likely supposition, and that it was
carried there by sailors or merchants from Central and Southwest Asia. Needham points
out further that the references suggest that the introduction of the windmill took place no
earlier, because it never before received a speciic character or speciic wording; it might
have been confused with the rotary winnowing fan, however, which is much older.
The irst European to report windmills in China was Jan Nieuhoff in 1656. These mills
had a distinctive form, with eight junk slat-sails mounted on masts around a vertical axis
and disposed so that they could be positioned automatically. Figure l-4(a) shows the
arrangement of the masts (for clarity, only one sail is rigged), supported from the rotor shaft
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