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Question: In The Regency Underworld, by Donald Low, there is an
engraving of Brixton Prison in London, which I believe was drawn in about 1800.
On the roof is what appears to be a vertical-axis windmill. The framework is a
horizontal 鈥淗鈥�. From the halfway mark to the outer ends of the arms are six
vertical slats on each side. How common were these mills, how did they work and
what were they used for?

The letters below suggest that the frame might have been either a
vertical-axis windmill or an 鈥渁ir brake鈥� designed to increase the labour of
prisoners on the treadmill shown in the picture. Although there is evidence that
both types of device existed in British prisons, the final letter writer seems
likely to be correct in thinking that the frame was too far from the treadmill
to be connected to it and was thus a windmill which provided ventilation. The
presence of a governor, noted in the first letter, supports this
惫颈别飞鈥抬诲

Answer: The device on the prison roof is a windmill with variable angle slats
controlled by a governor similar to those found on steam engines. The speed of
rotation of the frame is controlled by two heavy round balls on the end of
swinging arms. As the speed of rotation increases, the balls are thrown
outwards. The swinging arms are free at the lower end and fixed at the upper
end, and are connected to a slider on the central shaft. In turn, the slider is
connected to the six vertical slats so that as it rises, the angle of the slats
is automatically opened to lessen the effect of the wind. As the speed of
rotation decreases, the balls fall back, the slider drops and the angle of the
slats closes to improve the effectiveness against the wind. In this way the
speed of rotation is automatically controlled.

Nigel Mykura

Yeovil, Somerset

Answer: The vertical-axis windmill would probably have been used to provide
ventilation to the prison block in an effort to improve the inmates鈥� health. A
similar system involving a more common horizontal axis windmill was fitted at
Newgate Prison with funds provided by a local parson around the year 1750. The
obvious disadvantage of this type was that it needed to be turned to face the
wind.

Because vertical windmills are unidirectional they wouldn鈥檛 require constant
supervision, which would have made up for their lower efficiency. Fitting
ventilation fans seems to have been a public way for the authorities to say they
were improving the inmates鈥� health.

David Horton

West Launceston, Tasmania

Answer: The aerial-like structure on the top of the building was part of the
horizontally-slatted treadmill being worked by the 10 prisoners shown, all of
whom would have been serving sentences of 鈥渉ard labour鈥�. The first treadmill in
the country was installed at Brixton Prison from a design by William Cubitt in
about 1820, and was connected by appropriate shafts and gearing to a mill house
in which corn was ground.

Treadmills in prisons were designed to provide adjustable resistance and,
unlike the one at Brixton, many had no useful purpose whatsoever. The amount of
labour involved seems incredible to modern eyes. For example, at Coldbath Fields
Prison in London there were 20 treadmills鈥攐r more accurately treadwheels,
because none produced useful work鈥� and each prisoner was required to
produce an output of 鈥�1200 feet of ascent per day鈥�, which was the maximum that
could then be demanded without, it was thought, damage to health. The army,
however, eventually refused to send its offenders to Coldbath Fields because
they were returning unfit for service.

The structure on top of the Brixton building may had the advantage of
demonstrating to the surrounding population that the prisoners were labouring
hard. The treadmill was abolished throughout the country by rules made under the
1898 Prison Act. For further information see Prisons and Punishments of
London, by Richard Byrne (Grafton); and Judicial Punishment in
England, by J. A. Sharpe (Faber and Faber).

C. S. Richenberg

London

Answer: A windmill of the more usual type, with vanes set vertically in a
horizontal shaft, was erected on top of Newgate Prison in 1750. It drove bellows
to ventilate the cells below.

The treadmill, but not the vertical-axis windmill, in the circa 1890
engraving of Brixton Prison is mentioned on p 59 of The Windmills of Surrey
and Inner London by K. G. Fares and M. T. Mason. They write: 鈥淧rison
treadmills were occasionally put to the grinding of corn, and were sometimes
connected to vanes made to beat against the wind and nullify the effort applied
to them; neither of these practices were adopted at Brixton.鈥� In this engraving,
the windmill is perhaps too far from the treadmill to have been geared to it. It
might, however, have been connected to a ventilating system.

A larger and more sophisticated horizontal windmill was the one at Battersea
Built in 1788 to the design of Captain Stephen Hooper, it was used for grinding
linseed, corn and/or malt until around 1825. The drive to the grinding stones
was not direct as in its predecessors in medieval (and present-day) eastern
Iran, but instead was established via pinions meshing with a spur wheel at the
base of the tower.

The small vertical-axis windmill used for ventilating purposes survives in
modern times in the form of the Savonius rotor.

Michael Harverson

British representative for The International Molinological Society

Watford, Hertfordshire