When you visit Brixton Windmill on one of its open days, you will see a thick, heavy piece of canvas on display. This is a sample of sailcloth, which was originally hooked onto the frame of each sail to catch the wind.

In light winds the whole canvas would be unfurled; in stronger winds the sails would be “reefed”, or rolled back, so that only part of the surface was exposed (just like a sailing ship).

But if you look more closely at Brixton Windmill’s sails (or sweeps, as they were known south of the Thames), you’ll see that there are two different kinds.

The two trellis-like common sails, to which the sailcloth was attached, were simple, light, cheap and fairly efficient. This type of sail had been in use since medieval times. However, the disadvantage was that when the sailcloth had to be furled or unfurled, each sail had to be stopped in a vertical position and the miller, or a young apprentice, had to climb up to adjust the sailcloth. With four sails, this had to be repeated four times, which was rather inconvenient.

Brixton Windmill originally had four common sails, but in 1827 the Ashby family, who owned the mill, decided to replace two of them with a pair of patent sails. Patent sails have a series of small shutters, usually made of canvas, which can be opened or closed in one go – rather like a Venetian blind. They were invented by William Cubitt of Norfolk in the early 19th century.

As well as inventing patent sails, William Cubitt has another interesting connection with Brixton. In 1818, after a visit to his local prison in Bury St Edmunds, he invented a human treadmill to keep the “repulsive groups” of prisoners occupied.

Such was its success that in 1821 the Surrey House of Correction (now Brixton Prison) commissioned Cubitt to design a treadmill for Brixton. Prisoners started at one end of an elongated wheel and gradually moved across to the other end. They then dismounted and had a short rest before they went back and started again.

In 1822, the snappily titled Society for the Improvement of Prison Discipline and for the Reformation of Juvenile Offenders noted:

“…if twenty out of twenty-four are obliged to be upon the wheel, it will give to each man intervals of rest amounting to 12 minutes in every hour of labour…. At Brixton, the diameter of the wheel  being five feet, and revolving twice in a minute, the space stepped over by each man is 2193 feet, or 731 yards per hour.”

So six hours on the treadmill could be the equivalent of climbing more than 13,000 feet!

The treadmill was connected to subterranean machinery, which ground corn – the prison employed a miller and a baker. Its notoriety led to to several popular ballads and ditties, including the following punning verse:

“This Brixton Mill’s a fearful ill,
And he who brought the Bill in,
Is threaten’d by the cribbing coves
That he shall have a milling.
They say he shew’d a simple pate
To think of felons mending:
As every step which here they take,
They’re still in crime ascending.”

The treadmill was deemed such a success that by 1824 more than 50 prisons had adopted it. It was not outlawed until 1902.

It certainly did no harm to William Cubitt’s career. He served as President of the Institution of Civil Engineers and was knighted for his work on the Crystal Palace in Hyde Park for the Great Exhibition of 1851.

And although the inmates of Brixton Prison may have cursed his name while on the treadmill, his invention of the patent sail certainly made life easier for the Ashbys at Brixton Windmill!