A quick answer came up on Twitter, so here's the relevant passage from The Criminal Prisons of London ...
--As we were leaving the gate we caught sight, for the first time, of an immense machine situated in the paved court, which leads from the main or felons' prison to that of the vagrants'. In the centre of a mound, shaped like a pyramid, and whose slate covering and lead-bound edges resemble a roof placed on the ground, stands a strong iron shaft, on the top of which is a horizontal beam some twenty feet long, and with three Venetian-blind-like fans standing up at either end, and which was revolving at such a rapid pace that the current of air created by it blew the hair from the temples each time it whizzed past.
This is what is called the regulator of the tread-wheel. By this apparatus the resistance necessary for rendering the tread-wheel hard labour is obtained. Without it no opposition would be offered to the revolutions of the wheel; for, as that power is applied to no useful purpose,* [*We were assured that advertisements have often been inserted in the journals, offering to lease the treadmill power, but without any result.] the only thing which it is made to grind is, as the prisoners themselves say, "the Wind." Another method of increasing the resistance of this "regulator" consists in applying to it the apparatus termed by engineers a "governor." If the regulator revolves too quickly, the governor, similar in action and principle to that of a steam-engine, flies open from the increased centrifugal force, and by means of cog-wheels and levers closes the fans at the end of the beams, thus offering a greater resistance to the air, and, consequently, increasing the labour of the prisoners working at the wheel.
I love those brass ball governor things. Such a simple, yet clever engineering solution. Lots to be seen at Science Museum, Kew Bridge Steam Museum, &c. Presume the poor bastards on the treadmill were on the level directly below.
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