On Watt's fly-ball device, as the machine speeds up, the balls fly round faster. As they fly round faster, they move out from the central post. As they move out from the central post, they close the steam supply more firmly. As they do so, the machine slows down and the balls spin more slowly, and allow more steam into the cylinders. In addition, the system has a safety release valve to allow excess steam to escape without driving the wheels. 500 Boulton & Watt engines were built by the firm between 1775 and 1800.