Note 227 — Chapter XV · Watt again visits Cornwall—Invention of the Rotary Motion—The Patent Right again assailed.

Watt to Boulton, 30th August, 1781. In a subsequent letter he explained the invention as follows:—“The method I propose to stop an engine when the pump rods break is by means of an air bellows or forcing pump of a good large diameter fixed in the shaft and having a solid piston in it which is wrought constantly by the engine and quite easily while it goes at its ordinary speed, because there is a large valve open in its bottom or rather top, which suffers the air to pass and repass easily; but whenever the engine attempts to move quick, that valve shuts and all exit from the air is cut off, and it becomes a feather-bed to save the blow of the engine. This is exemplified by turning the valve-hole of a common bellows upwards and stopping the nozzle, then working the bellows first slowly and then quickly. I think this contrivance will be of great use and may prevent damage, especially those bangs which occur in setting on an engine.”—Watt to Boulton, 27th September, 1781.