[227] 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. [228] Boulton to Watt, 10th September, 1781. Boulton immediately proceeded with the erection of the new engine as secretly as possible. “The principles of the expansion engine,” said he to Watt, “you had invented before Dr. Small died, as Mr. Keir can testify as well as others. However, it is highly proper to execute every kind of beam that can be devised for the purpose of equalising the power. I have removed the little portions into the wooden house next the smith’s shop, and have blinded the window and barred the door. There is a convenient well that can be filled from the back brook, and the engine may be applied to the raising of water, which is the best sort of load to calculate from.” At the same time Watt was inventing his new jointed top-working gear, which he reported answered exceedingly well with the Dalcoath engine; and, in pursuance of an idea thrown out by Boulton, he perfected the model of a horizontal-axled elliptical with one pulley, which he described as performing _à merveille_, being free from all untoward frictions. He was also busy inventing a new method of an equalising beam, by causing the gudgeon to change its place; and another by means of a roller acting upon a curve in the nature of the working gear. Besides his experiments in mechanics, he was prosecuting investigations as to the properties of nutgalls in combination with various chemical substances, for the purpose of obtaining the best kind of ink for use with his copying machines; and at another time we find him contriving various iron cements for joints, confessing that he had “lost all faith in putty;” the result of which was his discovery of the well-known metallic cement.