[ [224] Watt to Boulton, 19th November, 1781.
[ [225] Watt to Samuel Ewer, jun., 9th July, 1781. Boulton MSS.
[ [226] Watt to Boulton, 30th August, 1781.
[ [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.”
[ [229] Watt to Boulton, 20th September, 1781.
[ [230] Watt to Boulton, 18th October, 1781.
[ [231] Watt, in a letter to Boulton, dated the 3rd July, 1782, speaks of it as an old plan of his own “revived and executed by William Murdock;” but we were informed by the late Mr. Josiah Parkes, that at an interview which he had with Mr. Watt at Heathfield, at which Murdock was present, Murdock spoke of the Sun and Planet motion as his invention, which Watt did not contradict. Boulton also attributed the invention to Murdock, as appears from his letter to Henderson, dated 22nd January, 1782; in which he says,—“Mr. Watt’s packet is not ready. I am to wait till his drawings [of the rotatory motion] are completed, which he is executing himself. There was some informality in those sent from Soho. Besides, he has another rotative scheme to add, which I could have told him of long ago, when first invented by William Murdock, but I did not think it a matter of much consequence.”
[ [232] Watt to Boulton, 26th Jan., 1782.
[ [233] “I have some time ago thought,” wrote Watt, “of a new expansive engine—a reciprocating engine with a heavy circular fly moved by a pinion from the end of the beam, so as to make three turns per down-stroke and as many contrariwise per return; so that in the first half of the stroke it may acquire a momentum which will carry it through the last half; and if a weight equal to half the load be put upon the inner end of the beam, and the engine be made to lift it during the return, by making a vacuum above the piston and using a rack instead of a chain, a cylinder of the present size may work to the same depth by half the steam; and I believe the engine will work very sweetly.”—Watt to Boulton, 16th January, 1782.