In the mean time Watt, notwithstanding his doubts, had been proceeding with the completion of his rotative machine, and by the end of the year applied it with success to a tilt-hammer, as well as to a corn-mill at Soho. Some difficulties presented themselves at first, but they were speedily surmounted. The number of strokes made by the hammer was increased from 18 per minute in the first experiment, to 25 in the second; and Watt contemplated increasing the speed to even 250 or 300 strokes a minute, by diminishing the height to which the hammer rose before making its descending blow. “There is now no doubt,” said he, “that fire-engines will drive mills; but I entertain some doubts whether anything is to be got by them, as by any computation I have yet made of the mill for Reynolds [recently ordered] I cannot make it come to more than 20_l._ per annum, which will do little more than pay trouble. Perhaps some others may do better.”[253] [253] Watt to Boulton, 28th November, 1782. [Illustration: “OLD BESS.”[254]] [254] The above illustration represents the first engine employed at Soho, with the alterations subsequently introduced, for the purpose of producing rotary motion. The old Kinneil engine, “Beelzebub,” as Watt called her, was entirely removed, and replaced by this engine, as explained by Watt in his MS. Memoir of Boulton now before us, wherein he states,--“The first engine of 18 inches cylinder, which was employed in returning the water to Soho mill, was replaced about 1778 or 1779 by a larger engine, the first on the expansive principle, which still remains there.” The engine became known at Soho as “Old Bess,” and she continued in regular work until within the last eight years. The illustration shows the state in which the engine now stands in South Kensington Museum. A. steam cylinder; B. steam pipe; C. throttle valve; D. steam valve; E. eduction valve; F. eduction pipe; G. valve gearing; H. condenser; I. air pump; K. air pump rod; L. foot valve; M. hand gear tappet rod; N. parallel motion; O. balance weight; P. rocking beam; Q. connecting rod; R. feed pump rod; S. sun wheel; T. planet wheel; U. fly wheel; W. governor; X. feedwater cistern.