"In reference to his great improvements in the steam-engine, I have a more particular recollection, and can confidently affirm that he was the first to introduce the high-pressure principle of working, thus establishing a way to the present high state of efficiency of the steam-engine, and forming a new era in the history of steam power. To the use of high-pressure steam, in conjunction with the cylindrical boilers, also invented by Mr. Trevithick, I have no hesitation in saying that the greatly-increased duty of our Cornish pumping engines, since the time of Watt, is mainly owing; and when it is recollected that the working power now attained amounts to double or treble that of the old Boulton and Watt engine, it will be at once seen that it is impossible to overestimate the benefit conferred, either directly or indirectly, by the late Mr. Trevithick on the mines of this county. I have often expressed my opinion that he was at the same time the greatest and the worst-used man in the county."[113] [Footnote 113: See letter of Michael Williams, chap. xix.] The late Sir John Rennie and other scientific persons were, about 1830, associated with Mr. Henwood[114] in examining the work performed by Cornish pumping engines: their reports are curtailed in the following comments on Wheal Towan engine, similar to Trevithick's Dolcoath engine of 1816, except perhaps that the last named was a little inferior in its detail movements, while much less care was taken to avoid unnecessary loss of heat. [Footnote 114: Henwood, 'Edinburgh Journal of Science,' 10.] [Illustration: STEAM DIAGRAM OF WHEAL TOWAN PUMPING ENGINE, ERECTED 1827.]