"And were that otherwise doubtful, it would, I think, be evident from the following quotation from a letter of his to me, dated 13th February 1783, where, speaking of an intended publication by a friend of mine, on subjects connected with the history of steam, he says, 'I think it is very proper for you to give him a short account of your discoveries and speculations; _and particularly_ _to assert clearly and fully your sole right to the honour of_ _the improvements of the steam engine_.' And in a written testimonial which he very kindly gave me, on the occasion of a trial at law against a piracy of my invention in 1796-7, after giving a short account of the invention, he adds, '_Mr. Watt was_ _the sole inventor of the capital improvement and contrivance_ _above mentioned._' "Under this conviction of his candour and friendship, it is very painful to me to controvert any assertion or opinion of my revered friend; yet, in the present case I find it necessary to say, that he appears to me to have fallen into an error; and I hope, in addition to my assertion, to make that appear by the short history I have given of my invention, in my [Pg095] notes upon Dr. Robison's essay, as well as by the following account of the state of my knowledge previous to my receiving any explanation of the doctrine of latent heat; and also from that of the facts which principally guided me in the invention. "It was known very long before my time, that steam was condensed by coming into contact with cold bodies, and that it communicated heat to them; witness the common still, &c. &c. "It was known, by some experiments of Dr. Cullen and others, that water and other liquids boiled in vacuo at very low heats; water below 100°. "It was known to some philosophers that the capacity or equilibrium of heat, as we then called it, was much smaller in mercury and tin than in water. "It was also known that evaporation caused the cooling of the evaporating liquid, and bodies in contact with it. "I had myself made experiments to determine the following facts:— "First, the capacities of heat for iron, copper, and some sorts of wood, comparatively with water. "Second, the bulk of steam compared with that of water. "Third, the quantity of water evaporated in a certain boiler by a pound of coals. "Fourth, the elasticities of steam at various temperatures greater than that of boiling water, and an approximation to the law which it followed at other temperatures. "Fifth, how much water in the form of steam was required every stroke by a small Newcomen's engine, with a wooden cylinder six inches diameter, and twelve inches stroke. "Sixth, the quantity of cold water required in every stroke to condense the steam in that cylinder, so as to give it a working power of about 7 lb. on the inch.