"Here I was at a loss to understand how so much cold water could be heated so much by so small a quantity of water in the form of steam; and I accordingly applied to Dr. Black, and then first understood what was called latent heat. "But this theory, though useful in determining the quantity of injection necessary where the quantity of water [Pg096] evaporated by the boiler, and used by the cylinder, was known, and in determining, by the quantity and heat of the hot water emitted by Newcomen's engines, the quantity of steam required to work them did not lead to the improvements I afterwards made in the engine. These improvements proceeded upon the old established fact, that steam was condensed by the contact of cold bodies; and the later known one, that water boiled in vacuo at heats below 100°, and consequently that a vacuum could not be obtained unless the cylinder and its contents were cooled every stroke to below that heat." [Illustration: LOCH LOMOND.] FOOTNOTES: [16] We are indebted for many of the anecdotes of the life of Watt to the _Eloge Historique_, recently published by M. Arago, who was furnished with all the documents and circumstances relating to this celebrated person which were considered proper for publication, by his son, the present James Watt, Esq., of Aston Hall, near Birmingham, and to the notes added to this memoir by Mr. Muirhead, a relative of Mr. Watt. [17] The following is the account of these experiments given in Watt's own words:—