Its principal features were two large cylindrical vessels, which were alternately filled with steam from an adjoining boiler and with cold water from the well or mine out of which the water had to be raised. When either of the hollow vessels was filled with steam, and then suddenly cooled by a dash of cold water, a vacuum was thereby created, and, the vessel being closed at the top and open at the bottom, the water was at once forced up into it from the well by the pressure of the atmosphere. The steam, being then let into the vessel from the top, pressed upon the surface of the water, and forced it out at the bottom by another pipe (its return into the well being prevented by a clack), and so up the perpendicular pipe which opened into the outer air. The second vessel being treated in the same manner, the same result followed; and thus, by alternate filling and forcing, a continuous stream of water was poured out from the upper opening. The whole of the labour required to work the engine was capable of being performed by a single man, or even by a boy, after very little teaching. Although Savery’s plans and descriptions of the arrangement and working of his engines are clear and explicit, he does not give any information as to their proportions, beyond stating that an engine employed in raising a column of water 3½ inches in diameter 60 feet high, requires a fireplace 20 inches deep. Speaking of their performances, he says, “I have known, in Cornwall, a work with three lifts of about 18 feet each, lift and carry a 3½-inch bore, that cost 42_s._ a day (reckoning 24 a day) for labour, besides the wear and tear of engines, each pump having four men working eight hours, at 14_d._ a man, and the men obliged to rest at least a third part of that time.” He pointed out that at least one-third part of the then cost of raising water might be saved by the adoption of his invention, which on many mines would amount to “a brave estate” in the course of a year. In estimating the power of his engine, Savery was accustomed to compare it with the quantity of work that horses could perform, and hence he introduced the term “horse power,” which is still in use.