The engraving of the engine was reproduced, with the description, in Harris's "Lexicon Technicum," 1704; in Switzer's "Hydrostatics," 1729; and in Desaguliers's "Experimental Philosophy," 1744. The sketch which here follows is a neater engraving of the same machine. Savery's engine is shown in Fig. 13, as described by Savery himself, in 1702, in "The Miner's Friend." _L_ is the boiler in which steam is raised, and through the pipes _O O_ it is alternately let into the vessels _P P_. [Illustration: FIG. 13.--Savery's Engine, A. D. 1702.] Suppose it to pass into the left-hand vessel first. The valve _M_ being closed, and _R_ being opened, the water contained in _P_ is driven out and up the pipe _S_ to the desired height, where it is discharged. The valve _R_ is then closed, and the valve in the pipe _O_; the valve _M_ is next opened, and condensing water is turned upon the exterior of _P_ by the cock _Y_, leading water from the cistern _X_. As the steam contained in _P_ is condensed, forming a vacuum there, a fresh charge of water is driven by atmospheric pressure up the pipe _T_. Meantime, steam from the boiler has been let into the right-hand vessel _P_, the cock _W_ having been first closed, and _R_ opened. The charge of water is driven out through the lower pipe and the cock _R_, and up the pipe _S_ as before, while the other vessel is refilling preparatory to acting in its turn. The two vessels are thus alternately charged and discharged, as long as is necessary. Savery's method of supplying his boiler with water was at once simple and ingenious. The small boiler, _D_, is filled with water from any convenient source, as from the stand-pipe, _S_. A fire is then built under it, and, when the pressure of steam in _D_ becomes greater than in the main boiler, _L_, a communication is opened between their lower ends, and the water passes, under pressure, from the smaller to the larger boiler, which is thus "fed" without interrupting the work. _G_ and _N_ are _gauge-cocks_, by which the height of water in the boilers is determined; they were first adopted by Savery. Here we find, therefore, the first really practicable and commercially valuable steam-engine. Thomas Savery is entitled to the credit of having been the first to introduce a machine in which the power of heat, acting through the medium of steam, was rendered generally useful. It will be noticed that Savery, like the Marquis of Worcester, used a boiler separate from the water-reservoir. He added to the "water-commanding engine" of the marquis the system of _surface-condensation_, by which he was enabled to charge his vessels when it became necessary to refill them; and added, also, the secondary boiler, which enabled him to supply the working-boiler with water without interrupting its work. The machine was thus made capable of working uninterruptedly for a period of time only limited by its own decay.