attached to the rod, _m_. The water which entered the cylinder from the cistern, together with that produced by the condensation of the steam, flowed out of the cylinder by the opening, _f_, the pipe from which was conducted downwards, and terminated under water, the surface of which was at least 34 ft. below the level of the cylinder; for the atmospheric pressure would cause the cylinder to be filled with water had the height been less. The improvements which Watt, reasoning from scientific principles, was enabled to effect on the rude engine of Newcomen, are well expressed by himself in the specification of his patent of 1769. It will be observed that the machine was formerly called the “fire engine.” [Illustration: FIG. 2.—_Newcomen’s Steam Engine._ ] [Illustration: FIG. 3.—_Watt’s Double-action Steam Engine._ ] “My method of lessening the consumption of steam, and consequently fuel, in fire engines, consists of the following principles:—_First._ That vessel in which the powers of steam are to be employed to work the engine (which is called the cylinder in common fire engines, and which I call the steam-vessel), must, during the whole time the engine is at work, be kept as hot as the steam that enters it; first, by enclosing it in a case of wood, or any other materials that transmit heat slowly; secondly, by surrounding it with steam or other heated bodies; and thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time.—_Secondly._ In engines that are to be worked either wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam-vessels or cylinders, although occasionally communicating with them,—these vessels I call condensers; and whilst the engines are working, these condensers ought to be kept at least as cold as the air in the neighbourhood of the engines by the application of water or other cold bodies.—_Thirdly._ Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam-vessels or condensers by means of pumps, wrought by the engines themselves or otherwise.—_Fourthly._ I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire engines. In cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the air after it has done its office.—_Lastly._ Instead of using water to render the pistons and other parts of the engines air- and steam-tight, I employ oils, wax, resinous bodies, fat of animals, quicksilver, and other metals in their fluid state.” [Illustration: FIG. 4.—_Governor and Throttle-Valve._ ]