It was in the year 1765, when Watt was twenty-nine years old, that he made his first revolutionary experiment, but his first patents were not taken out until 1769, by which time his engine had attained a relatively high degree of perfection. In furthering his idea of keeping the cylinder at an even temperature, he had provided a covering for it, which might consist of wood or other poorly conducting material, or a so-called jacket of steam--that is to say, a portion of steam admitted into the closed chamber surrounding the cylinder. Moreover, the cylinder had been closed at the top, and a portion of steam admitted above the piston, to take the place of the atmosphere in producing the down stroke. This steam above the piston, it should be explained, did not connect with the condensing receptacle, so the engine was still single-acting; that is to say it performed work only during one stroke of the piston. A description of the mechanism at this stage of its development may best be given in the words of the inventor himself, as contained in his specifications in the application for patent on his improvements in 1769. "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 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 at least to be kept as cold as the air in the neighborhood of the engines, by application of water or other cold bodies. "Thirdly, Whatever air or other elastic vapor 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 can not 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.