Having reflected on these circumstances, it became apparent to Watt, that a vice was inherent in the structure of the atmospheric engine, which rendered a large waste of power inevitable; this vice arising from the fact, that the condensation of the steam was incompatible with the condition of maintaining the elevated temperature of the cylinder in which that condensation took place. It followed, therefore, either that the steam must be imperfectly condensed, or that the condensation could not take place in the cylinder. It was in 1765, that, pondering on these circumstances, the happy idea occurred to him, that the production of a vacuum could be equally effected, though _the place_ where the condensation of the steam took place were not the cylinder itself. He saw, that if a vessel in which a vacuum was produced were put into communication with another containing an elastic fluid, the elastic fluid would rush into the vacuum, and diffuse itself through the two vessels; but if, on rushing into such vacuum, this elastic fluid, being vapour, were there condensed, or restored to the liquid form, that then the space within the two vessels would be equally rendered a vacuum;—that, under such circumstances, one of the vessels might be maintained at any temperature, however high, while [Pg121] the other might be kept at any temperature, however low. This felicitous conception formed the first step in that splendid career of invention and discovery which has conferred immortality on the name of Watt. He used to say, that the moment the idea of separate condensation occurred to him,—that is, of condensing, in one vessel kept cold, the steam coming from another vessel kept hot,—all the details of his improved engine rushed into his mind in such rapid succession, that, in the course of a day, his invention was so complete that he proceeded to submit it to experiment. [Illustration: _Fig._ 19.]