At the beginning of the eighteenth century every element of the modern type of steam-engine had been separately invented and practically applied. The character of atmospheric pressure, and of the pressure of gases, had become understood. The nature of a vacuum was known, and the method of obtaining it by the displacement of the air by steam, and by the condensation of the vapor, was understood. The importance of utilizing the power of steam, and the application of condensation in the removal of atmospheric pressure, was not only recognized, but had been actually and successfully attempted by Morland, Papin, and Savery. Mechanicians had succeeded in making steam-boilers capable of sustaining any desired or any useful pressure, and Papin had shown how to make them comparatively safe by the attachment of the safety-valve. They had made steam-cylinders fitted with pistons, and had used such a combination in the development of power. It now only remained for the engineer to combine known forms of mechanism in a practical machine which should be capable of economically and conveniently utilizing the power of steam through the application of now well-understood principles, and by the intelligent combination of physical phenomena already familiar to scientific investigators. Every essential fact and every vital principle had been learned, and every one of the needed mechanical combinations had been successfully effected. It was only requisite that an inventor should appear, capable of perceiving that these known facts and combinations of mechanism, properly illustrated in a working machine, would present to the world its greatest physical blessing. The defects of the simple engines constructed up to this time have been noted as each has been described. None of them could be depended upon for safe, economical, and continuous work. Savery's was the most successful of all. But the engine of Savery, even with the improvements of Desaguliers, was unsafe where most needed, because of the high pressures necessarily carried in its boilers when pumping from considerable depths; it was uneconomical, in consequence of the great loss of heat in its forcing-cylinders when the hot steam was surrounded at its entrance by colder bodies; it was slow in operation, of great first cost, and expensive in first cost and in repairs, as well as in its operation. It could not be relied upon to do its work uninterruptedly, and was thus in many respects a very unsatisfactory machine.