“Proceeding further with the experiment, if, instead of allowing the steam to blow into the water, you confine it until it gets to some pressure, then blow it into the water, it takes the same weight to raise the temperature to the same degree. This means that the total heat remains practically the same, no matter at what pressure. “This is James Watt’s discovery, and it led him to the use of high-pressure steam, used expansively.” Newcomen, in making his steam-engine, had simply made additions to Papin’s model. Watt had already done much more, for in trying to find how the engine might be made of greater service he had discovered at the outset the principle of the latent heat of steam. He knew that in Newcomen’s engine four-fifths of all the steam used was lost in heating the cold cylinder, and that only one-fifth was actually used in moving the piston. It was easy to see how this loss occurred. The cylinder was cooled at the top because it was open to the air, and was cooled at the bottom in condensing the steam that had driven the piston up so as to create a vacuum which would lower the piston for another stroke. Watt knew that what he wanted was a plan by which the cylinder could always be kept as hot as the steam that went into it. How was he to obtain this? He solved it by the invention of the “separate condenser.” This is how he tells of his discovery. “I had gone to take a walk on a fine Sabbath afternoon, early in 1765. I had entered the green by the gate at the foot of Charlotte Street and had passed the old washing-house, when the idea came into my mind that as steam was an elastic body it would rush into a vacuum, and if a communication were made between the cylinder and an exhausted vessel it would rush into it, and might be there condensed without cooling the cylinder. I then saw that I must get rid of the condensed steam and injection-water if I used a jet as in Newcomen’s engine. Two ways of doing this occurred to me. First, the water might be run off by a descending pipe, if an offlet could be got at the depth of thirty-five or thirty-six feet, and any air might be extracted by a small pump. The second was to make the pump large enough to extract both water and air.... I had not walked farther than the golf-house when the whole thing was arranged in my mind.” This was the discovery that gave us practically the modern steam-engine, with its countless uses in unnumbered fields. Newcomen’s engine was limited to the pressure of the atmosphere, Watt’s could use the tremendous force of steam under higher and higher pressure. He led the steam out of the cylinder and condensed it in a separate vessel, thereby leaving the cylinder hot. He closed the cylinder top, and prevented the loss of steam. The invention may seem simple enough as we study it, but as a matter of fact it was the attainment of this result of keeping the cylinder as hot as the steam that enters it that has given us our steam-engine.