422 



THE STEAM-ENGINE. 



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 communi- 

 cation 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 vapor, 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 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- taind in such rapid succession, that, in the 

 course of a day, his invention was. so complete that he proceeded to submit it 

 to experiment. ' . > 



To explain the first conception of this memorable invention ; let a tube or 

 pipe, S (fig. 6), be imagined to proceed from the bottom of the cylinder A B 



Fig. 6. 



to a vessel, C, having a stop-cock, D, by which the communication between 

 the cylinder and the vessel C may be opened or closed at pleasure. If we 

 suppose the piston P at the top of the cylinder, and the space below it filled 

 with steam, the cylinder and steam being at the usual temperature, while the 

 vessel C is a vacuum, and maintained at a low temperature. Then, on opening 

 the cock D, the steam will rush from the cylinder A B through the tube S, 

 and, passing into the cold vessel C, will be condensed by contact with its cold 

 sides. This process of condensation will be rendered instantaneous if a jet 

 of cold water is allowed to play in the vessel C. When the steam thus rushing 

 into C, has been destroyed, and the space in the cylinder A B becomes a 

 vacuum, then the pressure of the atmosphere being unobstructed, the piston 

 will descend with the force due to the excess of the pressure of the atmosphere 

 above the friction. When it has descended, suppose the stop-cock D closed, 

 and steam admitted from the boiler through a proper cock or valve below the 

 piston, the cylinder and piston being still at the same temperature as before. 

 The steam on entering the cylinder, not being exposed to contact with any 



