VAPORIZATION. 



exactly as in the instrument. But Mr. Watt introduced two 

 capital improvements into the construction of the condensing 

 engine ; the first was, the admitting steam, instead of atmos- 

 pheric air, to press down the piston through the vacuous cylin- 

 der, which steam itself could afterwards be condensed, and a 

 vacuum be produced above the piston, of which the same 

 advantage might be taken as of the vacuum below the piston. 

 The second was, the effecting the condensation of the steam, 

 not in the cylinder itself, which was thereby greatly cooled, 

 and occasioned the waste of much steam in being heated again 

 at every stroke, but in a separate air-tight vessel, called the 

 condenser, which is kept cool and vacuous. Into this con- 

 denser, the steam is allowed to escape from above and from 

 below the piston alternately, and a vacuum is obtained with- 

 out ever reducing the temperature of the cylinder below 212. 



A third and more recent improvement in the employment of 

 steam as a moving power, consists in using it expansively, a 

 mode of application which will be best understood by being 

 explained in a particular case. Let it be supposed that a piston 

 loaded with one ton, is raised four feet by filling the cylinder 

 in which it moves with low-pressure steam, or steam of the 

 tension of one atmosphere. An equivalent effect may be pro- 

 duced at the same expense of steam, by filling one fourth of 

 the cylinder with steam of the tension of four atmospheres, 

 and loading the piston with four tons, which will be raised one 

 foot. But the piston being raised one foot by steam of four 

 atmospheres, and in the position represented in the figure, the 



supply of steam may be cut off, 

 and the piston will continue to be 

 or l atmos. elevated in the cylinder by the 

 simple expansion of the steam be- 

 low it, although with a diminish- 

 ing force. When the piston has 

 been raised another foot in the 

 cylinder, or two feet from the 

 bottom, the volume of the steam 

 will be doubled, and its tension 

 consequently reduced from four 

 to T, or two atmospheres. At a 

 height of three feet in the cylin- 

 der, the piston will have steam 

 below it of the tension of -j. or 1| atmosphere, and when the 



or 



4 or 2 



4 atmos. 



