158 Steam-Engine — Physics, ^c. 



a state of water on losing its heat, by contact with any thing 

 cold, Savary easily produced his vacuum by the injection of a 

 little cold water. 



He also used (though in a very disadvantageous manner) 

 the expansive force of steam to drive the water out of the 

 chamber, through a pipe different from that by which it 

 entered. 



It is doubtful whether this kind of engine was ever erected 

 on a scale of any magnitude ; for, a few years later, Newco- 

 men and Crawley invented the first engine with a cylinder and 

 piston ; and Savary, abandoning his own, united with them in 

 bringing their engine into use. 



As steam drives out air, the principle of this engine was to 

 let steam into the cylinder beneath the piston, where (the pis- 

 ton having risen to the top of the cylinder) a jet of cold water* 

 condensed the steam, produced a vacuum, and the piston, 

 working air tight, descended by the pressure of the atmos- 

 phere upon it, this pressure being a weight of nearly fifteen 

 pounds to each square inch ; so that if the cylinder were two 

 feet diameter, it would amount to a weight of three tons. 



This mode of operation prevailed for about fifty years, and 

 though much used to pump water from mines, was found to 

 have great inconveniences and defects ; till, in the year 1762, 

 Mr. Watt, being employed to repair a working-model of an 

 engine at the University of Glasgow, was led to direct his 

 mind to the improvement of the machine ; and from his expe- 

 riments sprung the most essential change, viz. the condensa- 

 tion of the steam in the cylinder, by opening a communication 

 with a separate vessel, into which the injection of cold water 

 was made, thus allowing the cylinder to remain hot. 



On opening that communication, the steam instantly rushes 

 to the cold, or rather is destroyed by the instant loss or reduc- 

 tion of its heat, and the vacuum thus made allows the piston 

 to descend as before mentioned. 



* This jet of cold water being let into the cylinder itself, necessarily cooled it 

 at every stroke ; and then it was necessary to heat it again to the boiling point, 

 before the piston-would reascend, and thus a vast loss of heat occurred. Editor. 



