298 



HISTORY OF SCIENCE. 



steam upon the plug exceeds the pressure of the lever the plug is 

 raised, and permits the excess of steam to escape. It is curious that 

 the safety-valve was employed by Papin for this apparatus, which is 

 still called "Papin's Digester;" and yet he did not propose its use for 

 steam-boilers, nor was it in fact used for this purpose until many years 

 afterwards. 



FIG. 145. 



We come now to the engine invented by CAPTAIN THOMAS SAVERY 

 in 1698; and his apparatus was perhaps the first practical one of 

 steam power in any form, for Papin did not get beyond the construc- 

 tion of small models. Savery's arrangement was actually successfully 

 and extensively employed in raising water from mines, etc. His me- 

 thod was simply to create a vacuum within a capacious vessel by first 

 filling it with steam, and then condensing that steam by cold. A pipe 

 connected the vessel with the water to be raised, which was forced up 

 by the atmospheric pressure exactly as in the common pump. He 

 also employed the tension of steam acting directly upon the water to 

 force it up to a higher level. 



In 1711 THOMAS NEWCOMEN, an ironmonger, and JOHN CAWLEY, 

 a glazier, both of Dartmouth, perceiving the defects of Savery's ar- 



