STEAM. 211 



combustion of smoke. Doubly exhausting stop cocks, 

 of which "Watt and Leopold have made one of the 

 principal features in the high-pressure steam-engines, 

 where the barrel might be used for other purposes. 

 He also discovered a method for transforming the 

 reciprocating motion into a rotary motion. Papin 

 invented the first piston engine. He was the first to 

 note that vapour of water affords a very simple 

 means for obtaining a vacuum in the capacity of the 

 barrel. He was the first to whom it occurred to 

 combine in the same engine the action of the elastic 

 force of steam with the power which, as he pointed 

 out, this same vapour possesses of condensing itself as 

 it cools." 



Captain Savery, an Englishman, who lived at the 

 end of the seventeenth century, made some inventions 

 in the same line, which are referred to by Arago as 

 under : 



" We have no proof that Salomon de Caus ever 

 constructed his steam-engine. I might say the same 

 of the Marquis of Worcester. Papin's engine in 

 which the action of the steam and its condensation 

 are successively brought into play was only executed 

 in miniature and with a view to make an experi- 

 mental trial of the exactitude of the principle upon 

 which it was based. So that although there was 

 nothing very new in Savery's steam-engines, it 

 would be very unjust not to mention them, as they 

 are really the first which were put into practical 



