THE GROWTH OF THE STEAM-ENGINE. 



27 



The two vessels thus are alternately charged and discharged as 

 long as is necessary. Savery's method of supplying his boiler with 

 water was at once simple and ingenious. 



The small boiler D is filled with water from any convenient source, 

 as from the stand-pipe S. A fire is then built under it, and, when the 

 pressure of steam in D becomes greater than in the main boiler , a 

 communication is opened between their lower ends and the water 



Fig. 8. Savery's Engine, a. d. 1699. 



passes under pressure from the smaller to the larger boiler which is 

 thus "fed" without interrupting the work. G and iVare gauge-cocks 

 by which the height of water in the boilers is determined, and these 

 attachments were first adopted by Savery. 



19. Here we find, therefore, the first really practicable and com- 

 mercially valuable steam-engine. Thomas Savery is entitled to the 

 credit of havinsr been the first to introduce into s^neral use a machine 

 in which the po wer of heat, acting through the medium of steam, was 

 rendered useful. 



It will be noticed that Savery, like the marquis of Worcester, and 

 like Porta, used a boiler separate from the water-reservoir. 



He added to the " water-commanding engine " of the marquis the 

 system of surface condensation, by which he was enabled to change 

 his vessels when it became necessary to refill them ; and the secondary 

 boiler, which enabled him to supply the working boiler with water 

 without interrupting its action. 



The machine was capable of working uninterruptedly for a period 

 of time only limited by its own endurance. 



