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 L, a communication is opened between their lower ends and the water
passes under pressure from the smaller to the larger boiler which is thus "fed" without interrupting the work. G and N are 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 commercially valuable steam-engine. Thomas Savery is entitled to the credit of having been the first to introduce into general use a machine in which the power 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.