The case _G_ of the instrument has two inlets, one for steam, the other for water, the two being separated by the plate _F F_. Steam passes into the _receiving tube A_, and its escape is regulated by a taper-plug in the end of the rod _B_, moved by the handle _H_. At the upper end of the _combining tube_ _C_, where it swells out into a bell mouth, is a piston _N N_, sliding in the case. The lower end at _C_ is guided by the upper end of the _delivery tube_ _D_. The delivery tube _D_ is stationary. The _overflow_ opening is at _O_. The action of this instrument may be thus described: Steam entering the receiving tube _A_ escapes through its lower end when the plug _B_ has been drawn back. It unites with the water surrounding it in the space _N N_, is condensed and passes with the feed-water into the delivery tube _D_, and thence into the boiler. If too much water enters the combining tube, some will escape at the overflow _O_, and filling the space below the piston _N N_, will force the combining tube up toward the receiving tube, and, thus contracting the space between them, will diminish the water supply; while, if it gets too little water in this space, it will take some in at the overflow _O_, and thus draw down the piston _N N_, and enlarge the space, giving more water to the instrument. This self-regulating principle enables the instrument to continue working efficiently, no matter how much the steam pressure in the boiler varies. [Illustration: Fig. 70.] QUESTION 139. _How do you start this kind of injector?_