As steam is used from a boiler the water is slowly exhausted and it must be replenished with a fresh supply. In early days of the steam engine, water was pumped in against the boiler pressure by the use of powerful pumps, but in 1858 a man named Giffard invented a most ingenious apparatus by which steam of the boiler was used to force water in directly against its own pressure. This seems like lifting oneself by one’s boot straps. When the injector was first invented it seemed so impossible for it to work that engineers would not accept it until it had repeatedly demonstrated its operativeness. Even after it was accepted and in common use its mysterious operation was a subject of discussion for years. A sectional view of a Giffard injector is shown in Figure 45. Steam from the boiler comes down the tube A and passes out in a jet from the nozzle B. A needle valve C may be moved into the nozzle to reduce or shut off the jet of steam. The jet enters a conical chamber D which has a tube E that runs down into the water reservoir. The steam jet blows the air out of chamber D, producing a partial vacuum which draws water up the tube E and into the chamber D. When the water reaches the steam jet it is driven out of the chamber across a short open space into a slightly diverging tube or receiving cone F and through a check valve G into the boiler. At H there is a glass window through which the action of the water jet as it rushes into the receiving tube may be watched. I is an overflow pipe leading back to the reservoir. When the steam flows into the cone B it gathers momentum and issues from the nozzle in a jet of high velocity. On striking the water it combines with the water and condenses, but at the same time it imparts its momentum to the water so that the water is given more than enough momentum to drive it into the boiler against the pressure in the boiler. The pressure of steam is utilized to drive the piston of an engine or the steam may be set in motion by letting it issue from a nozzle, when its momentum may be employed to drive a steam turbine in the same way that a water jet drives a Pelton wheel but in either case it is heat that does the work. The steam expands as it moves the piston and as it passes through the nozzle its expansion is accompanied by a corresponding loss of heat. As we have already noted, it was for the purpose of raising water that inanimate powers were first set to work. It was with the same object in view that steam was first employed. HERO’S STEAM ENGINE