Fire and garden engines are only applications of the pump to
different purposes. The fire-engine has generally two cylinders and
pistons, and has moreover an air-chamber for the purpose of making
the stream of water continuous. It acts in this way:--The water
is forced by the power of those who are pumping the engine into
a vessel air-tight and full of air, having an opening which joins
the “hose” at its lower part; the result is, that as the water is
forced in faster than it can well escape, the air above it--becoming
greatly compressed, and by its expansion between each stroke of the
pistons--forces the water out, and so continues the stream or jet.
Fig. 3 shows this air-chamber; _a_ joins to the hose _c_, and _b_ is
in union with the forcing-pumps of the engine. The air is represented
as it would be compressed to about half its bulk, for it at first
filled all the air-chamber down to the openings.