In the standard steam fire-engine (Fig. 120) reciprocating engines and pumps are adopted, as seen in section in Fig. 121, in which _A_ is the furnace, and _B_ the set of closely-set vertical fire-tubes in the boiler. _C_ is the combustion-chamber, _D_ the smoke-pipe, and _R_ the steam-space. _E_ is the steam-cylinder, and _F_ the pump, which is seen to be double-acting. There are two pairs of engines and pumps, working on cranks, set at right angles, and turning a balance-wheel seen behind them. _G_ is the feed-pump which supplies water to the boiler, _H_ the air-chamber which equalizes the water-pressure, which reaches it through the pipe, _I J_. _K_ is the feed-water tank, under the driver's seat, _L_, which, with the engines and boiler, are carried on the frame, _M M_. The fireman stands on the platform, _N_. When it is necessary to move the machine, an endless chain connects the crank-shaft with the rear-wheels, and the engine, with pumps shut off, is thus made to drive the wheels at any desired speed. [Illustration: FIG. 121.--The Amoskeag Engine. Section.] [Illustration: FIG. 122.--The Silsby Rotary Steam Fire-Engine.] A self-propelling engine by the Amoskeag Company had the following dimensions and performance: Weight, 4 tons; speed, 8 miles per hour; steam-pressure, 75 pounds per square inch; height of stream from 1-1/4-inch nozzle, 225 feet; 1-3/4-inch nozzle, 150 feet; distance horizontally, 1-1/4-inch nozzle, 300 feet; 1-3/4-inch, 250 feet--a performance which contrasts wonderfully with that of the hand-worked fire-engine which these engines have now superseded. It has recently become common to construct the steam fire-engine with rotary engine and pump (Fig. 122). The superiority of a rotary motion for a steam-engine is apparently so evident that many attempts have been made to overcome the practical difficulties to which it is subject. One of these difficulties, and the principal one, has been the packing of the part which performs the office of the piston in the straight cylinder. Robert Stephenson once expressed the opinion that a rotary engine would never be made to work successfully, on account of this difficulty of packing. The most palpable of the advantages of the rotary engine are the reduction in the size of the engine, claimed to result from the great velocity of the piston; the avoidance of great accidental strains, especially noticed in propelling ships; and a great saving of the power which is asserted to be expended in the reciprocating engine in overcoming the inertia while changing the direction of the motions. These advantages adapt the rotary engine, in an especial manner, to the driving of a locomotive or steam fire-engine. [Illustration: FIG. 123.--Rotary Steam-Engine.] [Illustration: FIG. 124.--Rotary Pump.]