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CHAP. VIII.

ON BOILERS.

Vessels for the generation of steam for supplying steam en-gines are of a great variety of forms, and are usually denom-inated boilers. These vessels require great care and judgmentin their construction, in order that the fuel may be mosteconomically applied, the waste and nuisance of smoke avoided,and the enormous force which steam is capable of exerting athigh temperatures, safely restrained.

The boiler is, in fact, to the steam engine what the livingprinciple is to animated existence. Like the stomach, it re-quires food to maintain the temperature, circulation, and con-stant action, which constitute the energy of the steam engineas a motive power. To keep up the temperature we have tofeed, stoke, and replenish the furnace with fuel, and we maysafely consider it a large digester, endowed with the functions ofproducing that supply of force required in the maintenance of

Fig. 165;

the action of the steam engine.

The boiler has undergone great changes ofform and construction to adapt it to use. Atfirst it was hemispherical, fig. 165, as when em-ployed by Newcomen , which shape was retainedfor many years with certain modifications. Sub-sequently it was altered by Watt to the form ofa parallelopipedon with a semi-cylindrical top,as shown in fig. 166. This form of boiler wasextensively used by Watt in the early stagesof his steam engines, and continued to takeprecedence of every other description of vesselemployed for the production of steam. It was,however, modified by the introduction of acentral flue, and a slight modification of its ex-terior shape to enable it to withstand greater pressures. Fig .