CHAP, ix.] VARIOUS APPLICATIONS OF STEAM. 501 



production of steam which thus takes place is sufficient to cause 

 an explosion. 



2. The same accident may happen from the presence of incrusta- 

 tions left by the water upon the sides. This chemically-deposited 

 crust prevents the contact of the water with the metal, which grows 

 red hot, and then, if the crust happens to be detached, the meeting 

 of the water with the red-hot surface causes a sudden and considerable 

 production of steam, and the explosion of the boiler may be the 

 consequence. 



3. Water deprived of air and in a state of rest may be heated 

 without boiling to a temperature far above 100 C., but the least disturb- 

 ance determines a sudden ebullition, and a dangerous, because exces- 

 sive, production of steam, as we have already seen in recording Donny's 

 experiment. 



The above are causes of accident independent of the good state of 

 repair of the engine, or at least of its solidity of construction, inde- 

 pendent also of the due care and supervision of the stoker, the first 

 cause excepted, which is, however, one of the most frequent. The pre- 

 ventive measures for the latter are attentive watching of the water 

 level, and, if it is low, taking care not to replenish without precau- 

 tion and letting down the fire ; the choice of soft water, or if this is 

 impossible, the frequent cleaning of the inner surfaces is to be 

 recommended to stokers and managers. 



4. The steam may reach a pressure above the limits of resistance 

 if the safety valves are insufficient, work badly, or, what is worse still, 

 although unhappily too frequently the case, if they are stopped and 

 prevented from working at all. These apparatus ought therefore to be 

 constantly looked to. A mechanic who fastens down his safety valves, 

 says the celebrated engineer Mr. Fairbairn, with the energy of con- 

 viction, is comparable to the madman who throws himself into a 

 powder magazine with a lighted torch in his hand. Ignorance alone 

 explains so deplorable a practice, and it is the strict duty of managers 

 and engineers to stop it by employing only competent men, and 

 instructing those that are ignorant. 



5. One more cause of explosion is the bad construction of the 

 boiler, or, which comes to the same thing, the bad state of repair of its 

 different parts, owing to its age or long use. We have seen in describ- 

 ing the different types of boilers which are those that have the least 



