500 TEE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



The total number of sea-going steamers in the British navy was, 

 in 1874, 109, of which 16 were ironclad line-of-battle ships or 

 frigates. 



In France while the mean tonnage of sailing vessels is 60 tons, it 

 reaches 280 as an average for steamohips. The total number of 

 French vessels in 1873 reached 14,750 ; of these, 462 were steam 

 vessels, of 141,000 tons in all, and 57,000 horse-power. 



IX. EXPLOSION OF STEAM-BOILERS. 



We have recounted the benefits for which civilization is indebted 

 to the invention of the steam-engine, and the progressively increasing 

 introduction of this powerful force into every kind of industry. We 

 must now make mention of the mischiefs it has occasioned, the 

 lamentable accounts of which we read from time to time in the papers. 

 Every medal has its reverse. All explosions of steam-engines have 

 in reality but one simple cause : for one reason or another the 

 pressure of the steam produced in the boiler exceeds the limit of 

 resistance of the sides, the metal is torn asunder, bursting under the 

 irresistible force of the gas, and casting about its fragments, covers 

 the neighbourhood with the ruins and its victims. To the mechanical 

 effects of this terrible outburst are added those which a volume of 

 steam at a high temperature cannot fail to produce. The stoker, the 

 assistants, the drivers, everyone in fact whom the metallic debris 

 or the scalding steam encounters are horribly wounded, burnt, or 

 scalded. 



What are the causes of the explosion ? Only that we have just 

 mentioned. 



An abnormal increase of pressure may arise from the following 

 causes : 



1. Depression of the water level, the consequence of which 

 is an elevation of temperature at the metallic surfaces subjected 

 to the action of the incandescent gases of the fire, without their 

 being cooled by the water of the boiler within. These surfaces 

 become red hot, their resistance decreases, and they are deformed 

 and torn; the danger is greater still, if then by the filling of the 

 boiler, water is brought suddenly into contact with them, and trans- 

 formed thus into steam under abnormal conditions. The excessive 



