502 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



danger of explosion, but the choice of boilers not being guided by this 

 sole consideration, accidents are to a certain extent inevitable. It is 

 in factories, where fixed engines are employed, and on board steamers, 

 where the engines are exposed to more numerous causes of destruction, 

 that explosions are most frequent and terrible ; they are much more 

 rare on locomotives, which are doubtless subject to more careful super- 

 vision. They are also less dangerous in this case, because they are 

 often limited to the bursting of a tube, an accident which the attendant 

 can immediately remedy by plugging it up. 



