330 Review of^ Renmck 



er safety depends on hum&n vigilance, it will sometimes be jeopar- 

 dized ; for that vigilance will sometimes slumber. 



But notwithstanding the dangers inseparable from so powerful an 

 agent as steam, yet in fact but few of the explosions of the steam en- 

 gine which have ever taken place are attributable simply to the ener- 

 gies of the moving force. 



In all cases, where fatal accidents have occurred, the explosion ap- 

 pears to have been due to other causes than the mere expansive force 

 of the steam that would be formed when the boiler is in proper order, 

 and supplied with water. — p. 97. 



This, however, it must be acknowledged, is only saymg, that noth- 

 ing need be apprehended from the energies of the moving force em- 

 ployed, if we always guard it sufficiently ; while the improbability of 

 employing such a guard at all times is such, as to render the employ- 

 ment of powerful agents always more or less dangerous. 



It would seem at first view a very easy matter to provide against 

 any danger from the active energies of steam, by the means of safety 

 valves. These, however, sometimes, either by accident or design, 

 become too heavily loaded ; they are also liable to lose their sensi- 

 bility by becoming rusty, and in various other ways ; and such vast 

 quantities of steam are sometimes suddenly generated, that. the safety 

 valves are inadequate to afford any relief. Our author strongly re- 

 commends that every boiler should be furnished with two safety 

 valves, one of which should not be under the control of the fireman. 



It would alsa appear to be within our power to provide against 

 hazard from the. expansive force of steam by prot^i'no- the boilers ; 

 that is, by previously subjecting them to a pressure much greater 

 than that under which they are intended to work. 



It has been proposed to apply a pressure five or six times as great as 

 the boiler is intended to bear. Nor is this too great a precaution, for the 

 water proof is performed when cold, and the metal is then more tena- 

 cious than when heated, and the proportion, of six to one, at least, is 

 necessary before this difference is obviated. If a boiler be not sub- 

 jected to such proof, it may be possible that when heated, its limit of 

 rupture may be reached before the safety valve opens. The water 

 proof having been performed, the boiler should next be subjected to 

 a similar trial by steam, say of twice the force that is usually to be 

 generated in the boiler without causing its safety valves to act. In 

 France, it is required by law, that all high pressure boilers be subject- 

 ed to a proof five times as great a^ the boiler is intended to bear when 

 in service.- — p. 85, 



