144 OF THE GENERATION AND [SECT. in. 



permanent in alloys which are regularly exposed to a heat so nearly approaching 

 to that at which they fuse when newly formed. 



Alloys and Metals. Melting point. 



An alloy of lead 1 part, tin 3 parts, bismuth 5 parts, melts at 212 



Lead 1 tin 4 - - bismuth 5 - 246 



Tin 1 - - bismuth 1 - 286 



Tin 2 - - bismuth 1 - 336 



Lead 2 - - tin 3 - 334 



Tin 8 - - bismuth 1 - 392 



Tin 442 



Bismuth 472 



Lead 612 



Zinc 648 



But if a range of about two atmospheres above the working pressure be ne- 

 cessary to fuse the plug, and with a less range than that it is scarcely probable it 

 would withstand the working pressure, this mode of obtaining safety ought not to 

 be relied upon in practice. As an additional precaution the fusible plug may be 

 adopted, but not as a principal one, certainly not as one in which great dependence 

 may be placed. 



272. It has also been proposed to add a pipe to some part of the boiler, of 

 such thin metal that it may burst rather than the boiler ; but this plan, like that of 

 the metallic plugs, can only be useful in cases where the ordinary safety valves do 

 not act, for if it be made at first so that it would break on a small increase above 

 the working pressure, it would be constantly failing at that pressure, it being well 

 known that a metal strained to near its ultimate force will gradually break. 

 Besides, it is exceedingly difficult to determine the strain such a pipe will bear 

 without fracture, within the limits that would render it safe to depend on where 

 life is at hazard. 



273. The risk on high pressure boilers, even at their working pressure, be- 

 comes considerable in proportion as that pressure is high, and therefore too much 

 caution cannot be employed about them. At least one good safety valve, and a 

 mercurial gauge of sufficient diameter to allow the escape of the steam, should 

 be applied to each ; but it is the practice of careful engineers to apply two safety 

 valves. 



On the common safety valve an improvement might be made by constructing it 

 so as to be relieved of part of the load on the valve as it rises. 



