142 OF THE GENERATION AND [SECT. in. 



could never boil dry without exposing the lead to be melted, and consequently 

 making an opening for the steam to escape. This contrivance he expected to pre- 

 vent the boiler being burst, by suddenly forcing water into it when it had been 

 allowed through inattention to boil dry and become red hot. 



267. A plug of fusible metal riveted into a hole in the bottom of a boiler, so 

 that it may melt and allow the water and steam to escape into the fire, whenever 

 the contents of the boiler attain that degree of heat which produces steam of 

 a dangerous elasticity, is a method of a like nature. 



268. The mercurial steam gauge is generally applied to boilers to show the 

 state of the steam ; it is a curved tube, or inverted siphon, in which the mercury 

 rises by the force of the steam, and indicates the pressure. See Section vm. 

 When this steam gauge is applied to a high pressure boiler, it requires a tube of 

 considerable length, and is an additional security against the bursting of the 

 boiler, because when the steam is too strong, the mercury will be displaced into 

 a proper receiver, and the steam escape through the tube, when the pressure 

 exceeds that which the boiler is designed to sustain. This steam gauge is a most 

 desirable appendage to a high pressure boiler, because it shows at once the state of 

 the steam ; but as a means of safety we had better inquire how far either it or 

 metallic plugs are likely to be effective, lest, under an impression of being secure, 

 the reliance may involve us in more of these fatal accidents. 



269. In the first place, it is obvious that the aperture or apertures by which 

 the steam is to escape, should be so large that it may escape as fast as the fire can 

 generate it ; if it does not, it must accumulate, and eventually explode. Now it is 

 possible to convert a cubic foot of water into steam from somewhat less than 1'5 

 feet of fire surface, (see art. 200.) and it is making only a small allowance for 

 security to admit that each foot of surface may convert a cubic foot of water into 

 steam. 



Hence we derive the following RULE : Let the density of the steam cor- 

 responding to the pressure be found : then multiply 7'5 times this density, by 

 the square root of the quantity the density is greater than 1, and divide the feet of 

 fire surface by the product ; this quotient is the square of the diameter of the 

 narrowest part of the valve in inches. Or, divide the area of the fire surface by 

 the number corresponding to the pressure or temperature, under the head divisor 

 in the following table ; and the quotient will be the square of the diameter of the 

 valve, in the narrowest part, in inches. 



