SECT. III.] 



CONDENSATION OF STEAM. 



143 



The rule, it is to be remarked, is for the smallest aperture that ought to be used, 

 but there is much reason either to use two valves, or to double the area determined 

 by the rule. 



Example I. Required the area of a safety valve for a low pressure boiler, fifteen 

 feet long by 4 feet wide, the fire surface being considered equal to the area of 

 the bottom of the boiler. In this case 15 x 4 = 60, and the divisor is 5 for low 

 pressure steam ; hence, 



60 



= 12, the square of the diameter of the aperture ; 



and the square root is 3| inches nearly, for the diameter. And either two valves 

 of this diameter, or one of about 5 inches diameter, ought to be used. 



Example II. A high pressure boiler with 60 feet of fire surface is used for gene- 

 rating steam of four times the atmospheric pressure, what should be the least 

 diameter of the safety valve aperture ? 



In this case the divisor is 45, and 



60 

 45 



= 1-34 = the square of the diameter, 



and the square root is 1'16. 



270. Hence we find that the diameter of a mercurial gauge capable of giving 

 passage to the steam, is not so large as to prevent it being applied in practice and 

 with success, as a means of rendering boilers safe. A safety valve in addition 

 should of course be used, as the bends in the pipe would in some degree retard the 

 escape of the steam. 



271. The use of fusible metal plugs I do not think so likely to afford security, 

 for were the plug fusible at the pressure for which the boiler was adapted, it would 

 be so softened by the continued temperature of the working state, as to be incapable 

 of retaining the steam when made of sufficient power to be useful. Lead 

 would be wholly unfit, its melting point being at 612, a temperature at which the 

 force of the steam would be about 150 atmospheres. Tin melts at 442, and at this 

 temperature the force is upwards of 25 atmospheres. Alloys may be formed to 

 melt from 212 to 600, but we have no evidence that the melting points remain 



