252 OF THE PARTS OF [SECT. vn. 



The excess of strength for risk being - (see art. 523.), we have 



t= ap 



120 n 



For copper, f= 11000 and 



525. RULE for the upper plates of long rectangular and cylindric boilers. 

 Multiply the load in Ibs. per circular inch on the safety valve, by the greatest dia- 

 gonal of the section of the boiler in inches, and for wrought iron divide the product 

 by 120 times the cubic contents of the boiler per horse power ; the result is the 

 thickness in inches. For capper, divide by 72 instead of 120. 



The bottom plates should be as much thicker as will compensate for wear ; 

 usually twice the thickness of the top ones. 



Example 1. In a rectangular boiler the greatest diagonal being 8 feet, and 

 consequently equivalent to a radius of curvature of 96 inches, the load on the valve 

 3^ Ibs. per circular inch, and the space for steam for each horse power 16 feet ; 

 required the thickness for the top plates of wrought iron. 



In this case, 



= ^'175 inches, for the top plates. 



~16 



The bottom plates should not be less than 2 x -175 = -35 inches. 

 This nearly corresponds with the practice of the best makers. 



526. Example 2. If the boiler be a long cylinder, of which the diameter is 

 60 inches, and the pressure on the safety valve 30 Ibs., the boiler containing 

 20 feet for each horse power of the engine ; then, 



30 x 60 



120^20 = ' 75 mches - 



In practice boilers of this kind are made barely equivalent to the working 

 pressure : can we wonder that they sometimes fail ? 



The same rule applies to internal flues, with addition for the effect of the fire. 



527. OF SPHERICAL BOILERS. A spherical boiler has its dimensions equal, 

 and consequently (art. 521.) its thickness should be, 



_ 3-81 ap 



t - -^j . 



Hence for wrought iron, t = 



and for copper, t = ~~ . 

 RULE for spherical boilers. Multiply the diameter in inches by the pressure on 



