252 
OF THE PARTS OF 
[sect. vit. 
The excess of strength for risk being ~ (see art. 523.), we have 
For copper, f — 11000 and 
a p 
Î2ÏT» 
a p 
W~n * 
525. Rule for the upper plates of long rectangular and cylindric boilers. 
Multiply the load in lbs. 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 copper, 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|- lbs. 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, 
i2o x V(l == ^’175 inches, for the top plates. 
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 lbs., the boiler containing 
20 feet for each horse power of the engine; then, 
30 x 60 _ A>K7£r . i 
120 x~20 ~ 0 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, 
381 ap 
a p 
Hence for wrought iron, t — 
and for copper, t = . 
Rule for spherical boilers. Multiply the diameter in inches by the pressure on