A small locomotive boiler with a barrel of in. long by 2 1/2 in. diameter has its iron plate 1 /16 in. thick. If the tensile strength of the material is 20 tons per square inch, and the joints are single riveted, the bursting pressure of the shell may be (2x1/16x20x2,240x55/100)/ 2 1/2

= 1,332 lb. per square inch; so that as far as the shell is concerned the working pressure may be 2001b. per square inch. If copper were used for the barrel, the working pressure with the same factor of safety might be 120 lb. per square inch.