Divide the thickness of the plate in inches by the diameter of the boiler in inches, and multiply the quotient by 5000 for a copper boiler with single-riveted shells; by 6400 for a copper boiler with double-riveted shell; by 7600 for a wrought-iron boiler with single-riveted shell; by 9000 for a wrought-iron boiler with double-riveted shell; by 10, 000 for a steel boiler with single-riveted shell; and by 12, 000 for a steel boiler with double-riveted shell.

Thus, to find the safe pressure for a boiler 32 inches in diame ter, the shell being made of wrought-iron plates 1/4 of an inch thick, single-riveted: First divide 1/4 by 32, which gives 1/128, and multiply this by 7600, the product, 59-3/8 lbs. per square inch, being the pressure required.