Our correspondent S. B. cannot comprehend that the strength of iron
for a cylindrical boiler should Be in direct proportion to the
diameter thereof, in order to sustain an equal pressure per square
inch; wherefore, we must reason with him on the long scale. The
cohesive strength of good iron is 64,000 lbs. per square inch; and of
course, a strip of boiler-iron plate 1/8th inch thick will sustain
8000 lbs. If a boiler made of thin iron is 14 inches in diameter, or
44 inches in circumference, each inch of its length will contain 44
square inches, and either half thereof will contain 22 inches, and as
the pressure on this portion is sustained by at least two inches of
width of plate,--one inch on each side,--it follows that it will
sustain a pressure of at least 700 lbs. per square inch, in the
direction of circumference. If the diameter is double, the number of
square inches will be double, and will require double the thickness to
sustain equal pressure. With regard to the pressure endwise, the area
of a cylinder head 14 inches in diameter is 154 inches, and the
strength of the 44 inches of circumference would be sufficient to
sustain 352,000 lbs., which, divided by the area, is 2,275 lbs. per
square inch. If the diameter is 56 inches, the circumference being
172, would sustain a pressure endwise of 555 lbs. per inch. Thus it
will be seen that if the cylinder were even 20 feet in diameter, the
iron would better sustain the pressure on the head that on the
periphery. With regard to the requisite strength of the cylinder's
head, if they are made in a semi-spherical convex form, they will
require no more thickness of plate than the cylinder: but if they
consist of plane disks, the thickness thereof should bear the same
proportion to that of the periphery that the area in square inches
does to three times the circumference. But in general, no other rule
is observed for the thickness of the heads, than to make them
extravagantly heavy, without much regard to theoretic calculation.
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