ar inch corresponding to 80 lbs. per square inch) = 6224.1379,
and this, divided by 17,800, gives 0.349 as the thickness in inches,
instead of 0.3125, or 5/16ths, the actual thickness. If we take the
pressure per square inch instead of per circular inch, we obtain the
following rule, which is somewhat simpler:--multiply the internal diameter
of the cylinder in inches by the pressure in pounds per square inch, and
divide the product by 8,900; the result is the thickness in inches. Both
these rules give the strain about one fourth of the elastic force, or 4,450
lbs. per square inch of sectional area of the iron; but 3,000 lbs. is
enough when the flame impinges directly on the iron, as in some of the
ordinary cylindrical boilers, and the rule may be adapted for that strain
by taking 6,000 as a divisor instead of 8,900.

303. _Q._--In marine and wagon boilers, which are not of a cylindrical
form, how do you procure the requisite strength?

_A._--Where the sides of the boiler are flat, instead of being cylindrical,
a sufficient number of stays must be introduced to withstand the pressure;
and it is expedient not to let the strain upon these stays be more than
3,000 lbs. per square inch of section, as the strength of internal stays in
boilers is generally soon diminished by corrosion. Indeed, a strain at all
approaching that upon locomotive boilers would be very unsafe in the case
of marine boilers, on account of the corrosion, both internal and external,
to which marine boilers are subject. The stays should be small and numerous
rather than large and few in number, as, when large stays are employed, it
is difficult to keep them tight at the ends, and oxidation of the shell
follows from leakage at the ends of the stays. All boilers should be
proved, when new, to twice or three times the pressure they are intended to
bear, and they should be proved occasionally by the hand pump when in use,
to detect any weakness which corrosion may have occasioned.

304._ Q._--Will you describe the disposition of the stays in a marine
boiler?

_A._--If the pressure of steam be 20 lbs. on the square inch, which is a
very common pressure in tubular boilers, there will be a pressure of 2,880
lbs. on every square foot of flat surface; so that if the strain upon the
stays is not to exceed 3,000 lbs. on the square inch of section, there must
be nearly a square inch of sectional area of stay for every square foot of
flat surface on the top and bot