never have read it through. He would have
become discouraged because he could not understand it. Now to illustrate
what I mean, we will go a little deeper and then still deeper, and you
will begin to appreciate the simple way of putting the things which you
as a plain engineer are interested in.

For example on page 114 we talked about the safe working pressure of
different sized boilers. It was most likely natural for you to say "How
do I find the safe working pressure?" Well, to find the safe working
pressure of a boiler it is first necessary to find the total pressure
necessary to burst the boiler. It requires about twice as much pressure
to tear the ends out of a boiler as it does to burst the shell, and as
the weakest point is the basis for determining the safe pressure, we
will make use of the shell only.

We will take for example a steel boiler 32 inches in diameter and 6 ft.
long, 3/8 in. thick, tensile strength 60,000 lbs. The total pressure
required to burst this shell would be the area exposed times the
pressure. The thickness multiplied by the length then by 2 (as there
are two sides) then by the tensile strength equals the bursting
pressure: 3/8 x 72 X 2 x 60,000 = 3,240,000 the total bursting pressure
and the pressure per square inch required to burst the shell is found by
dividing the total bursting pressure 3,240,000 pounds by the diameter
times the length 3,240,000 / (32 x 72) = 1406 lbs.

It would require 1406 lbs. per square inch to burst this shell if it
were solid, that is if it had no seam, a single seam affords 62 per cent
of the strength of shell, 1406 x .62 = 871 lbs. to burst the seam if
single riveted; add 20 per cent if double riveted.

To determine the safe working pressure divide the bursting pressure of
the weakest place by the factor of safety. The United States Government
use a factor of 6 for single riveted and add 20 per cent for double
riveted, 871 / 6 = 145 lbs. the safe working pressure of this particular
boiler, if single riveted and 145 + 20 per cent=174 double riveted.

Now suppose you take a boiler the same length and of the same material,
but 80 inches in diameter. The bursting pressure would be 3,240,000 /
(80 x 72) = 560 lbs., and the safe working pressure would be 560 / 6 =
93 lbs.

You will see by this that the diameter has much to do with the safe
working pressure, also the diameter and different lengths makes a
difference in working pressure.

Now all of thi