When a boiler is under steam of say eighty pounds per square inch, the
body of water in it will have a temperature of about 324 degrees Fahr.,
and the shell plates will necessarily be somewhat hotter, especially on
the bottom (just _how_ much hotter will depend entirely upon the quantity
of scale or sediment present). Now introduce a large volume of cold water
through an opening in the bottom, and what becomes of it? Does it rise at
once, and become mixed with the large body of water in the boiler? By no
means. It _cannot_ rise until it has become heated, for there is a great
difference between the specific gravity of water at 60°, or even 212°
Fahr., and water at 324°. Consequently, it "hugs" the bottom of the
boiler, and flows toward the _front_ end, or hottest portion of the
shell. Now let us examine the effect which it produces.

We know that wrought iron expands or contracts about 1 part in 150,000
for each degree that its temperature is raised or lowered. This is
equivalent to a stress of _one ton_ per square inch of section for every
15 degrees. That is, suppose we fix a piece of iron, a strip of
boilerplate, for instance, ¼ of an inch thick and 4 inches wide, at a
temperature of 92 degrees Fahr., between a pair of immovable clamps.
Then, if we reduce the temperature of the bar under experiment to that of
melting ice, we put a stress of four tons upon it, or one ton for each
inch of its width.

[Illustration: FIG. 1]

Now this is precisely what happens when cold water is fed into the bottom
of a boiler. We have the plates of the shell at a temperature of not
less, probably, than 350° Fahr. A large quantity of cold water, often at
a temperature as low as 50° Fahr., is introduced through an opening in
the bottom, and flows along over these heated plates. If it could produce