A. It is when the water is insufficient to cover all parts
exposed to the flames.

Q. What is the first thing to do on discovering that you have
low water?
A. Pull out the fire.

Q. Would it be safe to open the safety valve at such time?
A. No.

Q. Why not?
A. It would relieve the pressure on the water which being
allowed to flow over the excessive hot iron would flash into
steam, and might cause an explosion.

Q. Why do boilers sometimes explode just on the point of
starting the engine?
A. Because starting the engine has the same effect as
opening the safety valve.

Q. Are there any circumstances under which an engineer is
justified in allowing the water to get low?
A. No.

Q. Why do they sometimes do it?
A. From carelessness or ignorance.

Q. May not an engineer be deceived in the gauge of water?
A. Yes.