A. No faster than it is evaporated into steam, unless just before a hard
pull; or when shutting off with a heavy bright fire in the fire-box to
prevent waste of steam at the pops.

15. Q. What is the difference between priming and foaming of a
locomotive boiler?

A. Priming is caused by carrying the water too high in the boiler so
that when the throttle valve is opened some of it passes over with the
steam in the form of a spray. Foaming is caused by the water becoming
dirty from animal or alkaline matter, so that heat makes it foam like
soap suds. Muddy water or certain vegetable matters will also make a
boiler foam.

16. Q. What should you do in a case of foaming? What in a case of
priming?

A. In a case of foaming, if possible, allow the boiler to cool off a
little, increase the supply of feed water to prevent water getting too
low, and whenever possible blow some of the dirty water out of the
boiler, replacing it with clean water. In case of priming, shut off the
supply of feed water until the water level drops to the proper height in
the boiler.

17. Q. What danger is there when the water foams badly? When it primes
badly?

A. There is danger of knocking out cylinder heads, cutting the valves,
stalling on some grade or getting on some train's time because the