20. Q. State why such temperature is necessary and at what place in the
fire-box it is most required.

A. The center is the hottest part of the fire-box. There is a much lower
temperature in the fire-box at the sides and end sheets, due to the
water on the opposite sides of the sheets being of a lower temperature
than the fire-box; therefore if we get as high a temperature as possible
at the side and end sheets, we will increase the steam-making efficiency
of the boiler. The gases which are liberated from the coal as soon as it
becomes heated must attain a temperature of about 1,800 degrees
Fahrenheit, known as the "temperature of ignition," before they will
unite with air which must also be heated up to that point.

21. Q. How is draft created through the fire?

A. Exhaust steam escaping through the stack reduces the pressure in the
smoke-box below the pressure of the atmosphere outside, therefore the
air tends to force itself into the smoke-box through all openings; with
everything in good condition, the easiest and largest passage for it is
through the grates and other openings into the fire-box and from it
through the tubes into the smoke-box and up the stack.

22. Q. Is smokeless firing practicable?

A. Yes, but it is necessary in order to obtain good results that boiler
and fire-box be in good condition, coal broken to the proper firing
size; then, with the hearty and intelligent co-operation of both
engineer and fireman, smokeless firing is both economical and
practicable.