A. Air, like coal and its gases, must be heated before they will unite
to form what is known as combustion and so as not to reduce the
temperature of the fire-box below the igniting point of the gases.

17. Q. Why is it necessary to provide for combustion a supply of air
through the fuel in the furnace?

A. In order to supply the oxygen necessary for combustion.

18. Q. What is the effect upon combustion if too little air is supplied?
If too much air is supplied?

A. If too little air is supplied, combustion is not complete, and only
one-third as much heat is obtained. If too much air is supplied,
combustion is complete; but the excess air must be heated, resulting in
a lower temperature. If twice the amount of air required for complete
combustion be supplied, the temperature of the fire-box will be about
one-half as high.

19. Q. Give a practical definition of the igniting temperature.

A. In all ordinary combustion there is a definite temperature, called
the ignition or kindling temperature, to which combustible substance
must be heated in order that it may unite with the gas in supporting the
combustion. The burning substance must not only be heated up to the
kindling temperature, but it must be kept as high as this temperature,
or combustion will cease.

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 u