Afterward one of the engineers said to me: "Do not let it be known
that I told you what you have hauled or I shall lose my place, but you
have drawn 50 per cent more than the maximum load of one of our 40 ton
engines." I said: "You attempted to 'stall' us, and when you try it
again, be fair enough to give me a flat of pig iron, and as you pack
cars on one end I will pack pig iron upon the engine until she will
stick to the track, but rest assured that you will not be able to get
that steam down." The experience with that engine proves conclusively
to my mind that the general principles of steam making are the same
for both stationary and locomotive practice. The grand secret of the
success of that Wootten engine was the enormous area of the grate
surface, being, if I remember correctly, 7 by 9 feet, permitting thin
fires to be carried and complete combustion to be obtained before the
gases reached the boiler tubes. An enormous crown sheet was presented,
and that is where the bulk of the work of any boiler is done.

Thin fires accomplish this. As already stated, a given amount of coal
generates a given amount of gas, and this gas requires a given amount
of air or oxygen. This air must be supplied through the grate bars and
then pass through the interstices of the mass of heated coal. It
requires about 10 cubic feet of air to consume one cubic foot of gas.
In stationary boilers we find that if we use "pea" and "dust" coal, an
extremely thin layer must be used, or the 10 feet of air per foot of
gas cannot pass through it; if "chestnut" coal be used, the thickness
may be increased somewhat; "stove size" allows a thickness of six
inches, and "lump" much thicker, if any wise man could be found who
would use that coarse, uneconomical size. Of course, I am speaking of
anthracite coal. Opinions differ about "soft coal," but the same
general principle applies as regards an unobstructed passage of air
through the hot bed of coal.