unnoticed; but they logically point to the truth that no furnaces
should present a cooling medium in contact with fuel which is
undergoing this process of digestion, so to speak. It will be very
evident, I think, from these facts that water-legs in direct contact
with a fire are a mistake. They tend to check a fire as far as their
influence extends, as a thin sheet of ice upon the stomach after
dinner would check digestion, and for the same reason, namely, the
abstraction of heat from a chemical process. If fire-brick could be
laid around a locomotive furnace, and the grate, of course, kept of
the same area as before, it is my belief that a very important
advantage would be at once apparent. An old-fashioned cast iron heater
always produced a treacherous fire. It would grow dead around the
outside next to the cold iron; but put a fire-clay lining into it, and
it was as good as any other stove.

If I have now made clear what I mean by making heat, we will next
consider the steam boiler. What is a steam boiler? It is a thing to
absorb heat. The bottom line of this science is the bottom of a pot
over a fire, which is the best boiler surface in the world; there is
water upon one side of a piece of iron and heat against the other. One
square foot of the iron will transmit through it a given number of
units of heat into the water at a given temperature in a given time;
two square feet twice as many, and three, three times as many, and so
on. Put a cover upon the pot, and seal it tight, leave an orifice for
the steam, and that is a steam boiler with all its mysteries.

The old-fashioned, plain cylinder boiler is a plain cylindrical pot
over the fire. If enough plain cylinder boilers presenting the
requisite number of square feet of absorbing surface are put into a
cotton mill, experience has shown that they will make a yard of cotton