proportions, form what is called in bottle-makers' talk the "batch" or
"dope." This batch is put into a specially constructed furnace--a brick
box about thirty feet long by fifteen wide, and seven feet high at the
crown of the arched roof. This furnace is made of the best refractory
blocks to withstand the fierce heat necessary to bring the batch to a
molten state. The heat is supplied by various fuels--producer-gas is the
most common, tho oil is sometimes used. The gas is forced into the
furnace and mixed with air at its inception; when the mixture is ignited
the flame rolls down across the batch, and the burnt gases pass out of
the furnace on the other side. The gases at their exit pass thru a brick
grating or "checkerboard," which takes up much of the heat; about every
half hour, by an arrangement of valves, the inlet of the gas becomes the
outlet, and vice versa, so that the heat taken up by the checkerboard is
used instead of being dissipated, and as little of the heat of
combustion is lost as is possible. The batch is put into the furnace
from the rear; as it liquefies it flows to the front, where it is drawn
off thru small openings and blown into shape.

The temperature in the furnace averages about 2100 degrees Fahrenheit;
it is lowest at the rear, where the batch is fed in, and graduates to
its highest point just behind the openings thru which the glass is drawn
off. This temperature is measured by special instruments called thermal
couples--two metals joined and placed in the heat of the flame. The heat
sets up an electric current in the joined metals, and this current is
read on a galvanometer graduated to read degrees Fahrenheit instead of
volts, so that the temperature may be read direct.

All furnaces for the melting of sand for glass are essentially the same
in construction and principle. The radical differences in bottle
manufacturing appear in the methods used in drawing off the glass and