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 blowing it into shape.

Glass is blown by three methods:  hand-blowing, semi-automatic blowing, and automatic blowing.  The first used was the hand method, and tho the introduction of machines is rapidly making the old way a back number, there are still factories where the old-time glass blower reigns supreme.

One of the great centers of the bottle industry in the United States is down in the southern end of New Jersey.  Good sand is dug there—­New Jersey was part of the bed of the Atlantic before it literally rose to its present state status—­and naturally the factories cluster about the source of supply of material.  Within a radius of thirty miles the investigator may see bottles turned out by all three methods.

The hand-blowing, while it is the slowest and most expensive means of making bottles, is by far the most picturesque.  Imagine a long, low, dark building—­dark as far as daylight is concerned, but weirdly lit by orange and scarlet flashes from the great furnaces that crouch in its shelter.  At the front of each of these squatting monsters, men, silhouetted against the fierce glow from the doors, move about like puppets on wires—­any noise they may make is drowned in the mastering roar of the fire.  A worker thrusts a long blowpipe (in glassworkers’ terminology a wand) into the molten mass in the furnace and twirls