Rumford fireplaces are more popular now than at any time since 1850. Inventor Count Rumford, an expert on the nature of heat, published essays on fireplace construction in the 1790s. Count Rumford understood that the only useful heat generated by a fireplace is radiant heat. So he designed a fireplace with a tall, wide opening; a very shallow firebox; and widely splayed covings, or jambs, to reflect as much radiant heat into the room as possible. Count Rumford also streamlined the throat, or in his words, "rounded off the breast" in order to "remove those local hindrances which forcibly prevent the smoke from following its natural tendency to go up the chimney." He essentially created a venturi that, like an inverted carburetor, shot the smoke and air up through the throat and into the receiving smoke chamber. Testing a Rumford at a brick manufacturing plant, we wanted to see if the flow through the throat is laminar-that is, with the air and combustion gases in layers rather than mixed together. The test proved the flow was laminar: The room air coming in over the fire doesn't mix with the hot products of combustion; rather, it acts like an invisible glass door, keeping the smoke behind it, as they both go up through the throat together. STRAIGHT FIREBACK ADVANTAGE By keeping the fireback straight and rounding the breast to achieve streamlined air flow, we can build Rumford fireplaces with throats less than half the size of a modern fireplace and with openings almost a foot taller. No wonder Rumfords are more efficient. They radiate more heat and waste less heated room air.