_A._--They are too various for enumeration, but most of them either operate
upon the principle of admitting air into the flues to accomplish the
combustion of the uninflammable parts of the smoke, or seek to attain the
same object by passing the smoke over or through the fire or other
incandescent material. Some of the plans, indeed, profess to burn the
inflammable gases as they are evolved from the coal, without permitting the
admixture of any of the uninflammable products of combustion which enter
into the composition of smoke; but this object has been very imperfectly
fulfilled in any of the contrivances yet brought under the notice of the
public, and in some cases these contrivances have been found to create
weightier evils than they professed to relieve.

154. _Q._--You refer, I suppose, to Mr. Charles Wye Williams' Argand
furnace?

_A._--I chiefly refer to it, though I also comprehend all other schemes in
which there is a continuous admission of air into the flues, with an
intermittent generation of smoke.

155. _Q._--This is not so in Prideaux's furnace?

_A._--No; in that furnace the air is admitted only during a certain
interval, or for so long, in fact, as there is smoke to be consumed.

156. _Q._--Will you explain the chief peculiarities of that furnace?

_A._--The whole peculiarity is in the furnace door. The front of the door
consists of metal Venetians, which are opened when the top lever is lifted
up, and shut when that lever descends to its lowest position. When the