furnace door is opened to replenish the fire with coals, the top lever is
raised up, and with it the piston of the small cylinder attached to the
side of the furnace. The Venetians are thereby opened, and a stream of air
enters the furnace, which, being heated in its passage among the numerous
heated plates attached to the back of the furnace door, is in a favorable
condition for effecting the combustion of the inflammable parts of the
smoke. The piston in the small cylinder gradually subsides and closes the
Venetians; and the rate of the subsidence of the piston may obviously be
regulated by a cock, or, as in this case, a small screw valve, so that the
Venetians shall just close when there is no more smoke to be consumed;--the
air or other fluid within the cylinder being forced out by the piston in
its descent.

157. _Q._--Had Mr. Watt any method of consuming smoke?

_A._--He tried various methods, but eventually fixed upon the method of
coking the coal on a dead plate at the furnace door, before pushing it into
the fire. That method is perfectly effectual where the combustion is so
slow that the requisite time for coking is allowed, and it is much
preferable to any of the methods of admitting air at the bridge or
elsewhere, to accomplish the combustion of the inflammable parts of the
smoke.

158. _Q._--What are the details of Mr. Watt's arrangement as now employed?

_A._--The fire bars and the dead plate are both set at a considerable
inclination, to facilitate the advance of the fuel into the furnace. In
Boulton and Watt's 30 horse power land boiler, the dead plate and the
furnace bars are both about 4 feet long, and they are set at the angle of
30 degrees with the horizon.