e
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.

159. _Q._--Is the use of the dead plate universally adopted in Boulton and
Watt's land boilers?

_A._--It is generally adopted, but in some cases Boulton and Watt have
substituted the plan of a revolving grate for consuming the smoke, and the
dead plate then becomes both superfluous and inapplicable. In this
contrivance the fire is replenished with coals by a self-acting mechanism.

160. _Q._--Will you explain the arrangement of the revolving grate?

_A._--The fire grate is made like a round table capable of turning
horizontally upon a centre; a shower of coal is precipitated upon the grate
through a slit in the boiler near the furnace mouth, and the smoke evolved
from the coal dropped at the front part of the fire is consumed by passing
over the incandescent fuel at the back part, from which all the smoke must
have been expelled in the revolution of the grate before it can have
reached that position.

161. _Q._--Is a furnace with a revolving grate applicable to a steam
vessel?

_A._--I see nothing to prevent its application. But the arrangement of the
boiler would perhaps require to be changed, and it might be preferable to
combine its use with the employment of