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 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.

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.