Mr. William Pritchard took out a patent in 1821 for the same object. He fixed a small cylinder in some convenient place contiguous to the furnace, with an air-tight piston to rise and fall within it. At the upper end of the piston-rod a chain is attached, which passes over pulleys, and its reverse end is connected to the top of the fire door or air-flue doors, by means of which connexion, when the fire door is raised, the piston descends in the cylinder by its own gravity; and when the fire door is shut down, the piston rises. On the outside of the cylinder is placed a branch pipe or channel, through which the air passes (as the piston ascends or descends) from the upper to the lower part of the cylinder, and vice versa. In the middle of this branch pipe is a valve or stop-cock, which may be so adjusted as to suffer the air to pass slowly, or by a very small stream, through the channel; by this means the ascent of the piston is retarded, and hence the entire descent, or closing of the fire doors, or air-flues, does not take place, until the air is nearly all expelled from the upper part of the cylinder, allowing time for the requisite quantity of atmospheric air to pass into the air-flues over the fire, for the purpose of consuming the smoke; the time of closing the doors is regulated, as above, by the valve or stop-cock in the branch pipe. - London Journal of Arts.