ney of the engine; and being introduced into it through a confined jet presented upwards, formed a powerful steam-blast up the chimney, and a draft of corresponding power was consequently produced through the furnace. This admirable contrivance forms one of the most important features in the recent improvements of locomotive engines. Its efficiency will be more fully appreciated when it is considered, that in proportion to the velocity of the engine, the discharge of steam from the cylinder will be more rapid, and thus the draft in the furnace will be most powerful at the moment when its power is most wanted.
An unlimited power of draft in the furnace being thus obtained, a fire of adequate intensity may always be supported. The next object is to expose the water to the action of this fire, under the most advantageous circumstances. A great variety of contrivances have been from time to time suggested for the attainment of this end. All, however, consist in subdividing the water by some means or other, so as to expose an extensive surface of it to the action of the fire. Some have distributed the water in small tubes, through and around which the fire plays. Others have disposed it between thin plates of metal, upon the external surface of which the fire acts, so that a number of thin sheets of water are exposed upon both sides to the action of the fire. Others again have proposed to place the water between two cylinders, nearly equal to one another, so as to have a thin cylindrical shell of water between them, the fire acting both inside and all round the cylinders. A number of such concentrical cylindrical shells of water may thus be exposed to the action of the furnace; the space between the con-
