action of the fire, becomes more intensely heated, and acquires a tendency to ascend. It is to give play to this tendency, that the tubes in the roof are placed in a direction sloping upwards, as already described. The position of the tubes forming the grate-bars is attended with a like effect. When the engine is in operation, therefore, the water in the boiler is kept in a state of prodigiously rapid circulation round the furnace. The water in the tubes forming the grate-bars, rushes constantly from the front towards the back of the furnace; thence it ascends with rapidity through the upright tubes at the back, and passes from them with equal speed through the tubes in the roof, into the cylinder placed above the fire-door,—a corresponding descending current being continually maintained from this cylinder through the vertical tubes at each side of the fire-door. The steam bubbles which are formed in the tubes surrounding the furnace are carried with this circulating current into the cylinder above the fire-place; whence ascending by their levity, they pass into the vessel already mentioned called the separator. The boiler is kept continually filled by a force-pump, which injects water into one of the cylinders which surround the fire-door.
One of the most obvious advantages of this arrangement is, that every part of the metal exposed to the action of the fire, not excepting the grate-bars themselves, is in contact with a rapid stream of water. As fast, therefore, as the metal receives heat from the fire it imparts that heat to the water; and can never itself receive that excessive temperature which would cause its destruction by burning; besides which, all the heat which would thus be expended in producing an injurious effect is here consumed in
