every side surrounded by a sheet of water, upon which its radiant heat acts. The blast of air which rises through the grate-bars, and passes through the burning fuel, is carried by the draft through the 100 tubes which traverse the boiler longitudinally. This highly heated air, in passing through the tubes, imparts its heat to the water in the boiler by which they are surrounded; and when it issues into the chimney, it is reduced to nearly the same temperature as the water itself. By these means, the greatest portion of the heat, whether radiated by the fire, or absorbed by the air which passes through it, is imparted to the water; the shell of water surrounding the furnace receiving the radiant heat, while the water surrounding the tubes and the boiler receives as large a portion of the heat absorbed by the air as can be communicated to it. The shell of water surrounding the furnace upon which the heat acts being below the level of the water in the boiler, and being generally heated somewhat more highly than that water, has a tendency to ascend, a current is accordingly established, running from the intermediate space surrounding the furnace to the cylindrical boiler, and a corresponding returning current must of course take place. Thus there is a constant circulation of water between the spaces surrounding the furnace and cylindrical boiler.
A close chamber of some magnitude is constructed at the opposite end of the boiler under the chimney, and in this chamber are placed the working cylinders. In the earlier engines used on the railroad, these cylinders were placed outside the boiler, and were consequently exposed to the atmosphere. A considerable portion of heat was thus lost, the saving of which was completely accomplished by transferring
