centrical cylinders forming the flues. Others propose to place the water in flat horizontal pans, disposing it in thin strata, the lower surface of which should be exposed to the action of the fire, the upper forming the evaporating surface. It would be impossible, were it even expedient, within the limits of this article, to explain the details of all these various contrivances. I shall, therefore, confine my observations to one or two of those which have either come into practical use, or which I consider to be on the point of doing so.
The locomotive engines constructed by Mr. Stephenson, and used on the Liverpool and Manchester railroad, consist of a cylindrical boiler placed upon its side; the furnace being at one end, and the chimney at the other. This boiler has circular ends, and its length (seven feet) from end to end, is traversed by about 100 copper tubes, each an inch and a half in diameter. These tubes form the only communication between the furnace and the chimney; and therefore through them the draft from the furnace towards the chimney must pass. The furnace is a square chamber, of considerable size, the back of which is connected with the end of the boiler. The sides and top, as well as part of the front, are formed of a double plating of iron, with a small intermediate space. The bottom contains the grate-bars which support the fuel. The space between the plating just mentioned, is filled with water, which communicates with the water in the boiler; and every part of this intermediate space being below the level of the water in the boiler, must necessarily be always filled.
Under these circumstances it will be apparent, that the surface of fire on the grate-bars is upon
