"The hearth was a thick iron plate placed above the old hearth, with an interval, n, of 3 inches between them. The two sides, or covings of the fire-place, were also formed of thick iron plates, placed 3 inches from the jambs. The space, n, at the back, and the spaces at the sides, communicated with the space, n, under the hearth ; two pipes, or channels, i, communicating with these hollow spaces, opened into the room at c, as shown by the dotted line in the section; these spaces could be closed at pleasure. When the fire was burning, the iron hearth, and the plates which formed the sides or covings, and the back, became very hot. The cold air at the floor, entering by the openings at a, into the space, n, was heated by the hearth, and rising into the spaces at the back and sides, had its temperature further increased; it then entered the channels, i, and escaped at c, thus diffusing an agreeable warmth over the whole room."