This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Fig. 1332 shows the manner of supporting the hearth-stone of a fireplace when timber joists are used. Into the front wall a or chimney breast, below the grate, is built the hearth-stone b, supported at one end by the wall and at the other by a trimmer arch c having its base situated in the wall a, and its crown abutting against the trimmer joist d.