The Civil engineer and architect's journal, scientific and railway gazette (1839)
London : [William Laxton] · leaf 401
ly left for its occurrence, a part or the whole of the crown bars insinking occupy the position of the intended brickwork ; and therefore, in or-der to insert a tunnel of the required dimensions, the bars and poling boardsmust be raised to their proper level: whicli is only to be done piecemeal,by… … When the heading is driven, it is widened atthe top along one side, to form, as it were, ashelf, \ipon which a crown bar may be laidlengthways. When this is done, the centrecrown bar is placed along the top heading, andsupported against the roof, by an upright propat the remote end, and by resting…
Architecture · Civil engineering · Science
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