"No. 8" is almost certainly a reference to number eight frame in the famous "shield" or excavation rig which Brunel Senior had patented in 1818. There were twelve frames in the rectangular shield. Each frame had three levels allowing three miners to work, one above the other, on the same vertical section of the tunnel face. A total work force of thirty-six miners could therefore excavate the tunnel at one time (See the illustration on p. 12 of this catalogue). The huge iron frames have been compared to "A row of hollow books -- each book representing a frame containing three cells -- and each capable of being advanced independently" (Paul Clements, Marc Isambard Brunel, 1970, p. 92). When tunnelling was re-started on 25 March 1835, after a lapse of six and a half years, this was with an improved shield manufactured by John Rennie.